Mechanism and application of rock dilation-induced injectivity enhancement in injection wells for offshore oil field

To reveal the elastic-plastic deformation characteristics of rock expansion and the development law of microcracks, and evaluate their expansion and permeability effects, this paper conducts triaxial rock mechanics seepage coupling experiments to explore the expansion mechanics parameters and permeability evolution law, establish expansion-induced microcrack morphology judgment criteria, define effective expansion radius to evaluate expansion and permeability effects, and conduct expansion numerical simulation research using injection wells in the west oilfield of South China Sea as an example. Research has shown that Athabasca oil sands and Bohai loose sandstones have the highest shear dilation potential. In contrast, low-permeability sandstones in the western South China Sea have the lowest shear dilation potential. Saturated samples have higher shear dilation potential than unsaturated samples, and temperature has little effect on shear dilation potential. Based on the relationship between tensile strength, cohesion, internal friction angle, and the state of stress, the type of expansion microcracks in offshore sandstone can be quickly determined. The volumetric strain of tensile expansion is smaller, but its permeability-increasing effect is better than that of shear expansion. The author proposes effective permeability models for water induced by shear dilation, damage permeability models, and permeability evolution models considering interface chemical enhancement. The (effective) dilation radius based on super pore pressure, porosity (or volumetric strain), permeability improvement, and microcrack development zone is defined. Rapid prediction of microcrack morphology and detailed evaluation of full-size wellbore numerical simulation are carried out for water injection wells in low-permeability offshore oil fields. It is found that after expansion, tension shear composite microcracks are generated, with an effective expansion radius of 12.83m. The research results can provide basic theoretical support and construction design guidance for water injection and expansion technology in offshore oil fields.