High-Fidelity Microseismic-Data Acquisition in the Midland Basin Wolfcamp Shale Play
Authors: Wilson Adam 
Published: January 2017

This article, written by Special Publications Editor Adam Wilson, contains highlights of paper SPE 178715, “High-Fidelity Microseismic-Data Acquisition in the Midland Basin Wolfcamp Shale Play,” by Robert Hull, Robert Meek, Brian Wright, Hallie Meighan, Jake Lempges, and Austin von der Hoya, Pioneer Natural Resources, prepared for the 2015 Unconventional Resources Technology Conference, San Antonio, Texas, USA, 20–22 July. The paper has not been peer reviewed.

In the Permian Wolfcamp shale formation in west Texas, density fields of microseismic events were mapped in four dimensions and variations were noted in the geometry of the hydraulic stimulation as well as in the development of pressure away from the perforations. In addition to aiding wellspacing decisions, these data were used to study individual-well geometries and compare variations in the microseismic response between adjacent wells.

Results

The data sets demonstrate that high-fidelity microseismic data can be acquired by use of downhole tractored and multiobservational well-imaging techniques to understand stimulations and the stress fields better as indicated by microseismic data. The data are called high-fidelity because, in general, they are excellent data that are consistent and conform to standard understandings of stimulations. Beyond the robustness in event counts, the data typically have a high signal/noise ratio with high-quality waveforms for picking and consistent hodograms across the tools within the array. Additionally, the P- and S-wave picks are orthogonal to each other and often include the P amplitude and the horizontal (Sh) and vertical (Sv) S amplitudes.

The data demonstrate

Clearly defined nodal planes indicating good P- and S-wave amplitude stability indicative of uniform radiation patterns directly related to the hydraulic mechanics and fracture networks

Variations in observed complexity of event clouds related to formations

Variations in event characteristics related to stress shadows from adjacent prior stimulations

Expanding on the first observation, for a number of the data sets with an offset stimulation, P- and S-wave amplitude ratios vary azimuthally with respect to the observation well. Fairly uniform radiation patterns appear to be caused by consistent failure mechanisms. These composite P/Sh amplitude displays provide low-cost source mechanism information and highlight a common nodal plane as imaged back to the observation well.

The amplitude ratios suggest a uniform mechanism that occurs during hydraulic stimulation that may be related to in-situ conditions of altered rock within the formation or the hydraulic stimulation itself. It is difficult to determine if the natural fractures are aligned in a certain orientation or the propagation of stress and slippage is related to the developing fracture. The microseismic data suggest a dominance of strike/slip failures because the log of Sh/P changes with a 90° rotation. If the data had plotted more as with a 180° rotation, two possibilities would exist for interpretation, including a vertical dip/slip mechanism or a horizontal bedding-plane slip. In the latter, the fracture opening is accommodated by shearing along the bedding planes as the fracture opens vertically.