Comparison of Single-and Dual-Array Microseismic Mapping Techniques in the Barnett Shale

Abstract

Abstract This paper shows a comparison of dual-array microseismic maps with single-well maps for horizontal wells in the Barnett shale.Results from two test cases showing gel and water fracturing maps are given and compared with initial production. Dual-array mapping provides for a much larger areal coverage and increased accuracy when accurate bottom-hole locations and velocity structure are available, but do have trade-offs that need to be considered. Introduction The stunning success of gas development in the Barnett shale of north central Texas is largely due to the optimization of hydraulic fracturing in this unusual reservoir environment.[1,2]In particular, the migration from crosslinked-gel, propped-fracture treatments to waterfracs with higher rates and lighter proppant loadings has provided both technical and economic benefits by improving connectivity within the reservoir while reducing costs.While these benefits have been seen directly in both gas production and AFE reductions, a full understanding of the mechanisms for these improvements has primarily been provided by microseismic monitoring of the treatments. A classical hydraulic fracture is generally a planar crack feature that is designed to minimize leakoff into the formation, create extensive fracture length, and provide a propped channel through which hydrocarbons can more easily migrate back to the wellbore.In such a case, the value of high-viscosity fluids that can minimize leakoff, generate width, and effectively carry the proppant to the fracture extremities is clearly founded. The Barnett shale, however, is a complicated naturally fractured reservoir where large-volume waterfracs pumped at high rates have been shown to effectively stimulate substantial volumes of the reservoir through the development of an interconnected fracture system.[3–5]Evidence for the development of a three dimensional stimulated region, as opposed to a two-dimensional region associated with a classical planar fracture, was first provided by the loading-up of offset wells during waterfracture treatments.These offset wells, located both along the fracture azimuth and perpendicular to it, generally showed improved productivity when the fluids were unloaded and the well returned to production. Microseismic mapping is the technology that has provided the details of the development of the process and the overall size of the stimulated network away from other offset wells.3–5It is the ability to observe the development of microseismic patterns that has given considerable insight into the mechanism for the success of waterfracs.In horizontal wells, where many more fracturing configurations are available (e.g., cemented or uncemented, number of stages), microseismic monitoring has been a key provider of information from which to make optimization decisions.However, the monitoring of waterfracs in horizontal wells is also a much more difficult task because the monitoring must cover the full extent of the well in addition to the full extent of the fracture.One successful solution to this coverage issue is to use two monitor wells, which does, however, provide some other monitoring trade-offs.The application of dual-well monitoring in the Barnett shale and the integration of the microseismic data with production and fracturing results is the subject of this paper.