Article pubs.acs.org/EF
Experimental Evaluation of a Surfactant/Compound Organic Alkalis Flooding System for Enhanced Oil Recovery Yingrui Bai,*,† Zengbao Wang,† Xiaosen Shang,*,† Changyin Dong,† Xiutai Zhao,† and Pingde Liu‡ †
School of Petroleum Engineering, China University of Petroleum (East China), Qingdao 266580, P. R.China Research Institute of Petroleum Exploration & Development, PetroChina, Beijing, 100083, P. R.China
‡
ABSTRACT: Monoisopropanolamine (MIA) and monoethanolamine (MEA), which are two kinds of organic alkalis, present favorable potential for enhanced oil recovery. When MIA and MEA are respectively applied, the minimum oil/water interfacial tension (IFT) can be reduced to about 1 mN/m, the crude oil can be emulsified, and the originally oil−wet sand surface can be altered to weak water-wet status. To display the synergistic effect of surfactant and organic alkalis, a surfactant/compound organic alkalis (MIE/MEA) flooding system (SMM), which consists of 0.10 wt % SDBS, 0.15 wt % MIA, and 0.10 wt % MEA, is screened. This system can reduce the minimum oil/water IFT to an ultralow value which contributes to the emulsion stability. Results of sandpack flooding tests indicate that although the incremental oil recovery can be continuously enhanced with the increase of the SMM slug size, there is an optimal injection size (0.8 PV in this study) when the expense is taken into consideration. As for the effect of the injection type, the highest incremental oil recovery (21.86% OOIP) is achieved when organic alkalis (MIA/MEA) are simultaneously injected with the SDBS. During the SMM flooding, emulsification is a crucial flooding mechanism, and most of the incremental oil is extracted in the form of emulsified oil droplets. Moreover, the injection rate is optimized (0.5 mL/min) for the SMM flooding system to achieve a satisfactory incremental oil recovery. flooding, especially in the oil-wet reservoir. He thought that the capillary pressure is negative and the capillary force opposes the oil displacement in the oil-wet medium; by means of altering the rock surface wettability from oil-wet to water-wet, the capillary barrier can be eliminated or reduced, and hence the displacing fluid can improve oil recovery in an oil-wet reservoir. It has been widely accepted that surfactant and alkali are two kinds of effective chemicals to reduce the IFT and emulsify crude oil. Therefore, alkaline (A)/surfactant (S) (AS) flooding was once considered to be a kind of promising flooding method for enhanced oil recovery. In the AS flooding process, the basic function of injected surfactant is to adsorb at the oil/water contact to reduce the IFT. Alkali plays an important role in reducing IFT through reacting with saponifiable components in the crude oil to generate the so-called in situ soap which combines with the injected surfactant to synergistically lower the oil/water IFT and emulsify the oil.11−13 James14 and Liu15 summarized the functions of alkali during the AS flooding, including reducing the adsorptive capacity of surfactant, generating the in situ soap, and working as a sacrificial agent to protect the added surfactant by reacting with the divalent and trivalent metal ions. Li et al.16 conducted a series of core flooding experiments by using surfactant/NaOH system for tertiary oil recovery and found that the residual oil recovery of AS flooding was obviously higher than that of A or S flooding. Although the oil extraction effectiveness of AS flooding in field tests is favorable, some shortcomings such as scale buildup and clay swelling emerge because of the application of inorganic
1. INTRODUCTION Water flooding is the main development method after the natural recovery stage during the development of oilfields. According to results of field tests, water has good sweep efficiency.1 However, during the water flooding process, because of the natural heterogeneity of reservoirs, water easily breaks through high permeable channels and forms a fingering pattern which affects the sweep efficiency and finally reduces the ultimate oil recovery.2 Conformance control in injection wells and water shut-off in production wells are two commonly applied methods to eliminate the water fingering, and they have shown significant effectiveness,3,4 especially in oilfields of China. Moreover, because of the adverse mobility ratio of water to oil, the displacement efficiency of water flooding is relatively low, and a large quantity of residual oil which cannot be produced via water flooding still is retained in the pores of water swept zone.5 Chemical flooding, which includes polymer flooding, surfactant flooding, alkaline flooding, etc., is an effective technology for tertiary oil recovery, and it has been widely used in oilfields of China, such as Daqing oilfield and Shengli oilfield.6,7 Austad et al.8 considered that the key problem in chemical flooding is to keep the oil/water interfacial tension (IFT) as low as possible, and this can be done by injection of surface active chemicals, such as surfactant. Ding et al.9 pointed out that the IFT reduction and oil emulsification are two main mechanisms of chemical flooding for enhanced oil recovery. He also used sulfonate surfactant and sodium hydroxide (NaOH) to verify the relationship between IFT and emulsification: the lower the IFT is, the easier the emulsification becomes. In addition to the IFT reduction and emulsification, Al-Maamari et al.10 reported that wettability alteration is also an important mechanism to enhance the sweep efficiency of chemical © XXXX American Chemical Society
Received: February 1, 2017 Revised: April 22, 2017 Published: April 24, 2017 A
