Research On Stress Sensitivity And Prediction Of Fracture Width In Naturally Fractured Sandstone Reservoirs

The naturally fractured reservoirs, in which the resource amount is very abundant, are distributed extensively. On one hand, fractures have high conductivity; on the other hand, working fluid loss during well drill-in and completion becomes more serious due to strong stress sensitivity of fracture. This increases severity of formation damage. The research of fracture width by stress sensitivity of fracture and the situation of working fluid loss provide basic parameter for design of formation damage control and the exploitation of fractured reservoirs.This paper studies Shaximiao and Lianggaoshan group oil reservoir in Gongshanmiao structure in center of Sichuan Basin. The geological characteristic of research area is with porosity Shaximiao 3-5 percent mostly, Lianggaoshan 1-3 percent mostly, both permeability (0.005-0.05) ×10^-3μm² mostly, thin pore throat and high capillary pressure. The types of clay mineral are various, including chlorite, illite, and Kaolin, and a little illite/smectite mixed layer mineral. There are micro-cracks and high angle structure fractures developed, which are up to one piece per meter in average.The effective stress model of porous media is discussed. How sandstone granularity and water saturation affect stress sensitivity is analyzed. The relationship between fracture width and effective stress was analyzed, too. It is well known that the permeability measured by formation fluid is regarded as baseline permeability for fluid sensitivity evaluation. The insufficiency of oil industry standard in stress-sensitive evaluation was pointed out. Hence, the permeability at in-situ effective stress is a datum mark in the evaluation of stress sensitivity. The contrasts are made among three methods of in-situ effective stress, oil industry standard, and stress sensitivity coefficient. Results demonstrate that stress-sensitive damage is magnified by oil industry standard and the fact is accorded by in-situ effective stress. The relativity between in-situ effective stress and stress sensitivity coefficient is well. In research area, the stress sensitivity of matrix is from middle to weak, fracture sample from middle to strong.Based on the radial diffusion hypothesis of drill-in fluid, the model of facture width was established. The program of predicting fracture width was compiled by Visual Basic. The fracture width calculated by this model is from 393 μm to 776 μm when positive pressure difference is beyond 10 MPa. Results of temporary plug experiment show that drill-in fluidsbegin to leak when fracture width is greater than 150-200 urn. And the fracture width computed by loss model is generally greater than 200 um. This explains the result of loss model accords with the precondition of loss. Finite-element analysis demonstrates that the fracture width of borehole wall is from 200 um to 350 urn when positive pressure difference in drill-in process is 3 MPa. Take the stress sensitivity of fracture into consideration; fracture width is up to 400 - 700 um, which is in accord with the result computed by loss model.Fracture width calculated by loss data enriches the means of fracture detection. Associated with other technology of fracture detection, it can provide important parameters for working fluids design of reservoir protection and dynamic analysis of oil-gas well.