The waterflooding effect evaluation of carbonate fractured–vuggy reservoirs constitutes a comprehensive multiple-information decision-making process involving quantitative unascertained measure theory. This paper establishes a novel comprehensive methodology to evaluate the waterflooding effects of carbonate fractured–vuggy reservoirs for the first time. A new evaluation grading criteria is proposed using the Mahalanobis distance method based on the multi-index comprehensive unascertained measure theory derived from a modified five-scale analytical hierarchy process–entropy weight method. The actual field data from the carbonate fractured–vuggy reservoirs and the nine evaluation indices are specifically applied to demonstrate the calculation process for the construction of the grading system model on the waterflooding effects and to verify the accuracy of the Mahalanobis distance method by comparing the calculation results with the Minkowski and Euclidean distance methods. The proposed methodology facilitates the effective evaluation of the waterflooding strategies implemented in carbonate fractured–vuggy reservoirs with three categories; the ones with outstanding performance usually demonstrated favorable index characteristics, with substantial contributions to the enhanced oil recovery, manifesting with complete well patterns, a balance in the injection–production dynamics, excellent waterflooding utilization, and control competence. In contrast, fractured–vuggy units with fair waterflooding performance revealed limitations in the enhanced oil recovery. It can also be inferred that the mediocre waterflooding performance of the fractured–vuggy units is associated with incomplete well patterns, an imbalance in the injection–production dynamics, low waterflooding utilization, and a negligible waterflooding effect. The results in this study show that this newly proposed integrated model can effectively assess the waterflooding effects quantitatively and provide a more precise scientific basis for evaluating the waterflooding effects in carbonate fractured–vuggy reservoirs, with potential applicability in other fields.

The paper presents a novel waterflooding technique, coupling cyclic high-pressure water slug injection with an asynchronous injection and production procedure, to address the inefficient development of low-permeability oil reservoir in Shengli Oilfield, a pilot test with 5-spot well pattern. Based on the first-hand data from the pilot test, the reservoir model is established. With an in-depth understanding of the mechanism of the novel waterflooding technique, different simulation schemes are employed to screen the best scheme to finely investigate the historical performance of the pilot test. The production characteristics of the pilot test are both qualitatively and quantitatively investigated. It is found that the novel waterflooding technique can provide pressure support within a short period. And the formation around the injector is significantly activated and deformed. Once passing the short stage of the small elastic deformation, the reservoir immediately goes through the dilation deformation accompanied with the opening of microfractures so that the reservoir properties are significantly improved, which leads to better reservoir performance. With the multicyclic dilation-recompaction geomechanical model, the impact of pressure cyclic evolution on the reservoir properties and performance under the novel waterflooding mode of cyclic high-pressure water slug injection is taken into consideration. The historical data of the pilot test is well matched. In the study, a high-precision simulation scheme for the novel waterflooding technique in low-permeability reservoirs is proposed, which provides significant technical support for further optimization of the pilot test and large-scale application of the novel waterflooding technique.

Revealing the impact of core mineral composition on the initiation pressure of waterflood-induced fractures (WIFs) in tight sandstone reservoirs is a crucial aspect of studying the initiation mechanism of WIFs. In this paper, through quantitative characterization of the core mineral composition from six samples of the Chang 6 reservoir in the Wuqi oilfield, western Ordos Basin, and modified experimental cores and displacement equipment for WIF experiments, the influence of the core mineral composition on the initiation pressure of WIFs in tight oil reservoirs is investigated. The conclusions are as follows. (1) The rock mineral composition of the Chang 6 reservoir in the Wuqi oilfield, western Ordos Basin, includes quartz, feldspar calcite, and clay, characterizing it as a typical feldspar sandstone reservoir. Quartz and calcite are considered as brittle minerals, while feldspar and clay are categorized as lithologic minerals. (2) For feldspar sandstone reservoirs, including quartz, feldspar, calcite, and clay minerals, when the combined content of quartz and feldspar exceeds 600% of the total mineral content, the changes of quartz and feldspar content will affect the initiation pressure of WIFs. As the ratio of the quartz content to feldspar content R-qf increases, the initiation pressure of WIFs exhibits a logarithmic function decrease. (3) Considering the contribution of diagenetic minerals to rock brittleness, the calculation method for the brittleness index of feldspar tight sandstone reservoirs is improved. (4) The relationships between R-qf, brittleness index, and initiation pressure of induced fractures suggest that an increase in R-qf leads to a power-law increase in the brittleness index, while the initiation pressure of WIFs relative to the brittleness index shows a power-law decrease. This phenomenon indicates an increased likelihood of WIFs occurring during the long-term water injection process in feldspar sandstone reservoirs. This work contributes to understanding how core minerals affect the initiation pressure of WIFs in tight sandstone reservoirs.