DOI: 10.1021/acs.energyfuels.7b00322 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels alkaline agents (NaOH, Na2CO3, etc.). Inorganic alkali can react with high valent metallic ions (Ca2+, Fe3+, Ba4+) to form precipitates; it can also react with the silica in the rock to form scale in formation pores and oil pipes. Umar17 and Cheng18 individually found the formation of silicate scale and calc− silicate scale during the ASP flooding. Cheng also reported that the scale formation rate of Daqing West-33 oil well was so fast that scale removal treatment in oil pipes was required every three months. Chavali et al.19 investigated the effect of alkaline solution on the swelling behavior of kaolinitic clays, and he found that alkaline ions can penetrate into kaolinitic clay; it can enhance the repulsion force of clay layers and results in the clay swelling. Both the scale buildup and clay swelling can cause formation damage and fluid permeability reduction. Therefore, the inorganic alkali-relevant flooding has been limitedly and cautiously applied in many oilfields, and the alkali-free flooding system is the hot topic nowadays.20,21 Common inorganic alkalis are responsible for the AS flooding limitation; organic alkalis, which can make the aqueous solution alkaline and have the application potential for enhanced oil recovery, have been reported by several researchers. Berger22 earlier proposed two basic performances of an organic alkali: (a) did not form precipitates with high valent metallic ions; (b) performed well as inorganic alkalis for enhanced oil recovery. Li et al.23 measured the oil/water IFT of triethylamine/sulfonate surfactant system and Shengli oil, and the IFT could be reduced to 10−3 mN/m when the triethylamine concentration was proper. Bai24 and Fu25 individually investigated the performance of the ethanolamine in AS or ASP flooding system, and both their results show that ethanolamine has not too poor performance of reducing the IFT and emulsifying crude oil, but the formula combination of ethanolamine and surfactant should be further optimized. Moreover, only ethanolamine has been reported to form the flooding system up to now; more other organic alkaline agents should be studied. In addition, the synergistic effect of organic alkali and surfactant for enhanced oil recovery should be also investigated. In the present study, monoisopropanolamine (MIA) and monoethanolamine (MEA) were applied as organic alkaline agents with sodium dodecylsulfonate surfactant (SDBS) to develop a surfactant/compound organic alkali flooding system. Both MIA and MEA are a kind of hydramine, and they are rarely reported for tertiary oil recovery; therefore, their EOR performances and flooding mechanisms remain unclear. Two main objectives of this work are (1) to explore the application potentials of MIA and MEA for enhanced oil recovery; (2) to screen an effective surfactant/monoisopropanolamine/monoethanolamine (SMM) flooding system and investigate its performances for enhanced oil recovery. What needs to be pointed out is that necessary protective measures must be taken during the experiments because of the hydramine toxicity to humans.
Table 1. Molecular Structure of Chemicals
was 946 kg/m3, and its acid number was 2.32 mg KOH/g of oil. The oil was centrifuged to remove water and solids before experiments. The formation brine of Shengli reservoir was sampled after filtration and used in this study, and its compositional analysis is shown in Table 2. 2.2. Interfacial Tension Measurement. The oil/water interfacial tension was measured by using the Texas-500 spinning drop tensiometer according to the following equation.26 The average reservoir temperature of Shengli oilfield is 50 °C. To more realistically simulate the formation temperature, the IFT measurements were conducted at 50 °C.
⎛ D ⎞3 L A = 1.2336(ρw − ρo )w 2⎜ ⎟ , ≥4 ⎝n⎠ D A is the oil/water interfacial tension (mN/m); ρw is the density of the water phase (g/cm3); ρo is the density of the oil phase (g/cm3); w is the rotational velocity (rpm); D is the measured drop width (mm); L is the length of oil droplet (mm); n is the refractive index of water phase. 2.3. Emulsification Measurement. Certain concentrations of Chemicals were first mixed with the formation brine to prepare chemical flooding system, and then it was stirred with crude oil using the electric mulser (SFS-S400, SieheChina Corp.) with a stirring speed of 3000 rpm for 10 min at 50 °C. The oil/water volume ratio was 1:1. To investigate the emulsion stability, each fresh emulsion was transferred into a transparent test bottle at 50 °C. Because of the density difference between water and oil, water separated from the emulsion and settled at the lower layer of test bottles. Separated water ratio, which is the ratio of the separated water volume to the total used water volume, was calculated and plotted in a graph as a function of time. Micrographs of each fresh emulsion and separated water were taken with the use of a microscope (XSP-8CE, Changfang Corp., China) to observe the status of the emulsified oil. 2.4. Sandpack Flooding Test. Sandpack cores were prepared using a coreholder with a diameter of 1 in. (2.54 cm) and a length of 1.48 ft (45 cm). The fresh quartz sand (80−120 mesh) was packed to ensure the same initial status of sand wettability and the homogeneity of the core. The experimental procedure was shown as follows: the sandpack core was saturated with the formation brine, and the water permeability of each core was measured; the sandpack core was saturated with the crude oil until the water production became negligible (water cut
