Measurements of Hydraulic-Fracture-Induced Seismicity in Gas Shales
Authors: Denney Dennis 
Published: February 2015

This article, written by Senior Technology Editor Dennis Denney, contains highlights of paper SPE 151597, ’Measurements of Hydraulic-Fracture-Induced Seismicity in Gas Shales,’ by N.R. Warpinski, SPE, J. Du, SPE, and U. Zimmer, Pinnacle - A Halliburton Service, prepared for the 2012 SPE Hydraulic Fracturing Technology Conference, The Woodlands, Texas, 6-8 February. The original paper was peer reviewed. See SPE Production & Operations August 2012, pages 240-252.

Hydraulic fracturing is essential for hydrocarbon extraction from both conventional and unconventional reservoirs. Recently, concern has developed regarding induced seismicity generated in association with multistage fracturing of horizontal wells in shale reservoirs. A review of thousands of fracture treatments that have been monitored microseismically shows that the induced seismicity associated with hydraulic fracturing is very small and not a problem under normal circumstances.

Introduction

Hydraulic fracturing is important throughout the world. In unconventional reservoirs, such as ultralow-permeability shales, hydraulically fracturing a well is essential to obtain economic levels of production. Contrary to media and general-public perception, hydraulic fracturing is not a new technology, having been applied in the late 1940s. There also is a perception that hydraulic fractures are much larger than ever, but the massive hydraulic fractures performed in the 1970s were of similar size to fracture treatments conducted in horizontal wells today. Also, these early large treatments were performed in shales in the eastern United States (to prove up the resources in the Devonian shales of Appalachia) and in the western tight gas sandstones of the Rocky Mountains.

Results from thousands of monitored fractured treatments demonstrate that fractures will not propagate vertically thousands of feet and intersect potable-water sources. In all the shale basins studied, fractures remain several thousand feet below the deepest potable-water aquifer. Recently, however, there has been considerable attention focused on earthquakes associated with hydraulic fracturing. Microseismic monitoring is a valuable technology for assessing the earthquake potential of fracturing operations. The object of this study was the very large suite of microseismic measurements available from the major shale basins of North America. The study showed that earthquakes are not a threat in nor-mal situations.

Injection Geomechanics

It is well-understood that long-term injection of fluids into the deep subsurface can induce earthquakes. Many cases of minor earthquakes are documented that likely were induced by local injection operations, the most notable of which was the US Army’s injection of chemical waste into a 12,000-ft-deep interval at the Rocky Mountain Arsenal in Colorado in the 1960s. Similarly, geothermal injections are potential sources of induced seismicity, often because optimal geothermal sites are in areas where faults and tectonics are likely to be conducive to Earth movement. Other long-term injection operations, such as solution mining, water disposal, and waterfloods, are potential sources in areas where the geologic conditions are favorable.