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Gulong shale oil is found in a typical continental shale oil reservoir, which is different from marine shale oil reservoirs. The Gulong shale oil reservoir is a pure shale-type oil reservoir with abundantly developed nanoscale pores, making it extremely difficult to unlock fluids. Pressure drive does not easily achieve fluid unlock conditions; thus, it is necessary to utilize imbibition to unlock nanoscale pore fluids. In this study, experiments were conducted on oil displacement by high-speed centrifugal pressure and imbibition under different conditions, respectively, and simulations were used to evaluate the effects of pressure differential drive and imbibition efficiency on the utilization of crude oil following fracturing. Combined with the mixed wettability of the reservoir, the imbibition efficiency was analyzed, and the imbibition efficiency at different soaking stages was evaluated. When the fracturing pressure was higher than the matrix pore pressure, the imbibition efficiency was the most obvious, which was 27.9%. Spontaneous imbibition depending solely on capillary force had poor efficiency, at 16.8%. When the fracturing pressure was lower than the matrix pore pressure, the imbibition efficiency was the lowest, at only 1.3%. It is proposed that strengthening fracture pressure and promoting pressurized imbibition are the keys to improving shale oil development. [ABSTRACT FROM AUTHOR]

The global greenhouse effect makes carbon dioxide (CO2) emission reduction an important task for the world, however, CO2 can be used as injected fluid to develop shale oil reservoirs. Conventional water injection and gas injection methods cannot achieve desired development results for shale oil reservoirs. Poor injection capacity exists in water injection development, while the time of gas breakthrough is early and gas channeling is serious for gas injection development. These problems will lead to insufficient formation energy supplement, rapid energy depletion, and low ultimate recovery. Gas injection huff and puff (huff-n-puff), as another improved method, is applied to develop shale oil reservoirs. However, the shortcomings of huff-n-puff are the low sweep efficiency and poor performance for the late development of oilfields. Therefore, this paper adopts firstly the method of Allied In-Situ Injection and Production (AIIP) combined with CO2 huff-n-puff to develop shale oil reservoirs. Based on the data of Shengli Oilfield, a dual-porosity and dual-permeability model in reservoir-scale is established. Compared with traditional CO2 huff-n-puff and depletion method, the cumulative oil production of AIIP combined with CO2 huff-n-puff increases by 13,077 and 17,450 m3 respectively, indicating that this method has a good application prospect. Sensitivity analyses are further conducted, including injection volume, injection rate, soaking time, fracture half-length, and fracture spacing. The results indicate that injection volume, not injection rate, is the important factor affecting the performance. With the increment of fracture half-length and the decrement of fracture spacing, the cumulative oil production of the single well increases, but the incremental rate slows down gradually. With the increment of soaking time, cumulative oil production increases first and then decreases. These parameters have a relatively suitable value, which makes the performance better. This new method can not only enhance shale oil recovery, but also can be used for CO2 emission control. [ABSTRACT FROM AUTHOR]

For unconventional reservoirs, water flooding performs poorly because of low displacement efficiency; gas flooding shows limited enhanced oil recovery (EOR) capability due to gas breakthrough. Carbonated water injection (CWI) and active CWI (ACWI) are promising EOR methods which combine advantages of water and gas flooding. This paper provides experimental and numerical studies of carbonated water and surfactant injection based on a case study in Changqing Oilfield, China, which is the first time to investigate the feasibility of CWI and ACWI for tight oil reservoirs. This study compares performances of active water injection (AWI), CWI, Water altering gas (WAG), and ACWI. Experimental results reveal that the oil recovery of CWI is 2.7% more than WAG. ACWI achieves the highest incremental oil recovery (9.43%) among four methods. The sensitivity analyses of ACWI + WAG is further implemented experimentally, which demonstrates that ACW as a pre-flood improves 7% of oil recovery during WAG process. For Changqing tight reservoir cores, the optimal injection volume of ACW is 0.8 pore volume. Numerical simulations are conducted to validate the capability of cubic Equation-of-State coupled with Henry's law (EOS/H model) for CWI in tight oil reservoirs, indicating EOS/H model is applicable to correlate phase behavior of carbonated water and oil system. This paper, for the first time, investigates the EOR performance of ACWI in tight oil reservoirs. These results explore the feasible application of using CWI/ACWI in tight oil reservoir development. • Active carbonated water injection has potential application for tight oil EOR. • ACWI followed by WAG can further improve oil recovery for tight oil reservoirs. • The critical injected active carbonated water volume is about 0.8 PV during ACWI+WAG. • The cubic equation of state coupled with Henry's law can simulate CWI more accurately. [ABSTRACT FROM AUTHOR]

• Allied In-Situ Injection and Production (AIIP) is a suitable method for shale oil. • AIIP can better supply formation energy and have a better sweeping efficiency. • The sensitivity analysis of AIIP is used to study the effect of AIIP parameters. • Asynchronous AIIP (A-AIIP) is a method that can delay gas breakthrough time. Due to the problem of low formation pressure in shale oil reservoirs of the Ordos Basin, oil production decreases rapidly during primary production stage. It is crucial to maintain formation pressure through fluid injection. However, field experiences indicate that traditional fluid injection in shale oil reservoirs is difficult, which results in low injection rate, low sweep efficiency and insufficient energy supplement. This paper presents an injection-production pattern, Allied In-situ Injection and Production (AIIP), which divides one horizontal well into 3 segments to improve fluid injectivity and sweep efficiency. Based on the field conditions of the Ordos Basin, a field-scale numerical model is built, and improving oil recovery (IOR) performance of AIIP is further investigated. Natural gas is used as the injection fluid due to its abundant reserve in Changqing Oilfield and its better IOR performance compared to nitrogen. This paper designs two schemes for AIIP, constituency AIIP (C-AIIP) and asynchronous AIIP (A-AIIP), to achieve the better IOR performance and to prevent the risk of gas breakthrough. The sensitivity analysis is further conducted, including injection-production segments length proportion, well spacing, half-length of hydraulic fracture, and soaking time. This study demonstrates that AIIP injection rate is more than six times that of vertical well injection, and the recovery of AIIP is 30.81% and 21.8% higher than that of huff-n-puff and vertical injection, respectively. The results show that AIIP method could better maintain the formation pressure and obtain a better IOR performance. A-AIIP not only achieves higher oil recovery, but also results in a later gas breakthrough time than C-AIIP. This paper explores the feasible application of AIIP for shale oil production, since it has potential to increase sweep efficiency and fluid injectivity. [ABSTRACT FROM AUTHOR]