Selection of Candidate Horizontal Wells and Determination of the Optimal Time of Refracturing in Barnett Shale (Johnson County)

The observed uneconomic production performance in many shale gas horizontal wells suggests refracturing as a restimulation treatment to revive economic gas production. To achieve high-performance re-stimulation, it is critical to select the right well among many other wells in the area and determine the proper time of refracturing. Well selection is challenging in shale gas horizontal wells because of the complexity of natural and induced fracture networks and in many cases due to insufficient reservoir and completion data. Selection of the candidate well and the time of refracturing can be made based on a thorough numerical simulation study developed by precise modeling of hydraulic fractures and refracturing process. Accurate modeling can only be accomplished by considering formation of fracture networks. Induced fracture networks are formed by an altered stress-field as a consequence of the fracturing process and are evidenced by micro-seismic hydraulic fracture monitoring techniques. Tavassoli et al. (2013) modeled gas production of a refractured well in the Barnett formation and validated their simulation methodology with the available field data. The validated model was used to predict gas production after refracturing the well. They then performed systematic sensitivity analyses to specify the characteristics of shale gas horizontal well suitable for refracturing and defined well screening criteria and optimal time of refracturing. In this study we extend their original work to study 188 horizontal wells in Barnett (Johnson County) to identify wells with potentials for refracturing. We found that among these 188 wells only 11 wells are suitable for refracturing and the best time to perform hydraulic fracturing is between 31⁄2 to 51⁄2 years after initial production. Introduction Shale-gas resources are an ever-increasing component of North American gas supply. Based on Gas Technology Institute (GTI) study, shales are considered the largest component of Western Canadian Sedimentary Basin (WCBS) with an estimated cumulative hydrocarbon shale volume in the order of 86 tcf. WSB shales have many similarities to US shale producing reservoirs (Faraj et al. 2004; Shaw et al. 2006). Barnett is amongst the largest shale gas resources in the US. Determining the hydraulic fracturing potential in this formation will contribute to production enhancement in all other shale plays. The Barnett Shale is a Mississippian-age marine shelf deposit with estimated ultimate gas recovery of 1-10 Bcf/well. It extends over 3.2 million acres with a thickness of 100-600 ft. The formation has ultra-low permeability (in the range of 10100 nanodarcies), total porosity of 4-5%, total organic content of 4.5%, and estimated 50-200 Bcf/ft of original gas in place (Cipolla 2010a, Potapenko et al. 2009). These resources are predominantly lithified clays with low permeability and classified as unconventional gas reservoirs. Despite their extremely low-permeability, gas production from these resources is much greater than anticipated owing to non-Darcy flows and different sources of gas in their formations. Gas flow is sourced from stored gas in nanopore networks and adsorbed gas on organic materials in the shale formations (Javadpour et al. 2007; Javadpour 2009; Swami et al. 2012; Rubin 2010). Recent advances in directional drilling and hydraulic-fracturing techniques have resulted in economic production from shale-gas reservoirs. There have been more than 12,000 horizontal wells drilled in the Barnett Shale since 2001 (IHS 2013). Effective fracturing techniques make for successful economic production from extremely low (on the order of nanodarcies) permeability formations. New fracturing techniques significantly improve reservoir-wellbore connectivity by creating a large, stimulated reservoir volume (Warpinski et al. 2009; Soliman et al. 2012). Gas production from these reservoirs declines in the very first years of production after the initial fracturing due to fracture conductivity impairments. Closure stress and long-