Relationships Between Mechanical Properties and Fracturing Conductivity for the Eagle Ford Shale

Abstract

Abstract Experimental measurements of the fracture conductivity and rock mechanical properties are presented for the five different geological facies of the Eagle Ford shale (A, B, C, D and E) as defined by Donovan el al. (2012). In this classification, facies A lies right above the Buda limestone, followed by facies B which has the highest total organic content and production. This study places emphasis in this zone. Facies A and B are known as the lower Eagle Ford shale, whereas facies C, D and E are known as the upper Eagle Ford shale. Facies E underlies the Austin chalk. Conductivity samples in three orientations and core plugs in two orientations were obtained from outcrops samples. The orientations represent vertical fractures with flow in the horizontal direction (X0 orientation) or flow in the vertical direction (X90 orientation), and horizontal fractures with horizontal flow (Z orientation). Short term fracture conductivity experiments were performed using a modified API conductivity cell at room temperature. Unpropped and propped conductivities were measured with gas flow. Low proppant concentration was used in order to simulate a slick-water treatment. Proppants of various sizes were placed manually on a rough fracture face. The fracture faces were scanned with a surface profilometer to calculate the fracture roughness. Compressive triaxial tests were performed on the core plugs in order to determine the elastic rock properties such as Young's Modulus and Poisson's Ratio. Additionally, Brinell hardness tests were completed to determine the resistance to embedment. Mineral composition was acquired by X-ray Diffraction analysis. The obtained fracture conductivities were compared to the fracture face roughness, elastic mechanical properties, Brinell hardness and mineralogy of the samples. It was found that propped and unpropped conductivities decline exponentially with closure stress. Propped conductivity was found to be dominated by proppant characteristics. Conductivity is positively related to rock brittleness and inversely related to Poisson's ratio. For the samples cut perpendicular to the bedding, no significant difference was found for between horizontal flow (X0) and vertical flow (X90). The parallel-bedding samples (Z) in general have lower conductivity compared with the perpendicular-bedding (both X0 and X90). Within facies B, samples with flow on the bedding plane (Z) were found to have the lowest conductivity. The findings of this study provide useful information to understand the outcomes of fracture stimulation in Eagle Ford, and therefore to assist fracture treatment design.