Gas flooding is a promising way to enhance oil recovery in unconventional oil reservoirs, but accurate identification of the distributions of movable and residual oil during gas flooding is difficult. In this study, the movable and residual oil distributions of tight conglomerate oil reservoirs during gas flooding are monitored with low-field nuclear magnetic resonance (NMR). The NMR T 2 spectra are converted into pore throat size distributions using a nonlinear conversion method in conjunction with high-pressure mercury intrusion, and then the lower limit of the pore throat size of movable oil under different pressure differences is determined. In addition, a mathematical model is proposed to predict the lower limit of the pore throat size of movable oil during gas flooding based on the capillary tube model and the fractal characteristics of pore structures in conglomerates. The research studies show that the model prediction results are very close to the results measured from experiments. The lower limit of the pore throat size of movable oil decreases with the increasing pressure difference of gas injection and decreasing core permeability. This study will be beneficial for characterizing residual oil distribution after gas flooding in unconventional oil reservoirs.
Using molecular dynamics simulations, we investigate the miscibility process of hydrocarbon mixture gas and crude oil in Tarim Oilfield, which was not well understood from the microscopic points of view. The results show that the increase of the pressure of hydrocarbon mixture gas is favorable for the gas−oil miscibility until the interfaces vanish under a minimum miscible pressure (MMP). The identified MMP (37.3 MPa) is well consistent with the experimental value. With increasing gas pressure, the parallel diffusion coefficients of gas molecules in the gas− oil interfaces decrease, while the perpendicular ones increase. We also analyze the profiles of potential of mean force between different gas and various oil components, identifying the components which dominate the process of gas−oil miscibility from the perspective of molecular forces. In addition, the introduction of gravity in gas flooding slightly increases the MMP. These results can guide the optimization and design of the enhanced oil recovery technology with hydrocarbon mixture gas flooding.
Fault stability refers to the risk level of reactivation of the pre-existing fault in the stress field. Fault reactivation within the oilfield is mainly caused by the increase of fluid pressure in the fault zone. The quantitative evaluation index of the fault stability is the critical fluid pressure (that is, additional fluid pressure) required for fault reactivation under the current pore fluid pressure. When the formation pore pressure reaches the critical value, the corresponding fault part will be in the critical stress state. The sliding of the fault in the critical stress state will easily cause oil and gas leakage and casing damage at the edge of the fault. Therefore, it is of great significance to study fault stability for oilfield production. Ground stress is a key parameter for fault stability evaluation. There are many methods to calculate the geomechanics including hydraulic method, acoustic emission method, and the use of the logging data, among which the hydraulic fracturing method can be used to obtain the most accurate horizontal minimum principal stress. This paper calculates the continuous geomechanics by using the logging data. There are many methods available for evaluating fault stability, among which fault sealing analysis technology (FAST) method is most widely used. FAST can be used to not only quantitatively evaluate fault stability, but also evaluate the impact of fault cohesion on fault stability. There are many factors affecting fault stability. The relationship between the differential stress and tensile strength of the fault rock will affect the trend of the fault reactivation.The direction of the stress field also affects the fault stability greatly. The argillaceous material weakens the strength of fault rock. When a large amount of argillaceous material enters the fault zone, the fault tends to reactivate. The change of reservoir fluid pressure will also lead to the change of horizontal stress to affect the stability of the fault. In addition, the accuracy of seismic interpretation will also affect the evaluation results of fault stability. Based on the geological model framework and one-dimensional geomechanical model calibration, this paper establishes a three-dimensional geomechanical model by using the finite element simulation method to carry out four-dimensional geomechanical research to evaluate the fault stability in the development of the Donghe 1 Reservoir in Tarim basin. The research results show that the fracture sealing gradually strengthens during the development of Donghe 1 Reservoir, and the quantized critical fracture opening pressure is 67.38MPa.
This paper provide several improved miscible assistants, trying to mitigate the problem that CO2 miscible flooding is difficult to achieve in reservoirs because of the high miscible pressures, which leads to a lower recovery up to expectations. These miscible assistants could be easily mixed with crude oil by adding into CO2 and reduce the interfacial tension to drive down the minimum miscible pressure (MMP) in order to enhance sweep efficiency. Some efforts have been made to improve this situation. The effective method is to draw the experiences from structure characteristics of surfactants applied in micro emulsion of CO2-water systems. Amphiphilic organic assistants were designed to synthetize with fluoro-alkane chains and non-fluoro-OAc chains as the CO2-philic ends, as well as alkane structure as the lipophilic ends. The minimization of MMP of CO2 miscible processing assistants has been analyzed and optimized by surface tension testing in CO2-kerosene system and CO2-crude oil system. The crude oil was obtained from field test pilot in L Reservoir. The results of interfacial tension tests show that per-acetylated glucose dodecyl ester molecules have the ability to lower the interfacial tension most in these five kinds of new miscible processing assistants in two categories. Citric acid triisopropyl ester molecules take the second place, and others almost make no difference. The probable reason is that kerosene is mainly composed of C12 and lack of heavy components, which cause a weak interaction between independent hydroxyl of citric acid and tartaric acid assistant and hydrogen bond of kerosene. The results of interfacial tension tests show that all these miscible assistants possess good effects on minimizing the interfacial tension of the CO2-crudes system, and could also definitely reduce the MMP. Among these, per-acetylated glucose dodecyl ester molecules and citric acid isopropyl ester molecules perform most excellent, and could decrease the MMP of CO2 flooding by 27.5%. The assistants have been implemented in the CO2 flooding plan of L Reservoir with 4 gas injectors and 15 producers. After 15 years’ development simulation, cumulated oil production will reach 3.4 MMbbl with recovery increasing from 41.6% (only CO2 flooding) to 46.6%. Injection test shows that 1400 tons CO2 has been injected with 3800 bbl oil produced. The improved miscible assistants provided perform as well as other existing assistants in reducing interfacial tension and enhancing sweep efficiency in CO2 flooding. Compared with assistants of light hydrocarbon, these assistants require a little quantity to improve the miscible flooding, which could break the economic limits. Compared with the traditional fluoride assistants, these assistants are quite different in molecular structure and could cause little pollution and have been applied in field test.
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