Evaluation and improvement of velocity-prediction models and its application in in-situ stress estimation for shale gas

In-situ stress profile is the key to the hydraulic fracture stimulation of producing commercial volumes of gas. To calculate the in-situ stress profile, one must have an understanding of the mechanical rock properties, such as Poisson's ratio and Young's modulus. Due to the rarely and expensive laboratory tests, those are not available commonly. This paper proposes an effective and economic method using rock physics model to calculate the mechanical rock properties. In that way, appropriate rock physics model is quite important. Taking an organic shale well from southern China as a case, we evaluate the main rock physics models including Wyllie equation (Wyllie, 1956), Gassmann's equation (Gassmann, 1951), Kuster-Toksöz (KT) model (Kuster and Toksöz, 1974), and Xu-White (XW) model (Xu and White, 1996) in shale gas reservoir. And then, we modified XW model by applying Berryman's 3-D theory and DEM theory to describe the kerogen particle and the inclusion pores more precisely for shale gas reservoir. And then, the predicted mechanical rock properties are used as inputs for a continuous calculation of the in-situ stress. Calculated results show the estimated in-situ stress using the modified rock physics model is in good accordance with the measured data, which proves the applicability of these methods.