Experimental Investigation and Modeling of Flow ّunction Variation in Enhanced oil Recovery Using Low Salinity/Smart Water Injection in a Carbonate Reservoir

Farhadi, Hamed | 2021

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 53939 (06)
  4. University: Sharif University of Technology
  5. Department: Chemical and Petroleum Engineering
  6. Advisor(s): Ayatollahi, Shahaboddin; Fatemi, Mobin
  7. Abstract:
  8. Low salinity water flooding (LSWF) as an efficient enhanced oil recovery (EOR) technique is proved to affect both, fluid-fluid and rock-fluid interactions to potentially release the trapped oil from the porous rock. Despite more than one decade of extensive research works on the low salinity water effect (LSWE), there are still many parameters to be studied for very complex cases. Therefore, in the first part of the experimental study of the dissertation, a systematic investigation on the effect of the initial wetting state (water-wet or oil-wet) of pure calcite is conducted to study the importance of fluid/fluid and fluid/rock interactions for LSWE investigation. In the case of initially water-wet cores, additional oil recovery is observed as the salinity increases. On one hand, dynamic interfacial tension of crude-oil/brine was measured for different salinity to assess fluid-fluid interaction. Based on the results, an increase in salinity led to a decrease in the crude-oil/brine IFT, which is in agreement with the results of secondary waterflooding. The reason behind the phenomenon was attributed to the transport of asphaltene molecules under a drift-diffusion controlled regime (which was suggested and developed in this work). On the other hand, the dynamic contact angle (CA), zeta potentials of oil/brine and rock/brine surfaces, and effluent pH are measured. It is shown that in the case of the water-wet sample, short-term IFT effect and long-term wettability alteration toward neutral-wetness (due to change in the electric charge of the crude-oil/brine and calcite/brine) are the dominating recovery mechanisms.In the case of oil-wet cores, the results show that lowering the salinity of seawater (SW) leads to an increase in the oil recovery, which is different from the observed trend in the water-wet core, where the same salinity reduction leads to the reduction of oil recovery efficiency. This suggests that the involved dominating mechanisms in an oil-wet sample would be different. To assess the involved mechanism, static and dynamic contact angle measurement was designed to simulate the time effect and the aging process in secondary waterflooding. CA measurement shows that diluted seawater (dSW) has more ability to change wettability toward water-wetness. Using the results of effluent pH and the zeta potentials of the crude-oil/brine and the calcite/brine surfaces, it is found out that a more negative charge at the interfaces would increase the water film thickness and shift the wettability toward more water-wetness. Besides, the results of coreflood tests confirm that the oil recovery is a strong function of the initial wetting state. SW brine recovered less oil and experienced an early breakthrough in the initially mixed-wet core compared to the initially water-wet core. This was attributed to the finger-like advancement of SW in the mixed-wet cores. Unlike the previous case, dSW injection resulted in higher ultimate oil recovery and almost the same breakthrough time in the initially mixed-wet core compared to the initially water-wet core. The superior performance of the low salinity water in the mixed-wet core is a result of the wettability alteration toward more water-wetness which triggers more piston-like front movement.Additionally, the performance of tertiary dSW is compared in water-wet and mixed-wet systems. Tertiary dSW flooding recovered more additional oil in the initially mixed-wet core (3.9 % of IOIP) compared to the water-wet core (0.8 % of IOIP). . To evaluate the involved mechanism, static and dynamic contact angle measurement was designed to simulate the time effect and the aging process in tertiary waterflooding In the case of the initially mixed-wet state, it is found out that the additional oil recovery is due to the increased water-wetness of the rock. The real-time measurements of the pH of the effluent brine during LSWF in this core, along with the results of calcite/brine zeta potential, shows that the trend of additional oil recovery is consistent with the trend of pH rise, as it was measured at the exit port. Combining the results of dynamic behavior IFT and CA with the pH results reveals that although both fluid-fluid and rock-fluid contribute to the oil recovery, the pH development prevails other involved mechanisms. It was demonstrated that as the effluent pH increases (due to microscopic dissolution of calcite), the rock surface becomes more negative, repels more oil, and hence, significant additional oil is recovered for the mixed-wet calcite cases.The second part of the experimental study aims to investigate the possible effect of pore morphology and surface chemistry on the effectiveness of LSWF. Tertiary LSWF (after secondary SW injection) was conducted on two rocks, namely, pure calcite (PC, which is the same as the last part) and Indiana limestone (IL), mainly constructed of calcite. Coorflooding results show that 40-time diluted seawater (40xdSW) recovered more additional oil in the case of the IL rock (20.8 % of IOIP) compared to the PC rock (3.9 % of IOIP). Static contact angle (CA) measurement was not able to justify the much higher tertiary oil recovery of the IL core compared to the PC core. High-resolution FE-SEM and surface elemental analysis using EDX showed that the CA test, ruling out the effect of both pore morphology and grain surface composition, is not able to depict a real picture of in-situ wettability alteration. Therefore, wettability alteration was studied with viewpoints of rock morphology and surface chemistry. , to investigate the impact of rock morphology on in-situ wettability alteration, high-resolution FE-SEM imaging was conducted on both rocks. The results showed that the PC rock has a wider pore size distribution and more angular particles compared to the IL rock, which leads to the creation of more stagnant regions in the PC rock after secondary SW flooding. Dynamic contact angle measurement indicates that as the stagnant regions increase, low salinity water becomes less efficient to alter the rock wettability toward water-wetness, which is in agreement with the low oil recovery of the PC core. To assess the effect of surface chemistry, the rock/bine zeta potential was measured. Based on the results, the surface of the IL rock experienced significantly more electric charge reduction (13.8 mV) compared to that of the PC rock (6.3 mV). Therefore, the IL surface repels oil stronger than the PC. The charge reduction of the PC surface is attributed to the development of pH gradient; consequently, the adsorption of CO32- as a potential determining ion. However, pH development is not a principal mechanism for increased negativity of the IL surface. Therefore, the XRF and EDX analysis performed on both rocks to address the impact of total and pore surface composition, respectively. The total composition of both rocks didn’t show significant differences. However, EDX analysis showed that the presence of dolomite on the PC pores and anhydrite on the IL pores are the main reasons behind the lower electric charge of the IL surface
  9. Keywords:
  10. Enhanced Oil Recovery ; Wettability Alteration ; Low Salinity/Smart Waterflooding ; Low Salinity Water Flooding ; Water Injection ; Surface Tension ; Rock Morphology ; Surface Chemisty

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