REARRANGEMENT OF THE DEEP HYDROLOGIC CYCLE AS A RESULT OF OIL AND GAS PRODUCTION

Large volumes of often saline formation water are both produced from and injected into sedimentary basins as a by-product of oil and gas production. Despite this, the location of water production and injection wells has not been studied in detail, and the effects of long-term water injection on groundwater quality and reservoir pressures remain uncertain. Even where injection and production volumes are equal at the basin scale, local changes in hydraulic head will occur due to the distribution of production and injection wells. These changes in hydraulic head are important in understanding induced seismicity and can potentially act as drivers of fluid flow, possibly leading to contamination of overlying potable groundwater resources where high permeability pathways are present. Across the Western Canadian Sedimentary Basin (WCSB), approximately 29 km³ of water has been co-produced with oil and gas, and 30 km³ of water has been injected into the subsurface for saltwater disposal or enhanced oil recovery (EOR). This study evaluates the effects of production and injection wells on deep groundwater resources by examining wells within the southeastern WCSB. A comprehensive fluid budget was created for each formation, as well as maps of the spatial distribution of produced and injected water within each formation. By comparing spatial distributions and formation fluid budgets, it was possible to locate areas where high levels of injection pose the most substantial risk of contamination. In the Midale, areas with high injection volumes were found to be injecting at rates up to 6,000 times that of the estimated natural formational flow rate. Modelled pressures changes in the Midale were found to exceed >8 MPa at up to 250 m away from the injection well, and 2 MPa at up to 1.5 km away, which translates to hydraulic head values above the ground surface and potentially lead to upward leakage of fluids in the presence of permeable pathways. Increased formation pressures due to injection are not unique to the southeastern WCSB and have been recorded in several other regions including Oklahoma, Texas, and Kansas, in some cases leading to induced seismicity. While many of these settings have small changes in the overall fluid budgets, the distribution of production and injection wells can cause substantial changes to fluid pressures locally.