Tight sedimentary tuff reservoirs (TSTRs) are a new type of tight oil reservoirs, which are mainly developed by water huff-n-puff (WHP). However, there is no quantitative study on the effect of water injection pressure (WIP) and fracture density (FD) on the oil recovery effect of WHP, and the reasons for the low flow-back rate (FR) of the injected water are also not fully explained. In this study, the real cores of TSTRs were used to simulate the seepage state of the matrix-fracture systems of the reservoir, the effects of WIP and FD on the WHP were quantitatively studied, and the reasons for the low FR of the injected water were comprehensively analyzed. The result shows that in five cycles of WHP, the recovery factor (RF) of the core only increases from 8.72 to 10.91% with the WIP increasing from 25 to 30 MPa. However, when the WIP is 40 MPa (rock breakdown pressure), the RF of the core reaches 16.47%, indicating that overfracture-pressure water injection has an obvious improvement effect on the oil recovery effect of WHP in TSTRs. Increasing the FD can also significantly improve the RF and oil recovery efficiency (ORE) of WHP in TSTRs. When the FD of the core increases from 0.34 to 0.44 cm –1 , the RF of five cycles of WHP increases by 9.26%, the ORE increases by 8.61%, and the FR of the injected water decreases by 0.56%. The reasons for the low FR of the injected water in WHP in tight oil reservoirs are matrix water locking, fracture water locking, and reservoir nonconstant-volume water locking. The study can provide an important reference for the efficient development of the WHP in TSTRs.
Imbibition is an important mechanism to improve the recovery factor (RF) of a tight oil reservoir. Accurately evaluating the oil production capacity of tight oil reservoirs by imbibition is of great significance for the formulation of oilfield production plans and productivity prediction. However, there is currently no unified regulation on the selection of rock sample size in tight oil reservoir imbibition evaluation experiments, resulting in great differences in reservoir imbibition oil production capacity obtained from rock samples of different sizes, which brings great challenges to the efficient development of tight oil reservoirs. To clarify the law and mechanism of the rock sample size effect of tight core imbibition oil recovery, this paper takes the newly discovered tight sedimentary tuff (TST) oil reservoir as an example. First, several representative real cores were collected. Then, their wettability and pore structure characteristics were analyzed. Finally, physical simulation experiments of imbibition under different rock sample sizes were conducted. The results show that the TST has very favorable imbibition conditions, which are manifested in the following: (i) the wettability is weakly hydrophilic to hydrophilic; (ii) the mineral composition is tuffaceous minerals, calcite, and quartz, without clay minerals; (iii) micro-nanoscale pores are developed; and (iv) the pore throats are evenly distributed. In the imbibition experiments of rock samples of different sizes, the oil production characteristics of the core surface, the variation form of imbibition rate, pore production characteristics, and the influence mode of imbibition pressure on imbibition do not have the sample size effect. However, the RF of the spontaneous imbibition has an obvious sample size effect, and there is a good exponential function relationship between the imbibition RF and the specific surface area (SSA) of cores. The fundamental reason why the rock sample size effect of the TST imbibition oil recovery is relatively stable and has strong regularity is that its pore structure and wettability are relatively homogeneous and stable. The change of rock sample size does not have a great impact on the distribution of the core pore structure and wettability, resulting in no significant change in its imbibition power, resistance, and distance. Therefore, the main factor determining the imbibition RF of rock samples with different sizes is their SSA. The research results of this work can provide an important theoretical basis for understanding the law and mechanism of TST imbibition oil recovery and unifying the imbibition experimental results of small-sized rock samples.
The water injection huff and puff (WIHP) technology is regarded as one of the important means to improve the recovery factor (RF) of tight volcanic oil reservoirs (TVORs), but the influence of water injection pressure (WIP) and water injection method (WIM) on the oil recovery effect of WIHP has been rarely reported. In this paper, we first collected the real full-diameter cores from a TVOR and then simulated the distribution characteristics of fractures and matrix pores after hydraulic fracturing of the reservoir through the combination and cutting of the cores. Finally, we used the large-sized physical simulation device for tight oil WIHP that can bear high temperature and high pressure and a nuclear magnetic resonance instrument to conduct experiments of five cycles of constant pressure WIHP (CWIHP) with WIPs of 25, 32.5, and 40 MPa and step-by-step pressure rising WIHP (SWIHP) (the WIP was 25, 30, 33, 37, and 40 MPa in order) and obtained the liquid production law and mechanism of tight volcanic rock (TVR) under CWIHP and SWIHP. The result shows that under the CWIHP mode, the RF of TVR has a good power-law-positive correlation with the WIP. However, with the increase of WIHP cycles, the RF of CWIHP always decreases rapidly. In the WIHP of TVR, the injected water mainly collects oil in large pores (the pore radius is greater than 0.1 μm), and the closer the area to the outlet end of oil production and the higher the fracture density, the higher the RF. SWIHP can also effectively improve the RF of TVR, but compared with CWIHP with a WIP of 40 MPa, the amount of recovered oil decreases relatively slowly with the increase of WIHP cycles. In the first two cycles of the five cycles of WIHP, the RF of CWIHP was higher, but from the third cycle, the RF of SWIHP begins to be greater, and the more the number of cycles of WIHP, the more obvious the advantage of SWIHP. When the number of WIHP cycles exceeds 5, the oil recovery effect and the economy of SWIHP are better. This study can provide a solid theoretical basis for the efficient development of WIHP in TVORs.
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