Investigation on Preparation and Profile Control Mechanisms of the

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Investigation on Preparation and Profile Control Mechanisms of the Dispersed Particle Gels (DPG) Formed from Phenol-Formaldehyde Crosslinked Polymer Gel Yifei Liu, Caili Dai, Kai Wang, Mingwei Zhao, Mingwei Gao, Zhe Yang, Jichao Fang, and Yining Wu Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b00055 • Publication Date (Web): 06 Apr 2016 Downloaded from http://pubs.acs.org on April 24, 2016

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Industrial & Engineering Chemistry Research

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Investigation on Preparation and Profile Control Mechanisms of the

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Dispersed Particle Gels (DPG) Formed from Phenol-Formaldehyde

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Crosslinked Polymer Gel

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Yifei Liu1, Caili Dai1∗, Kai Wang2, Mingwei Zhao1, Mingwei Gao1, Zhe Yang1, Jichao Fang1, Yining

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Wu1∗

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1 School of Petroleum Engineering, State Key Laboratory of Heavy Oil, China University of Petroleum (East

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China), Qingdao, Shandong, 266580, People’s Republic of China.

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2 China National Offshore Oil Corporation Research Institute, Beijing, 100028, People’s Republic of China.

∗

Caili Dai

E-mail: [email protected]

∗

Yining Wu

E-mail: [email protected] Tel: +86-532-86981183

Fax: +86-532-86981161 1

ACS Paragon Plus Environment


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Abstract

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To achieve in-depth profile control and further improve oil recovery, a new profile control agent,

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termed a dispersed particle gel (DPG), has been developed and reported. In this paper, the DPG

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particles with sizes ranging from submicron to micron are prepared successfully from

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phenol-formaldehyde crosslinked polymer gel by the high speed shearing method. The preparation

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method is convenient and easy to scale up for the field application. The microscopic characteristics

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of the DPG have been investigated by transmission electron microscopy (TEM) and dynamic light

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scattering (DLS). Parallel sandpack tests and microscopic visualization tests have been conducted to

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gain insights into the profile control mechanism. Additionally, scanning electron microscope (SEM)

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has been used to study the distribution of DPG particles in porous media. The results show that the

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DPG particles can block the high permeability layers by accumulating in large pore spaces or by

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directly plugging small pore throats. Meanwhile, the viscoelastic DPG particles can achieve in-depth

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profile control due to the elastic deformation and migration into the porous media of the reservoir.

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Moreover, the oil displacement tests show that the DPG can increase the sweep efficiency and

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effectively enhance oil recovery.

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Key words: in-depth profile control; dispersed particle gel; preparation mechanism; profile control

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mechanism; enhanced oil recovery

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1. Introduction

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With the rapid growth of the world economy, global demand for crude oil as an important energy

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resource and chemical raw material is growing constantly. However, most oil reservoirs are not

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homogeneous, which makes the exploration difficult. Meanwhile, the low oil recovery and excess

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water production caused by the heterogeneity of the reservoirs have become a real challenge for field

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operators.1-3 Many oilfields have experienced water flooding for long periods of time after their early

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stages of development. Long-term erosion of the high permeability layers, caused by water injection,

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results in more serious heterogeneity of the reservoirs, which leads to water breakthrough into

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producing wells along the highly permeable channels or fractures.4 The water breakthrough results in

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low water displacement efficiency, low oil production, high water cut and high remaining oil saturation,

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making the water-flooding projects uneconomical.5-8 As reported, approximately 65-77% remaining

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oil is left in the unswept areas of heterogeneous reservoirs.1 Thus, increasing the sweep efficiency is

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crucial to the reduction of water production and enhanced oil recovery, which is the key to meet the

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increasing demand for crude oil.

