Material balance equation of shale gas reservoir considering stress sensitivity and matrix shrinkage

Abstract

As the formation pressure drops during the development process of shale gas reservoir, the effects of stress sensitivity and matrix shrinkage occur. Stress sensitivity effect decreases the porosity of reservoir formation, while matrix shrinkage effect increases the porosity. Therefore, the porosity will be influenced by the combined effect of both stress sensitivity and matrix shrinkage, which need to be considered in the material balance equation of shale gas reservoir. In this work, based on the material balance theory of shale gas reservoir, a material balance equation considering the influences of stress sensitivity and matrix shrinkage on shale gas reservoir that the porosity varies as formation pressure changes is established. Then, the material balance equation is linearized, and the original gas in-place (the controlled reserve) of single well can be calculated by the intercept and the slope of the straight line of the linearized material balance equation, which is convenient for field application. Production data of three gas wells of a certain shale gas reservoir are calculated with the proposed material balance equation in this work. The calculation results indicate that the stress sensitivity effect has the dominant influence on the porosity at the early stage of formation pressure dropping, while the matrix shrinkage effect dominates the influence on the porosity at the late stage of formation pressure dropping. Furthermore, the controlled reserves of the three gas wells have decreased by considering the effects of stress sensitivity and matrix shrinkage in the material balance equation.