Summary
The rock physics model is a significant tool for characterization of reservoir. Pore geometry of shale reservoir is usually complicated and pore aspect ratio is an important parameter which can be used to characterize the pore geometry. Hence, we intend to build a rock physics model for shale reservoir, and use it to inverse pore aspect ratio and predict S wave velocity. However, we find the error between predicted S wave velocity and measured S wave velocity increase as clay volume increase. In addition, the lamination of clay make a big contribution to shale anisotropy and it has a great effect on elastic properties. Hence we propose a double scan procedure in order to take clay lamination into consideration during S wave velocity prediction. In summary, we firstly propose a shale rock physics model for shale reservoir, and we introduce a new parameter (CL), the standard deviation of the distribution function, to model the degree of lamination of clay. Then we introduce a double scan procedure to inverse the CL and pore aspect ratio simultaneously which can be used to evaluate the lamination situation and pore geometry of target zone. And the predicted S wave velocity coincides well with measured data from Southwest China.
Introduction
The characterization of pore geometry and lamination of shale reservoir is very important, since these two factors can make great contribution to the anisotropy of shale (Johansen 2004, Sondergeld et al 2010, Guo 2014). Shale reservoir has been testified to be strong anisotropy (Vernik 1996). The anisotropy can influence the elastic properties of shale, and ignoring the shale anisotropy during reservoir evaluation will make evident mistake (Sayers 2013). The object of our paper is to build a shale rock physics model which takes clay lamination into consideration. And then use this model to inverse pore aspect ratio and lamination situation of shale.
2D rock physics templates are usually constructed to illustrate the relationship between pore aspect ratio and other elastic parameters (such as velocity or impedance) and the further inversion (Xu 2009, Jiang 2011, Guo 2012). The mineralogy of shale is very complicated, which may cause the situation that many data points cant covered by 2D template. This situation may due to the influence of mineralogy to elastic parameters is stronger than that of pore geometry. A 3D template has been constructed by taking mineralogy effect into consideration during pore aspect ratio inversion, which make the template more close to realistic and also make the inverted result more robust.
