Abstract
This paper presents a timely and detailed study of significant injection-induced seismicity recently observed in the Sichuan Basin, China, where shale-gas hydraulic fracturing has been initiated and the aggressive production of shale gas is planned for the coming years. Multiple lines of evidence, including an epidemic-type aftershock sequence model, relocated hypocenters, the mechanisms of 13 large events (MW > 3.5), and numerically calculated Coulomb failure stress results, convincingly suggest that a series of earthquakes with moment magnitudes up to MW 4.7 has been induced by “short-term” (several months at a single well pad) injections for hydraulic fracturing at depths of 2.3 to 3 km. This, in turn, supports the hypothesis that they represent examples of injection-induced fault reactivation. The geologic reasons why earthquake magnitudes associated with hydraulic fracturing operations are so high in this area are discussed. Because hydraulic fracturing operations are on the rise in the Sichuan Basin, it would be beneficial for the geoscience, gas operator, regulator, and academic communities to work collectively to elucidate the local factors governing the high level of injection-induced seismicity, with the ultimate goal of ensuring that shale gas fracking can be carried out effectively and safely.
Highlights
When any type of fluid is pressure-injected into an underground reservoir, as is done during fluid waste disposal and shale gas hydraulic fracturing, the pressure of the fluids underground increases, and the underground stress distribution may change
We focus on shale gas site blocks in the town of Shangluo and its environs in the Sichuan Basin, China, referred to hereafter as “the study area”
The lateral portions are commonly drilled in the direction parallel to hydraulic fracture opening, which is perpendicular to the maximum horizontal principal stress, in order to maximize the volume stimulated by the induced fractures
Summary
Multiple lines of evidence indicate that the hydraulic fracturing operations that began systematically in Dec. 2014 in the Shangluo shale gas site (mainly in the Ning-201 and Ys-108 blocks) caused a rapid increase in seismicity in the region. We suggest that (1) strong and brittle Pre-Triassic sedimentary rocks; (2) critical regional stress; (3) widely existing faults; (4) there are insufficient top and bottom seals and/or no fracturing barrier between the shale formation and the rocks above and below are the geologic reasons why earthquake magnitudes associated with hydraulic fracturing operations are so high in the study area. We assume that the study area will encounter with increasing likelihood additional injection-induced seismicity Taking these points into consideration, it would be beneficial for academic, oil industry, and regulator communities to work collectively in order to elucidate the governing factors behind the high level of injection-induced seismicity in the southern Sichuan Basin, thereby allowing shale gas hydraulic fracturing to be conducted effectively and safely. Our results are expected to be helpful for conducting risk assessments at other sites with similar geological and tectonic conditions
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