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However, the main methods to increase sweep efficiency are various profile control techniques,

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among which the chemical methods, including gel treatment, polymer flooding, and injecting foams,

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have been widely used.7-10 In recent decades, injecting a gelling system composed of polymer and

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cross-linker has been the most attractive profile control technology because of its low cost,

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controllable gelation time and adjustable gel strength.6,11-13 Nevertheless, the gelling systems have

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some intrinsic drawbacks. The gelation is process that is very sensitive to physical and chemical

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conditions such as temperature, pH, salinity, shear rates, etc. These parameters are usually very poorly

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known around the well bore, so it is very difficult to predict the actual gelling time, the final gel 3



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strength or the depth of the gel penetration, making the efficiency of the treatment lower.14-15

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A new trend in the gel treatment techniques is using preformed gel systems, which are prepared in

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surface facilities to overcome the drawbacks of the gelling systems.14-18 Based on published

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documents, the preformed gel systems that have been economically applied in the oilfields include

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preformed particle gel (PPG), micro-gels and swelling submicron-sized polymers (called bright

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water).19-24 Among these three preformed gel systems, PPG was the most extensively applied, and

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PPG has been successfully applied in more than 5,000 wells to reduce water production.14 Bai et al.

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have systematically studied the transport mechanism of PPG through porous media and factors

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affecting its properties and applications. Positive field application results have been reported.19-22

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However, PPG particles are polymer grains of millimeter size, which cannot pass through porous

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media with a permeability lower than several darcies.4,22

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In recent years, a newly developed preformed gel (dispersed particle gel (DPG)) has attracted

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significant attention because of its excellent properties such as controllable particle size and strength,

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simple preparation method, low costs, environmentally friendliness, etc.25-28 Our group has prepared

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the DPG successfully from the chromium acetate crosslinked gel using a peristaltic pump or colloid

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mill. The sizes of the prepared DPG particles, which range from nanometer to micrometer, can be

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adjusted by the shearing rate, the initial polymer mass concentration, and the salinity.29 Although

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several papers have reported the preparation of the DPG, systematic investigations of the preparation

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and profile control mechanisms are few. In this paper, the DPG products are prepared successfully by

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the high speed shearing method using the colloid mill. The bulk gel chosen to prepare the DPG is

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phenol-formaldehyde crosslinked polymer gel, owing to its excellent properties such as high elastic

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modulus, good thermal stability, salinity tolerance, etc.30,

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The detailed preparation process is

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proposed. The morphology and microscopic characteristics of the prepared DPG have been

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researched. Additionally, the profile control mechanism of DPG has been investigated by the parallel

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sandpack tests and microscopic visualization test. This research provides a scientific basis and

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technical support for the application of DPG in the fields.

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2. Materials and methods

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2.1 Materials

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The nonionic polyacrylamide (PAM) with an average molecular weight of 12,000,000 g/mol was

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provided by Beijing Hengju Chemical Group Co., Ltd. China. The phenolic resin crosslinking agent

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was provided by Yuguang Co., Ltd., Dongying, China. The crude oil sample from Dingbian reservoir

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(Changqing Oilfield, China) with a density of 0.73 g/cm3 and a viscosity of 1.34 mPas at the reservoir

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temperature of 80 °C was used for the displacement experiments. The eosin was used to dye the

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dispersed particle gels. The formation brine of Dingbian reservoir was used for all experiments. The

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formation brine was treated to eliminate the solids and floating oil droplets. The compositional

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analysis of the formation brine is shown in Table 1.

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2.2 Preparation of DPG

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The DPG was prepared by the high speed shearing method using a colloid mill (CMSD2000, IKN

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Equipment Shanghai Co., Ltd., China). First, 400 g of brine water and 400 g of bulk gel were put

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into the colloid mill. Then, the mixture was milled with different rotation speeds (1000-14000 rpm)

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for different times (2-15 min). Finally, the yellow liquid obtained from the colloid mill was the final

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DPG product. In this paper, the DPG was prepared using the colloid mill with 2000 rpm for 15

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minutes.

