A new perspective on the hydraulics of oilfield wastewater disposal: how PTX conditions affect fluid pressure transients that cause earthquakes†
Pumping oilfield wastewater into deep injection wells causes earthquakes by effective stress change and solid elastic stressing. These processes result from fluid pressure changes in the seismogenic basement, so it is generally accepted that pressure diffusion governs spatiotemporal patterns of induced earthquake sequences. However, new evidence suggests that fluid density contrasts may also drive local-scale (near-well) pressure transients to greater depths than pressure diffusion and over much longer timescales. As a consequence, the pressure, temperature, and composition (PTX) conditions of wastewater and deep crustal (basement) fluids may be fundamental to understanding and managing injection-induced seismicity. This study develops a mechanistic framework that integrates PTX-dependent fluid properties into the generally accepted conceptual model of injection-induced seismicity. Nonisothermal variable-density numerical simulation is combined with ensemble simulation methods to isolate the parametric controls on injection-induced fluid pressure transients. Results show that local-scale, density-driven pressure transients are governed by a combination of fracture permeability and PTX-dependent fluid properties, while long-range pressure diffusion is largely governed by fracture permeability. Considering this new conceptual model in the context geochemical data from oil and gas basins in the United States identifies regions that may be susceptible to persistent density-driven pressure transients.