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Experimental Investigation of Nano-laponite Stabilized Nitrogen Foam for Enhanced Oil Recovery Yingrui Bai, Xiaosen Shang, Zengbao Wang, Xiutai Zhao, and Changyin Dong Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b03798 • Publication Date (Web): 06 Feb 2018 Downloaded from http://pubs.acs.org on February 17, 2018
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Experimental Investigation of Nano-laponite Stabilized Nitrogen Foam for Enhanced Oil Recovery Yingrui BAI, Xiaosen SHANG*, Zengbao WANG, Xiutai ZHAO, Changyin Dong School of Petroleum Engineering, China University of Petroleum (East China), Qingdao 266580, P.R.China
ABSTRACT In this study, a modified nano-laponite was applied with foaming agent XR-1 to form a foam flooding system for enhanced oil recovery. The system formula was screened as 0.40 wt% XR-1+0.25 wt% nano-laponite, and both the foaming volume and the drainage half-time reach satisfactory values using the formula, proving the synergistic effect of the nano-laponite and XR-1 for the contribution to nitrogen foam properties. Micromodel displacement experiments were performed to investigate the micro-static stability and displacement behavior of this system. Results show high micro-static stability of nitrogen foam within the test time when XR-1 and nano-laponite are applied together. According to the matching rule between the foam bubble and pore throat, the nano-laponate stabilized nitrogen foam belongs to the ideal bubble’s throat blocking model, which can significantly improve the conformance efficiency. Moreover, the obvious oil dispersion and emulsification phenomena also contribute to the sweep efficiency. Single and double sandpack core displacement experiments were conducted to study the enhanced oil recovery (EOR) ability of this system in homogeneous and heterogeneous reservoirs, respectively. Results display that the nano-laponite stabilized nitrogen foam has favorable EOR ability, and the incremental oil recovery is considerable both in single and double core displacement experiments. Keywords: Nano-laponite; nitrogen foam; foam ability; displacement; enhanced oil recovery
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1 INTRODUCTION Water flooding is the main oil development method for the development of oilfields. However, because of the natural heterogeneity of reservoirs, water easily breaks through high permeable channels and forms viscous fingering, weakening the sweep efficiency and finally decreasing the oil recovery. 1,2 Therefore, a large quantity of remaining and residual oil which cannot be produced via water flooding still remains in the reserviors.3 Foam flooding is one of the promising chemical methods for enhanced oil recovery after water flooding and has been employed in thermal pilots in the late 1980s and several non-thermal applications in the middle 1990s.4 Many researchers have reported that foam has good displacement ability in heterogeneous reservoirs. Solbakken et al.5 found that the foam shows favorable conformance control performance in low permeability laminated sandstones compared to that of water, and the foam fluid diversion caused by bubbles blocking in porous media contributed to this performance. Siddiqui et al.6 speculated that the permeability contrast between the formation layers plays the most important role in foam diversion, and the degree of foam fluid diversion improved with increasing the permeability contrast. Nitrogen foam is one the widely applied foam flooding methods and has displayed satisfactory effectiveness in oilfields.7 Nitrogen is low-cost, green, and inexhaustible gas; it is also noncondensable and almost insoluble and is in the gaseous state under reservoir conditions and has high mobility and low flow resistance in porous media.8 Friedmann et al.9 studied the nitrogen foam flooding performance for crude oil in high temperature environment and proved that the enhanced oil recovery (EOR) ability of nitrogen foam was three times higher than that of hot water flooding. Pang et al.10 found that nitrogen foam
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made up the pressure deficit near the wellbore to slow bottom water coning in thin oil zones, called as foam anti-water-coning technology. Moreover, the presence of nitrogen bubbles can increase the gas effective viscosity, providing stability in the displacement process.11 The displacement performance of nitrogen foam mainly depends on foam properties in porous media, including foaming ability and foam stability. However, under the shearing action of formation, nitrogen foam bubbles usually experience the process of deformation, collapse, coalescence, and regeneration, eventually improving the complexity of the foam propagationand ability in porous media.12–14 The efficiency of foam flooding mainly depends on the stability of foam bubbles, which is a key factor to the mobility reduction and the swept efficiency in reservoirs. Both foaming ability and foam stability are well known to be significantly affected by the foaming agent. Surfactants including anionic surfactant, nonionic surfactant, and amphoteric surfactant are widely applied as the foaming agent.15,16 However, the single application of surfactant often faces challenges to maintain foam abilities under harsh conditions. Sun et al.17 reported that although surfactant had a good foaming ability for nitrogen gas, its foam stabilization ability was not satisfactory. The foam stability indicates the ability of foam to resist bubble breakdown, which can arise from bubble collapse or coalescence. Liquid drainage from foam can cause instability, other factors of instability are pressure reduction, heating, bubble coalescence, and etc.18,19 The instability of foam for long periods in formations makes it difficult to meet production requirements. The addition of foam stabilizer is the main method to increase the foam stability, and polyacrylamide is widely applied as the foam stabilizer. Xu et al.20 reported that the addition of partially hydrolyzed polyacrylamide (HPAM) at 1000 ppm concentration into the CO2