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2.3 Transmission electron microscopy (TEM) and scanning electron microscope (SEM) 5



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measurement

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The morphology of the prepared DPG particles was studied using the transmission electron

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microscope (TEM, Model JEM-100CXII, JEOL Ltd., Tokyo, Japan). A drop of the sample solution

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was placed on a formvar-covered TEM grid (copper grid, 3.02 mm, 200 mesh). The excess solution

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was removed by blotting with a filter paper. Then, the sample was observed using the TEM with the

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working voltage at 80-100 kV.

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The distribution of DPG particles in the core was observed by scanning electron microscopy (SEM,

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Model S-4800, Hitachi, Tokyo, Japan). One pore volume of DPG was injected into a core (Ф 25 mm

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× 100 mm). Then, the core was dried in a vacuum oven at 75 °C for 3 days. Next, the core was

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broken into small pieces with sizes of approximately 10 mm in length and 10 mm in width, and

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sputtered with gold to enhance their electrical conductivity. Subsequently, the fracture surface of the

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core sample was investigated using the SEM operating at 3.0 kV.

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2.4 Dynamic light scattering (DLS) measurement

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The particle size of DPG was measured using a temperature-controlled DLS device (Zetasizer Nano

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S, Malvern Instruments, Malvern, UK). Approximately 1.0 mL of the DPG sample was put into the

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sample pool slowly to avoid the formation of bubbles. Then, the DLS device was used to determine

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the particle size of DPG. The test temperature was 25 °C.

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2.5 Parallel sandpack test

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The fractional flow tests and oil displacement tests were conducted by heterogeneous parallel

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sandpacks to study the improvement in the sweep efficiency and the capacity for enhanced oil

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recovery of the DPG. The schematic diagram is shown in Fig. 1. In the experiments, high

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permeability and low permeability sandpacks were used to simulate the heterogeneity of reservoirs. 6


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A syringe pump (Model 100DX, Teledyne ISCO, Lincoln, NE, USA) was used to inject fluids at the

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desired flow rate. The experiment processes are described briefly as follows.

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In the fractional flow tests, first, the sandpacks were prepared by the dry-pack method. Second, the

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sandpacks were saturated with brine water, and the pore volumes were calculated by the weight

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method. Third, initial water flooding was conducted until the fractional flows of the sandpacks were

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constant. Fourth, a 0.3 pore volume (PV) DPG slug was injected into the sandpacks, and the

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sandpacks were put in an oven at 80 °C for three days. Finally, the extended water flooding was

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performed. During the tests, the brine water from the formation was used, and the fractional flows

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and pressures were recorded. The flow rates during the fractional flow tests were fixed at 1 mL/min.

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In the parallel sandpack oil displacement tests, the sandpacks were first saturated with brine. Then,

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the sandpacks were displaced with oil at a flow rate of 0.1 mL/min to set the initial oil saturation

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using the ISCO pump. Subsequently, the initial water flooding was performed at a flow rate of 1

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mL/min until the water cut reached 98%. Then, the DPG slug (0.3 PV) injection and the extended

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water flooding were conducted in sequence. The flow rates during the initial water flooding, the

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DPG injection and the extended water flooding were fixed at 1 mL/min, and the extended water

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flooding ended when the water cut again reached 98%. During the process, the oil recovery data

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were recorded.

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2.6 Microscopic visualization test

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The etched glass micromodel was used for the microscopic visualization test to investigate the

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profile control mechanisms of the DPG. Fig. 2 shows the schematic of the microscopic visualization

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test and the etched glass micromodel. The etched glass micromodel was prepared by etching channels

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of different sizes on a glass plate and sintering the etched plate with another glass plate, except for two 7



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pores on the diagonal of the model, which were used to simulate an injection well and an oil well. The

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pore network has one high permeability zone (diagonal region) and two low permeability zones

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(regions on two sides of the model), as shown in Fig. 2, which simulates the heterogeneity of the

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reservoir. The size of the model is 50 mm × 50 mm (length × width).

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The experimental procedure is as follows. First, the model was saturated with formation brine water.

