Actual and Optimal Hydraulic-Fracture Design in a Tight Gas Reservoir

Abstract

Summary Designing hydraulic-fracture stimulation to optimize well productivity requires a carrier fluid with a suitable leakoff coefficient and knowledge of the initial reservoir pressure and permeability, the closure-stress magnitudes in the reservoir and in the bounding formations, and rock properties such as the Young’s modulus and the Poisson’s ratio, which can be determined from cores. When key parameters are left unknown, the hydraulic-fracture stimulation is likely to be severely suboptimal. This study integrates pressure-buildup and production transient analyses with microseismic surveys and the recorded pumping schedule to estimate the aforementioned parameters in previously fractured wells in a tight gas reservoir that have been on production for up to 8.5 years. The first well drilled and completed in the block included a pressure-buildup test that enabled accurate estimation of the initial reservoir pressure and permeability. A post-fracture buildup test was also conducted, and annual pressure-buildup tests in the subsequent 6 years showed continuous changes in the fracture morphology, with fracture conductivity decreasing by a factor of three and fracture length increasing by approximately 50%. Many of the subsequent wells were drilled in two patterned well clusters, each designed to account for fracture-propagation behavior indicated from a microseismic survey. A comparison with an optimal hydraulic-fracture design that was intended to maximize well productivity indicates that most of the well stimulations were suboptimal, with rate and cumulative production approximately one-half that of an optimized design on the basis of the same proppant mass. The observed changes in fracture conductivity and length over time were not anticipated. Because such data are rarely recorded, the variations in fracture morphology may be fairly typical and should be of considerable interest to pressure-transient analysts. The production-data analysis shows the difficulties in determining formation and fracture parameters when the transient response lacks radial flow. The fracture-treatment analysis shows a comparison between the actual fracture treatment and one designed to maximize well productivity, and clearly illustrates the potential for well improvement through the use of modern hydraulic-fracture-design principles.