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foam can increase its drainage half-time by over 10 times. The main foam stabilization mechanism of polymer is that polymer molecules can be adsorbed onto the gas–liquid surface, enhancing the strength of the foam film and reducing the liquid drainage velocity, and these in turn enhance the foam stability.21 In recent years, nanoparticles have been applied in the EOR process as an alternative to surfactant to stabilize foam and have attracted interest of many researchers. Guo et al.22 reported that CO2 foam generated using the nanosilica and surfactant mixture exhibited superior formability and stability compared to that of only surfactant mixtures. Zhang et al.23 proved that the mixture of nano-laponite particles and surfactant (tetraethylene glycol monododecyl ether) contributed to the foam stability under certain conditions. Worthen et al.24 produced stable and viscous CO2-in-water foam using silica nanoparticles and caprylamidopropyl betaine surfactant. Singh and Mohanty25 reported the synergistic stabilization of the nitrogen foam by mixing surface modified silica nanoparticles and anionic surfactants. AlYousef et al.26 analyzed two main foam stabilization mechanisms of nanosilica: the nanoparticles arrangement during the film drainage and the increase in the maximum capillary pressure of coalescence. Wu et al.27 reported that the foam film with the adsorption of only surfactant molecules has weak strength, and thus it could not effectively reduce the fluid drainage and slow down the coalescence of bubbles; however, the nanosilica particles could form a strong solid adsorption layer onto the gas–liquid film, which would significantly reduce the film attenuation and greatly improve the foam stability. In general, the foam stabilization mechanisms of nanosilica are relatively clear. The synergistic effect of nanoparticles and surfactant may offer an effective method to overcome the shortcomings of foam. Besides nanosilica, many types of other nanoparticles
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can be applied. Moreover, extensive investigation should be carried out to further understand the stabilization effect of these nanaparticles on foam stability and EOR performance. Towards this effort, in this study, modified nano-laponite was applied together with self-synthetic surfactant XR-1 to form a nanoparticle-stabilized nitrogen foam flooding system for EOR. Both the nano-laponite and surfactant concentrations were optimized for best oil performance. Furthermore, micromodel displacement experiments were carried out to investigate the micro-static stability and displacement behavior of this foam system. In addition, single and double sandpack core displacement experiments were conducted to study the displacement performances of this foam system for EOR in homogeneous and heterogeneous reservoirs, respectively. 2 EXPERIMENTAL SECTION 2.1 Chemicals and Fluids Benzenesulfonates surfactant (XR-1, white powder, Self-synthesis in Lab) with a purity of 98 wt.% was applied as the foaming agent. Modified nano-laponite (Nanocor., USA) with an average lamellar structure diameter of 25 nm, an average lamellar thickness of 1 nm and a bulk density of 1000 kg/m3 was used as the foam stabilizer. The dispersion procedure for preparing XR-1/nano-laponite solutions is shown as follows: first, a certain weight of nano-laponite was added into formation brine and dispersed using an ultrasonic dispersive spectrometer for 10 min to achieve a uniform nano-laponite solution; then, a certain amount of XR-1 was added to the nano-laponite solution and dispersed also using the ultrasonic dispersive spectrometer for 3 min to finally form the uniform XR-1/nano-laponite solution.
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The crude oil was collected from Gudong reservoir of Shengli oilfield. It was degassed before experiments. The viscosity of the dead oil was 72 mPa·s at 55℃ (average temperature of Gudong reservoir) as measured by Brookfield DV-II+ Viscosimeter. The density of oil was 902 kg/m3. The oil was centrifuged to remove water and solids before experiments. The formation brine sampled from Shengli oilfield reservoir was used to prepare chemical solution after filtration in this study, and its compositional analysis is shown in the Table 1. 2.2 Bulk Foam Measurements Foamability and foam stability are two important characteristics to evaluate foam properties. In present study, two parameters, foaming volume and drainage half-time, were employed to evaluate foamability and foam stability, respectively. The experimental apparatus mainly included the Waing Blender (7012S, Waring Ltd., America) and a visual graduated cylinder with a cubage of 1000 mL. During foam property tests, 100 mL of prepared chemical solution was poured into the chamber of the Waring Blender and stirred at the speed of 2000 rpm for 3 min to generate foam at 55℃. Then, the foam was immediately transferred into the visual graduated cylinder which was put in an oven with the temperature of 55℃ to measure the foaming volume which was adopted to evaluate the foamability. After that, the volume change of foam was visually observed and the time was recorded. The time that foaming volume decreased to half of its maximum volume was the drainage half-life time. 2.3 Micromodel Displacement Experiments A square glass-etched micromodel (Figure 1) with the length of the diagonal of 7.0 cm was used to investigate microscopic displacement behaviors of foam. It was made according