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Second, oil was injected into the model to set the initial oil saturation. Third, an initial water flood was

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carried out using the brine water. At water flood residual oil saturation, the DPG injection was

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performed. A syringe pump (Model 100DX, Teledyne ISCO, Lincoln, NE, USA) was used to inject

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the fluids. The injection rate during the microscopic visualization test was fixed at 3.0 µL/min.

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3. Results and discussion

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3.1 Preparation of the DPG

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The DPG was prepared from an organically crosslinked polymer gel by the high speed shearing

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method. The bulk gel used to prepare the DPG was composed of 0.9% PAM and 1.0% phenolic resin

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cross-linker. The gelation time of the bulk gel is 24 h in an oven at 80 °C.

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3.1.1 Preparation mechanism of DPG

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The preparation and formation mechanism for DPG was proposed and is shown in Fig. 3. The whole

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preparation process can be divided into two major stages: the crosslinking reaction process of the

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bulk gel and the shearing process using the colloid mill.

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As shown in Fig. 3, the polymer and crosslinking agents are first mixed well (Fig. 3(a)). Then, the

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bulk gel crosslinking reaction process begins at 80 °C with the amide groups (-CONH2) from the

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polymer reacting with the hydroxyl groups (-CH2OH) from the crosslinking agents, forming the bulk

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gel with a three-dimensional network structure (Fig. 3(b)).

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The three-dimensional network

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structure is conducive to good viscoelasticity and perfect water holding capacity, which contributes

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to low water separating proportions and good thermal stability of the bulk gel.31,32 The crosslinking

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reaction period can be completed within 24 hours at 80 °C or in a shorter time at a higher temperature.

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Next, the bulk gel is added into the colloid mill mixed with the brine water, and the second period,

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the shearing process, is conducted. The large shearing force acts on the bulk gel, making it break into

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discontinuous gels that can be described as a partial dispersion (Fig. 3(c)). This process can be

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completed rapidly within approximately 1 minute with a shear rate of 5000 rpm, after which the bulk

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gel is only partially dispersed. Subsequently, the partially dispersed gel is pumped to flow through

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the circulation system of the mill, further crushed by the shearing force. This process lasts for

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approximately 8 to 10 minutes with a shear rate of 5000 rpm. As the shearing force, which is caused

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by the relative movement between the stator and rotor, continues to act on the partially dispersed gel,

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the size distribution of the particles becomes more and more uniform (Fig. 3(d)). Finally, the further

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rounding process of the particles occurs. During this process, the shearing force acts on the particles

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continuously. Because of high elastic modulus of the bulk gel,

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particles disappear, and the particles gradually turn into regular spheroids. After the further rounding

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process, the DPG particles with a regular shape are formed and uniformly dispersed in the solution.

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The yellow liquid obtained from the colloid mill is the final DPG product (Fig. 3(e)). The above

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discussion shows that the high speed shearing method using a colloid mill is a convenient way to

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obtain the DPG products, making this method a potential candidate for field application.

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3.1.2 Morphology and microscopic characteristics of the prepared DPG

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The TEM and DLS measurements were employed to study the morphology and characterize the

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microstructure of the prepared DPG. Fig. 4(a)-(c) shows the TEM images of the prepared DPG

28-30

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the edges and corners of the


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particles, in which the particles show regular spheroidicity. The size of a single DPG particle ranges

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from submicron to micron. Several particles may reunite in the DPG suspension, and the size of the

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reunited particles is approximately several micrometers.

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To verify the TEM results, DLS was performed to measure the particle size. Fig. 4(d) shows the

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radius distribution curve of the DPG particles derived from DLS measurement, where well-separated

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single distribution curves were observed. The size of the particles varies from 0.3 to 5 µm, and the

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average size of the particles is 1.59 µm, in good agreement with the TEM results.

