Research on water blocking and residue damage mechanism of fracturing fluid in Yongjin tight reservoirs

Abstract

The existing reservoir in the Yongjin block exhibits an extremely low matrix permeability, posing challenges in precisely evaluating the damage caused by fracturing fluid using traditional core flow experimental methods. Currently, there is no established quantitative method for characterizing the degree of damage. In this study, we integrated online nuclear magnetic resonance, microscopic computed tomography, and core displacement experimental techniques, pioneering a novel approach to evaluate damage in deep tight oil reservoirs subjected to hydraulic fracturing. We analyzed the influence patterns of key factors such as backflow pressure differential, shut-in time, invasion volume, and residual retention on rock matrix damage in the operational area. The study unveiled the mechanisms behind water sensitivity, water block, and fracturing fluid retention damage among oil, water, and rock. The results indicate that water sensitivity damage is less than 20%, primarily occurring within large pores. Water block damage can significantly reduce the residual oil permeability. Experimental findings suggest that optimizing liquid backflow with a pressure differential and well shut-in time set at 5 MPa and 9 days can markedly reduce the intrusion volume of gel-breaking fluid, restoring the residual oil permeability. Under high-pressure differential conditions, residual fracturing fluid can infiltrate the rock matrix, resulting in pore damage. Additionally, it can accumulate on the fracture surfaces, thereby reducing the permeability of microfractures.