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to the core sample collected from the Shengli Oilfield of China with a porosity of 27.35% and an average water permeability of 2120×10-3µm2. The average depth and width of pores in micromodel were about 45 and 35-80 µm, respectively. The microscopic model is placed in a holder with a pressure of less than 15 MPa and temperature of less than 95 °C, but the experimental temperature and pressure were 55℃ and 2 MPa, respectively. Take the diagonal as the middle line, five areas with the same width (1.4 cm) were divided and named as region a, region b, and region c, respectively, as shown in the Figure 1. The schematic of the micromodel displacement experimental apparatus is shown in the Figure 2. The foam generator was used to generate foam in experiments. It is actually a sandpack with a length of 15.00 cm and an inner diameter of 1.50 cm, which filled with quartz sand of 20−40 mesh. Procedures of micromodel displacement experiment were shown as follows: evacuated the micromodel; saturated the micromodel with formation brine; injected crude oil into the micromodel until the water production became negligible; injected water into the micromodel until the oil production became negligible; injected XR-1/nanolaponite foam; injected subsequent water. During foam flooding, the foam formula solution (0.0025 mL/min) and nitrogen (0.0025 mL/min) were injected into a foam generator simultaneously, and then uniform foam was injected into the micromodel. The displacement process was videotaped using video collection software. 2.4 Sandpack Displacement Experiments Sandpack cores were prepared using a coreholder with a diameter of 1 in.(2.54 cm) and a length of 0.98 ft (30 cm). The fresh quartz sand (40-120 mesh) was packed to ensure the same initial status of sand wettability and the homogeneity of the core. To pack cores with
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different water permeability, different meshes of fresh quartz sand was applied. The packing procedure was shown as follows: the coreholder, which was filled with formation brine, was positioned vertically and the quartz sand was added to fill it in ten steps. In each step after sand was packed, the coreholder was vibrated for 5 min. The experimental procedure was shown as follows: the horizontal sandpack core was vacuumed using a vacuum pump for 2 h; 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＜1%); the sandpack core was water-flooded until the oil production became negligible (oil cut＜1V%); the XR-1/nanolaponite foam were injected, and the subsequent water flooding followed until the oil production became negligible once again. The effluence was collected using a measuring cylinder to record the oil and water production. Two main differences between the sandpack and the micromodel displacement experimental apparatus are the replacement of the micromodel holder with the sandpack holder and the addition of the oven thermostats which can keep the apparatus constant temperature. Pressure drop was recorded using a set of pressure transducers with the measuring range 0-10 MPa and the accuracy of 10 kPa. A back-pressure regulator (BPR) with an accuracy of 10 kPa was used to maintain the pressure right downstream of the sandpack as 2.0 MPa.. All experiments were conducted at 55℃ with a constant injection rate of 0.5 mL/min. In the present study, four groups of single core (cores No. 1, 2, 3, and 4) and three groups of double core (cores No. 5, 6, and 7) sandpack displacement experiments were
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conducted, and their parameters were measured and shown in the Table 2. 3. RESULTS AND DISCUSSION 3.1 Screening of Nano-laponite Stabilized Foam Flooding System 3.1.1 Screening of Foaming Agent Concentration The foaming agent XR-1 is a kind of benzenesulfonates surfactant, it has been studied in our laboratory and applied in pilot tests for many years and shows relatively favorable foamability. The effect of the foaming agent XR-1 concentration on the foam properties is shown in Figure 3, indicating that when the XR-1 concentration is 0.1 wt%, the foam volume is 490 mL; with increasing XR-1 concentration from 0.1 wt% to 0.4 wt%, the foam volume increases to 550 mL and then levels off with a further increase in the XR-1 concentration, because the foam volume mainly depends on the surface tension between the foaming agent solution and N2 gas. The lower the surface tension is, the higher the foam volume becomes. The surface tension relationship between XR-1 solution with different concentrations and N2 gas is illustrated in Figure 4, indicating that the surface tension first decreases rapidly when the XR-1 concentration is less than 0.4 wt% and then levels off at ∼31 mN/m. This data prove that the critical micelle concentration (CMC) of the surfactant XR-1 is ∼0.4 wt%, and surface tension will not be further lowered with more increase in concentration. It also explains that the foam volume reaches the highest value when the XR-1 concentration is 0.4 wt%. The foam drainage half-life versus XR-1 concentration curve shown Figure 3 can be divided into three stages. Stage A: the drainage half-life obviously extends from 2.8 min to 5.7 min when the XR-1 concentration increases from 0.1 wt% to 0.4 wt%. Stage B: the drainage half-life remains stable when the XR-1 concentration increases from 0.4 wt% to 0.6
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wt%. Stage C: With further increase in the XR-1 concentration, the drainage half-life reveals gradually decreasing trend. The foam drainage half-life reveals the foam stability and is greatly affected by the adsorption ability of foaming agent molecules on the gas–liquid interface.28 When the XR-1concentration is