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3.2 Profile control mechanisms

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3.2.1 Fractional flows

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Two fractional flow tests were performed using heterogeneous parallel sandpacks to verify the

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sweep efficiency improvement capacity of the DPG. The amounts of fluid produced by both

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sandpacks were recorded, respectively, to calculate the fractional flows. The changes in the fractional

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flows of the parallel sandpacks serve as important parameters in characterizing the profile control

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treatments of the DPG particles. The fractional flow of a sandpack at a given time can be defined as: ݂ଵ (%) = ொ

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ொభ

భ ାொమ

× 100

(1)

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where ݂ଵ is the fractional flow of the No. 1 sandpack at a given time, ܳଵ is the volume flow rate of

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the No. 1 sandpack at the moment, and ܳଶ is the volume flow rate of the No. 2 sandpack at the

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moment.

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Fractional flow and pressure curves from the initial water flooding, the DPG slug injection and

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extended water flooding are plotted in Fig. 5. The vertical lines in the figure indicate the start and

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end of the DPG slug injection process. The permeability ratios of the heterogeneous parallel

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sandpacks in the two tests were 0.59:0.20 and 0.65:0.11 µm2/µm2, respectively. As shown in Fig. 5, 10


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in both tests, the fractional flow and pressure curves have similar changes, described as follows:

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(1) During the initial water flooding period, water breaks through into the high permeability

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sandpack. As shown in fractional flow curves, the fractional flows of high permeability

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sandpacks in both tests are more than 90%, while the fractional flows of low permeability

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sandpacks are less than 10%, similar to what happens in the development of the heterogeneous

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reservoirs.

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(2) When the DPG injection begins, the fractional flows of the low permeability sandpacks

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increase rapidly, while the fractional flows of the high permeability sandpacks decrease

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quickly. Meanwhile, the injection pressure rises sharply. At the end of the DPG injection, the

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fractional flows of the low permeability sandpacks exceed the fractional flows of the high

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permeability sandpacks in both tests. The reversion of the fractional flow ratios of the

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heterogeneous parallel sandpacks indicates that most of the injected DPG particles are forced

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into the high permeability sandpack. The high permeability channels are effectively blocked

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and the fluid is diverted into the low permeability sandpack, which increases the sweep

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efficiency significantly. Sweep efficiency indicates the ratio of the water flooding swept

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volume of the reservoir to the total reservoir volume. Hence, the increased sweep efficiency

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indicates that water can displace the oil in the initial unswept regions after the DPG treatment,

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and therefore, effectively improve oil recovery.

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(3) At the beginning of the extended water flooding stage, the fractional flow ratio of the low

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permeability sandpack continues to increase. In addition, the injection pressure rises. After a

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period of extended water flooding, the fractional flow ratio of the heterogeneous sandpacks

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and the injection pressure tend to be stable. Finally, the fractional flow ratio changes from 91:9 11



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to 25:75 in test (a), while for test (b), the fractional flow ratio changes from 93:7 to 10:90.

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Additionally, the final injection pressure of test (b) is higher than the final injection pressure of

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test (a). The above results show that as the permeability ratio becomes larger, the profile

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control is more effective. This can be explained as follows because of the more serious

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heterogeneity of the parallel sandpacks in test (b), more DPG particles flow into the high

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permeability sandpack, which leads to a better plugging effect and a better profile

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improvement effect.

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(4) Another important result is the fluctuating data of the injection pressure and the fractional flow

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ratios shown in Fig. 5. This fluctuation is likely attributed to the propagation of the viscoelastic

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DPG particles in porous media. The DPG particles can block pore spaces. However, because

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of their elasticity, some of them will change their shape and pass though pore throats under the

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displacement force of injected water when the injection pressure rises. The deformed particles

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will return to their original shape after they pass though pore throats entering other large pores,

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creating the re-plugging. The result shows the probable “blocking-passing-blocking” feature of

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the DPG system, which is conducive to in-depth profile control.

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3.2.2 Enhanced oil recovery

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Two oil displacement tests were conducted using heterogeneous parallel sandpacks to study the

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enhanced oil recovery capacity of the DPG. The oil recovery curves for the initial water flooding, the

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DPG slug injection and the extended water flooding are shown in Fig. 6. In each image of Fig. 6,

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three curves can be observed, representing the oil recoveries of the high permeability sandpack, the

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low permeability sandpack and the summed oil recovery. The permeability ratios of the

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heterogeneous parallel sandpacks in the two tests were 0.61:0.22 and 0.60:0.10 µm2/µm2, 12


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respectively.

