Summary
When imaging hydraulic stimulations, the industry requires alternative methods to acquire microseismic data when vertical wells are unavailable for downhole monitoring. Surface acquisition of microseismic data is one such method that can be used. This method may have lower costs in some cases when used for acquiring multiple wells on the same pad. It can also avoid impacting completions, in particular, during zipper stimulations when a downhole well is required for imaging. New processing techniques and acquisition methods in this field appear to show promising results for event detections and improved focal mechanism solutions (FMS) for fracture network classification.
Pioneer and numerous operators have shown that recording microseismic with a star-pattern surface array has proven to be successful in the Eagle Ford Shale Formation in South Texas. For Pioneer, this acquisition was paired with monitoring from a downhole array in a nearby vertical offset well. Both methods of observation recorded the overall stimulation and found a similar maximum stress azimuth and fracture orientation, which was also obtained from an oil base micro-imager (OBMI) tool. However, differences were noted in the event counts and fracture geometries, which are dependent upon the limitations of each acquisition method.
Acquiring microseismic data using surface arrays has proven in the past to be a challenge in the Midland Basin of West Texas. Pioneer Natural Resources (Pioneer) has previously used two vendors to record microseismic data in Midland and Reagan Counties using star- and patch-arrays at the surface. Both array types provided focal mechanism solutions (FMS) to help characterize the fractures and provide stimulation geometries, including average fracture length, height and azimuth. In one of these past jobs, Pioneer also acquired downhole along with the surface data. In this project, the patch- and nearby downhole-array recorded the same hydraulic stimulation and their microseismic datasets confirmed the same maximum stress azimuth. Yet, the same limitations of surface acquisition were apparent in both types of arrays, specifically the problem of very low event counts. It is suspected that the low event counts are due to energy attenuation in the shallow, high-velocity evaporites prevalent in the Midland Basin.
In an attempt to better understand the limitations of surface arrays in the Midland Basin, Pioneer along with two surface contractors (Dawson and NanoSeis) undertook a large scale project to better quantify differences between the downhole and surface methods as well as to test acquisition systems. These companies put together a large-scale (using four times the geophones of standard surface arrays) surface patch-array test program with the purpose of mitigating the shallow attenuation effects through signal enhancement via high fold, from an increased number of geophones. In this design, each patch was augmented with an inset array of 3-component geophones. This microseismic dataset was compared to one recorded by a nearby downhole array for the same project, and a portion of these data are shown and discussed in this paper. Beyond recognizing key similarities and differences in the microseismic attributes from the downhole and surface data in the context of the related stimulation, one of the most significant contributions of this test is that the data can be decimated to determine the minimum requirements for a successful surface acquisition of microseismic data in the Midland Basin.
