Sensitivity Studies of Horizontal Wells with Hydraulic Fractures in Shale Gas Reservoirs
Authors: Zhang Xu , Du Changan , Deimbacher Franz , Crick Martin , Harikesavanallur Arvind 
Published: May 2016

Abstract
Producing natural gas from shale gas reservoirs has gained momentum over the past few years in North America and will become an increasingly important component of the world's energy supply. A shale gas reservoir is characterized as an organic-rich deposition with extremely low matrix permeability and clusters of mineral-filled "natural" fractures. Shale gas storage capacity is defined by the adsorbed gas on the organic material within the shale matrix and free gas in the limited pore space of the shale rocks. Horizontal drilling and hydraulic fracturing are the primary enabling technologies to obtain economical production from the shale gas reservoir.
This paper presents a comprehensive reservoir simulation model to study the impact of reservoir and hydraulic fracturing parameters on production performance of a shale gas reservoir. The simulation model was constructed as a multi porosity system with matrix sub-grids to account for transient gas flow from the matrix to the fracture. The extended Langmuir isotherm was used to model the desorption process of multiple components during the production. Primary hydraulic fractures perpendicular to the horizontal wellbore were modeled explicitly with thin grid cells that preserved the finite conductivity. The hydraulically-induced fracture network around the horizontal well was characterized by the matrix-fracture coupling factor (sigma) and permeability of the fractures.
The study was aimed to quantify the influence of the reservoir and hydraulic fracture parameters using experimental design, including porosity and permeability of the reservoir matrix and fracture, matrix-fracture sigma factor, matrix subdivisions and, primary hydraulic fracture half-length, height, spacing and conductivity, rock compaction, non-Darcy flow coefficient, as well as gas content. Sensitivity tests were performed to identify the most influential reservoir and hydraulic fracture parameters and provided important insights into the impact of uncertainties on shale gas production forecasts, which can be critical for fracture treatment design and production scheme optimization.

Introduction
Fundamentals of unconventional gas reservoirs have been reviewed1 and a multidisciplinary workflow was established for geological characterization of shale gas reservoirs.2 Studies have been performed to detect natural fractures using 3D-seimic data and estimate natural fracture patterns in gas shales using production data.3,4 To enable production from a shale gas reservoir, the key is to stimulate the existing natural fractures or rock fabric by hydraulic-fracture treatments that pump large-volume, high-rate water with different size of proppents.5 The application of microseismic mapping has revealed complexities of the created fracture networks and improved our understanding of hydraulic-fracture growth in the gas shale formation. A semi-analytical pseudo 3-D geomechanical model has been developed to characterize the induced fracture network.6