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In the initial water flooding stage, oil recovery of the high permeability sandpack is always higher

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than the oil recovery of the low permeability sandpack, and the oil recovery of the low permeability

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sandpack will never catch up with the oil recovery of the high permeability sandpack just by the

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initial water flooding. Additionally, the difference between the oil recoveries of the high permeability

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sandpack and the oil recoveries of the low permeability sandpack increases with the rise of the

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permeability ratio. The summed oil recovery becomes lower when the permeability ratio is larger

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(49.1% in test (a) and 43.8% in test (b)). The results indicate a more serious nonuniform

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displacement in a more serious heterogeneous reservoir, leaving more unswept oil after the initial

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water flooding.

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With injection of the DPG slug, the oil recoveries of both high and low permeability sandpacks

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increase. However, the oil recovery of the low permeability sandpack increases much faster than the

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oil recovery of the high permeability sandpack because the DPG particles effectively divert the fluid

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to the low permeability sandpack, significantly increasing the sweep efficiency.

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In the extended water flooding period, the oil recovery of the low permeability sandpack still

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increases faster than the oil recovery of the high permeability sandpack because of the blocking

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effect of the DPG particles in the high permeability sandpack. During the extended water flooding

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period, the oil recovery of the low permeability sandpack even exceeds the oil recovery of the high

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permeability sandpack. The final summed oil recovery is 65.3% in test (a) and 64.6% in test (b). In

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addition, the enhanced oil recovery values are 16.2% and 20.8% in test (a) and test (b), respectively.

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The above results indicate that the DPG can effectively increase the sweep efficiency and recover

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more of the remaining oil. Meanwhile, the DPG shows better profile control performance and 13



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improves oil recovery more effectively in more seriously heterogeneous reservoirs.

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3.2.3 Microscopic visualization test

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The specifically designed etched glass micromodel was used to simulate the heterogeneous

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reservoirs, which can easily show the profile control effect and the enhanced oil recovery mechanism

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of the DPG. The model has three zones, one high permeability zone (diagonal region) and two low

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permeability zones (regions on two sides of the model), as shown in Fig. 2, which can well simulate

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the heterogeneity of the reservoirs. The black fluid was oil, and the DPG particles were dyed red.

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The test results are shown in Fig. 7. Fig. 7(a) displays the initial oil saturation in the model, Fig.

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7(b) illustrates the oil and water saturation distribution after initial water flooding. Obviously, the

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water breakthrough along the high permeability zone leaves a large unswept region, which is

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consistent with the development status of heterogeneous reservoirs. When the DPG slug is injected

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into the model, the particles selectively flow first into the high permeability zone, then slowly disperse

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to the low permeability zones (Fig 7(c)). Fig. 7(d)-(g) shows the distribution and plugging effect of the

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DPG particles in the etched channels of the micromodel. When the pore size is smaller than the particle

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size, the particle will plug the pore directly (Fig. 7(d)). However, when the pore size is larger than the

283

particle size, several particles will aggregate to become a larger particle, bridging on the pore surface

284

and plugging the pore (Fig. 7(e)). Moreover, the particles can deform and pass through the pores under

285

the displacement force in the plugging process (Fig. 7(f)-(g)). The propagation of the particles

286

contributes to in-depth profile control and a better enhanced oil recovery effect. Meanwhile, the

287

“blocking-passing-blocking” feature of the DPG system will form pressure pulses in the reservoir

288

formation. The pressure pulses will promote the displacement of blind side residual oil or the

289

residual oil attached on the rock surface, which plays an important role in improving the oil recovery. 14


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290

Fig. 7(h) shows that less oil remains after the DPG treatment, and Fig 7(i) shows that the DPG can

291

significantly improve the sweep efficiency.

