Interpreting Microseismic Measurements: Practical Guide

Abstract

This article, written by Senior Technology Editor Dennis Denney, contains highlights of paper SPE 144067, ’A Practical Guide To Interpreting Microseismic Measurements,’ by C. Cipolla, SPE, S. Maxwell, SPE, M. Mack, SPE, and R. Downie, Schlumberger, prepared for the 2011 SPE North American Unconventional Gas Conference and Exhibition, The Woodlands, Texas, 14-16 June. The paper has not been peer reviewed. Thousands of hydraulic-fracture treatments have been monitored in the past 10 years with microseismic mapping, providing measurements that show a surprising degree of diversity in event patterns. Interpretation of the microseismic data to determine the geometry and complexity of hydraulic fractures continues to focus on visualization of event patterns and on qualitative estimates of the stimulated volume, which has led to wide variations and inconsistencies. Comparing the energy input during a hydraulic-fracture treatment with the resultant energy released by microseismic events demonstrates that the seismic deformation is a very small portion of the total deformation. Introduction Hydraulic-fracture microseismic monitoring (MSM) has increased our understanding of fracture propagation, especially in unconventional reservoirs. MSM, which is a straightforward application of earthquake-seismology principles, consists of the detection, location, and further analysis of extremely small seismic events induced by the fracturing process. Typically, multiple-receiver arrays are positioned over a 300- to 1,000-ft vertical interval in a single nearby offset well. The microseismic events usually are not detectable as discrete events at the surface, but only from other wellbores within approximately 3,000 ft of the fracture-treatment well. Typically, microseisms are represented adequately by a double-couple source, which implies shear slippage that releases stress over some area. In competent rocks, shear slippages occur on pre-existing planes of weakness as a result of the changes in stress and pore pressure induced by the fracturing process. When fracture growth is relatively planar, these changes in stress and pore pressure usually result in a narrow band of microseisms that clearly defines the azimuth and geometry of the hydraulic fracture. However, complex fracture behavior is often interpreted from microseismic data, as evidenced by very intricate event patterns with width/length aspect ratios greater than 0.5. MSM data have revealed a large variation in hydraulic-fracture growth, from relatively simple planar fractures to very complex fracture networks. Understanding fracture growth and the degree and nature of hydraulic-fracture complexity is essential to selecting the appropriate stimulation design and completion strategy.