Matrix-Fracture Connectivity in the Eagle Ford Shale

Despite micro- to nano-Darcy matrix permeability, shales and mudrocks have become highly productive sources of hydrocarbons owing to advanced horizontal drilling and multi-stage hydraulic fracturing techniques. Production is attributed to an interconnected network of induced fractures that accesses the hydrocarbons stored in the rock matrix. It has been postulated that the induced fracture network results in part from reactivation of natural fractures. Natural fractures in these reservoirs are either lined or completely occluded with mineral cement with little to no connectivity among fracture pores or between the fracture and matrix pores. However, reactivation of natural fractures during hydraulic fracture stimulation may allow the interface between mineralized fracture and matrix to be broken, potentially resulting in increased well performance. In this investigation we use scanning-electron microscopy imaging in conjunction with ion-milling techniques to study pore space connectivity between the nanometer-sized pores in the matrix and reactivated natural fractures. A variety of natural fractures found in the Eagle Ford and Pearsall Formations are considered, varying in orientation, in-filling composition and appearance. The matrix of mudrocks and shales can appear fairly homogenous; however, variance in pore space geometry can be substantial, containing pore sizes that vary in size over several orders of magnitude. Furthermore, we apply direct pore-scale flow models (lattice Boltzmann and level set methods) to quantify this flow between matrix pores and natural fractures.