292

3.2.4 Distribution of DPG particles in core

293

Although the parallel sandpack test and microscopic visualization test have been conducted to

294

study the profile control mechanism, the distribution of DPG particles in the core and the interaction

295

that existed between the DPG and porous media remain unclear. The SEM is therefore used to

296

observe the distribution of particles in porous media, as shown in Fig. 8. The core used has a porosity

297

of 27~28% and a permeability of 0.50 µm2.

298

In the SEM images, large amounts of spherical particles with sizes ranging from submicron to

299

micron can be observed, consistent with the morphology and microscopic characteristics results.

300

Accumulation of the DPG particles in pore spaces, which will block the pore spaces and divert the

301

fluid to the unswept zones, is observed. Additionally, the particles can attach on the rock surface,

302

increasing the flow resistance of the high permeability layers. The above discussions show that the

303

DPG particles can block the pore spaces effectively, increasing the water flow resistance of high

304

permeability channels, which will improve the sweep efficiency and significantly enhance oil

305

recovery.

306

3.2.5 Enhanced oil recovery mechanism

307

Based on the above discussions, an enhanced oil recovery mechanism for the DPG system was

308

proposed, as shown in Fig. 9. After long-term water flooding, water breakthrough in high

309

permeability channels, resulting in ineffective circulation of injected water, happens in the

310

development of heterogeneous reservoirs (Fig. 9(a)), leading to low sweep efficiency and leaving

311

large amounts of remaining oil (Fig. 9(b)). To enhance oil recovery, DPG particles are injected, and 15



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312

the DPG particles first flow selectively into high permeability channels, accumulating in large pore

313

spaces or plugging the small pore throats directly (Fig. 9(c)). Meanwhile, the particles can deform

314

and pass through the pore throat by the displacement force of the injected water, so the particles can

315

enter the in-depth formation, forming the re-plugging, which contributes to in-depth profile control

316

and

317

“blocking-passing-blocking” feature of the DPG particles will form pressure pulses in the reservoir

318

formation, which can promote the displacement of blind side residual oil or the residual oil attached

319

on the rock surface. Fig. 9(e) shows that the subsequent injected water is diverted to unswept areas,

320

which will increase the sweep efficiency significantly and enhance the oil recovery.

321

4. Conclusions

better

enhancement

of

the

oil

recovery

effect

(Fig.

9(d)).

Additionally,

the

322

In this paper, the DPG has been prepared successfully, and laboratory tests have been conducted to

323

understand the enhanced oil recovery mechanism of DPG in heterogeneous reservoirs. The major

324

conclusions are summarized as follows:

325

(1) The DPG products are prepared successfully by the high speed shearing method. The prepared

326

DPG particles appear as regular spheroids with sizes ranging from submicron to micron. The

327

proposed preparation mechanism shows that the high speed shearing method using a colloid

328

mill is a convenient way to obtain the DPG products, thereby making the high speed shearing

329

method a candidate for field application.

330

(2) The parallel sandpack tests show that the DPG can significantly block a high permeability

331

sandpack, increasing the sweep efficiency and effectively improving oil recovery. Meanwhile,

332

as the permeability ratio of the heterogeneous sandpacks becomes larger, the profile control is

333

more effective. Additionally, the viscoelastic DPG particles can deform and pass through pore 16


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334

throats using the displacement force, forming the “blocking-passing-blocking” feature, which

335

is conducive to in-depth profile control.

336

(3) The microscopic visualization test shows that the DPG particles can selectively flow first into

337

the high permeability zone, accumulating in large pore spaces or plugging the small pore

338

throats directly. Meanwhile, the particles can deform and pass through the pore throats in the

339

plugging process, forming the in-depth re-plugging, and the “blocking-passing-blocking”

340

feature of the DPG particles will form pressure pulses in the reservoir formation, which can

341

promote the displacement of blind side residual oil or the residual oil attached on the rock

342

surface, leading to better enhancement of the oil recovery effect.

