Abstract
Gas production from unconventional shale gas reservoirs is controlled by multi-scaled fractures, i.e., hydraulic fractures, natural fracture, micro fractures, etc., as flow pathways. On the other hand, shale matrix provides the majority of shale gas storage of both free and adsorbed gas. In addition, field data show that there exists significant amount of micro fractures within low-permeable shale matrix and these micro fractures may connect to large, global-connected fracture network. However, these micro fractures have been either ignored or lumped into the "matrix block" in the current simulation models. In this paper, we present a physically based multiple-continuum concept to include both natural and micro fractures as well as low-permeability shale matrix. The multiple-continuum conceptual model implemented considers shale gas rock consisting of (1) globally connected natural fractures, (2) micro fractures locally connected between matrix and/or kerogen to global fractures, and (3) low-permeability shale rock matrix. Similar to the classic double-porosity concept, the global fracture continuum is responsible for global gas flow to hydraulic fractures or wells, while low-permeability shale matrix, providing main storage space, is locally connected to micro-fractures and interacting with globally connecting fractures. In addition, our model formulation also includes the following processes: (1) nonlinear adsorption/desorption effect, (2) Klinkenberg effect, (3) non-Darcy flow (at high flow rate and low flow rate), and (4) pressure-sensitive rock deformation. We use a hybrid modeling approach to describe different types of fractures, including hydraulic fractures, natural fracture network, and micro fractures. We will demonstrate model application to quantify flow behavior in fractured shale gas reservoirs.
