Cracked rock layers and pressure pockets riddle Montney Shale gas drilling territory with earthquake hazards in northern British Columbia (BC), according to a report completed for the province’s Oil and Gas Commission (BCOGC).
Horizontal wells and hydraulic fracturing (fracking) in the region face subterranean tangles that are “in a near-critical state” prone to sudden ground-shaking shifts if disturbed, said the study by Calgary-based Enlighten Geoscience Ltd.
“Only small fluid pressure increases are sufficient to cause specific sets of fractures and faults to become critically stressed,” according to the report. “It is generally believed that critically stressed faults are a key factor in induced seismicity.”
The hazards pervade a drilling and fracking hot spot known as the Kiskatinaw Seismic Monitoring and Mitigation Area (KSMMA), straddling southern legs of the Alaska Highway through Dawson Creek and Fort St. John.
The report, completed with the cooperation of producers, is the latest addition to increasingly detailed probes of Montney earthquake risks since shale drilling spread into northern BC a decade ago.
Vulnerability to earthquakes have evolved in repeated natural disturbances of KSMMA rock formations since the Precambrian youth of the planet, said Enlighten Geoscience.
The firm’s research assignment followed a trio of magnitude 3.4-4.5 tremors in November 2018 that were attributed to Montney fracking and prompted evacuation of a dam construction site near the well blamed for the shaking.
The BCOGC enforced earthquake prevention regulations enacted in 2015 to suspend Montney activity temporarily and made producers review and improve safety precautions.
The episode triggered widespread environmental protests. But neither the BCOGC nor the BC government have moved to ban fracking, which has been used in more than 12,000 BC wells.
Enlighten Geoscience described its hazard report as a contribution to fracking risk management, saying that “the results of this and anticipated subsequent studies may be used in the development of standardized pre-assessment methodologies and inform potential mitigation protocols.”
Much remains to be learned, the firm added. Unsolved geological mysteries still include explanations for variable hazards and the unpredictability of ground-shaking events across the Montney region, the report said in suggesting further research topics.
“Why do most [well] completion hydraulic fracturing activities have no events? Why do some completions induce many smaller events while others seem to induce less frequent but larger events?”
The new recital of unanswered questions about fracking echoes a report compiled earlier this year by a panel appointed by the BC government, which also recommended more research but no moratorium while the studies continue.
Only 0.3% of fracking and 1% of waste disposal in western Canada since 1985 has shook the ground. The panel also said that explanations remain elusive for shale drilling tremors known as anomalous induced seismicity (AIS)
“Differentiating why certain areas are more susceptible to AIS, understanding the variability within susceptible areas and accurately forecasting whether a fluid-injection operation is susceptible to AIS are key challenges,” said the panel.