Study on the Mechanism of High-Pressure Cyclic Water Injection for Far-Wellbore Oil Extraction in Cracks-Caverns Reservoirs

When applying the high-pressure cyclic water injection technique in injection and production wells belonging to cracks-caverns reservoirs for huff and puff oil production, it is crucial to effectively judge the reservoir type, accurately calculate the reservoir parameters, and reasonably set the high-pressure water injection parameters, which can effectively solve the low recovery efficiency and rapid oil production decline of the injection and production wells due to the differences in cracks and caverns and its complexity in spatial development. However, the imperfection of the existing technical system, resulting in the inability to ensure the rationality and effectiveness of high-pressure cyclic water injection parameter settings, followed by the difficulty in evaluating oil increment of the injection and production wells after multiple rounds of water injection and oil production, greatly limit the deep application of this technique. To solve this problem, we take the Tahe Oilfield (a typical cracks-caverns reservoir in China) as an example. First, we analyze the morphological characteristics of numerous high-pressure water injection indicating curves in Tahe Oilfield, and extract four typical indicating curves using differentiated classification. Second, based on the volume balance equations, we establish two mathematical models—the karst cavern mathematical model of water injection indicating curve and the cracks-caverns mathematical model of water injection indicating curve. Finally, by solving the two mathematic models and the correlation analysis of characteristic parameters belonging to the four extracted typical indicating curves, we can fulfill the reservoir types identification and quantitative calculation of the key reservoir parameters in the injection and production wells. Application of this technique in Well TH1021XX indicates that its far-wellbore crude oil reserves are 69.80×104 m3, its activation pressure of the interconnected fractures ranges from 6.25 MPa to 8.25 MPa, and the error between the actual accumulated oil production and its predictive value is less than 4% after four rounds of high-pressure water injection and oil production. Meanwhile, the error assessment results of numerous wells are all within 10%, which validates the effectiveness and practicality of the research findings in this article.