343

Acknowledgments

344

The work was supported by the National Key Basic Research Program (No. 2015CB250904), the

345

National Science Fund for Distinguished Young Scholars (51425406), the Chang Jiang Scholars

346

Program (T2014152), and the Postdoctoral Science Foundation of China (2015M580620). The

347

authors express their appreciation to technical reviewers for their constructive comments.

348

References

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treatments in China. Oil & Gas Science and Technology–Revue d’IFP Energies nouvelles 2010,

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Control Treatments in China Oilfields. Presented at SPE/DOE Symposium on Improved Oil Recovery,

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Tulsa, Oklahoma, 22-26 April 2006; Paper 99641.

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(22) Bai, B.; Huang, F.; Liu, Y.; Seright, R. S.; Wang, Y. Case study on prefromed particle gel for

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Particle Gel. PloS One 2014, 9(9), e100471-e100471.

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Aqueous Polyacrylamide/Formaldehyde Gel. Oilfield Chemistry 2001, 1, 008.

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Table 1. Composition of the Formation Brine -

Ions

K++Na+

Ca2+

Mg2+

Cl-

HCO3-

concentration (mg/L)

11470

2601

146

22649

259

total salinity (mg/L)

37125

22


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Figure captions：： Fig. 1: Schematic diagram of the parallel sandpack test. Fig. 2: Schematic diagram of the visual simulation experiment and the etched glass model. Fig. 3: Schematic diagram of the preparation mechanism of the DPG. Fig. 4: Morphology and microstructure characterization. (a)-(c) TEM micrographs of the DPG, (d) Radius distribution curve of the DPG particles. Fig. 5: Fractional flow and pressure curves of the parallel sandpack tests. Permeability ratio: (a) 0.59:0.20; (b) 0.65:0.11. Fig. 6: Enhanced oil recovery tests of the parallel sandpack tests. Permeability ratio: (a) 0.61:0.22; (b) 0.60:0.10. Fig. 7: Results of the microscopic visualization test in the etched glass model. The black fluid is oil, the red particles are DPG particles, and the yellow dotted lines indicate directions of fluid flow. Fig. 8: Distribution of DPG particles in core. (a) The core injected with one pore volume of DPG; (b)-(d) SEM micrographs of DPG particles in core. Fig. 9: The proposed profile control mechanism of the DPG.

23



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Fig. 1. Schematic diagram of the parallel sandpack test.

24


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Fig. 2. Schematic diagram of the visual simulation experiment and the etched glass model.

25



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Fig. 3. Schematic diagram of the preparation mechanism of the DPG.

26


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Fig. 4. Morphology and microstructure characterization. (a)-(c) TEM micrographs of the DPG; (d) Radius distribution curve of the DPG particles.

27



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Fig. 5. Fractional flow and pressure curves of the parallel sandpack tests. Permeability ratio: (a) 0.59:0.20; (b) 0.65:0.11.

28


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Fig. 6. Enhanced oil recovery tests of the parallel sandpack tests. Permeability ratio: (a) 0.61:0.22; (b) 0.60:0.10.

29



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Fig. 7. Results of microscopic visualization test in etched glass model. The black fluid is oil, the red particles are DPG particles, and the yellow dotted lines indicate directions of fluid flow. (a) The model saturated with oil; (b) water breakthrough along the high permeability zone; (c) injection of 0.3 PV DPG; (d) retention in larger pore space; (e) directly plugging the small pore throat; (f) retention and direct plug; (g) deformed DPG particles passing though the pore; (h) less remaining oil after the DPG treatment; (i) extended water flooding.

30


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Fig. 8. Distribution of DPG particles in the core. (a) the core injected with one pore volume of DPG; (b)-(d) SEM micrographs of DPG particles in the core.

31



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Fig. 9. The proposed profile control mechanism of the DPG. (a) initial distribution of crude oil in the reservoir; (b) water breakthrough along the high permeability channels after long-term water flooding; (c) plugging the high permeability channels by DPG; (d) deformed DPG particles pass through the pore throat; (e) increased sweep efficiency after the DPG treatment.

32


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For Table of Contents Only

33


Investigation on Preparation and Profile Control Mechanisms of the

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