Effective stress characteristics in shale and its effect on shale gas productivity

Abstract

Research of effective stress characteristics on the productivity of shale gas has a great significance. The samples of the black shale and bedding shale at the Lower Silurian Longmaxi Formation in Sichuan Basin are selected. Microscopic pore structure characteristics are described by scanning electron microscopy, and the effective stress coefficient is measured by the cross-plotting method. The experimental results show that the effective stress coefficient of black shale is 0.29 on average and the bedding shale is 0.71. There are micro-fractures in bedding shale; thus pore pressure has a more significant influence on bedding shale than black shale. Fitting the relationship between effective stress and permeability by an exponential function, and the calculated stress sensitivity constants of black shale and bedding shale are 0.268 MPa−1 and 0.355 MPa−1, respectively. Recovery model of the fractured horizontal well was established according to experimental results involving effect of geological stress and flow scale, and the abandoned pressure approach was used to evaluate the recovery. The results show that the geological stress greatly influences the reservoir with developed micro-fractures, the production pressure should be adequately controlled for the high production well. The fractured block size and micro-fractures in the fractured block of the construction area have a significant influence on gas recovery; then for the reservoirs with small amount of micro-fractures and large fractured blocks, wellbore pressure should be controlled to improve the recovery.