A note on induced stress changes in hydrocarbon and geothermal reservoirs

Earthquakes have been induced by oil and gas production, where pore pressures have decreased, in some cases by several tens of MPa. It has previously been suggested that such earthquakes are caused by poroelastic stressing of crust surrounding the reservoir. Induced earthquakes are also common in geothermal fields, such as The Geysers, where strong correlations between both steam production and condensate injection, and earthquake activity have been observed over the last several decades. Stress measurements within hydrocarbon reservoirs show that the least horizontal stress decreases with declining reservoir pressure, as predicted by poroelasticity. For circular disk-shaped reservoirs, isothermal reduction in pore pressure induces a relative horizontal tension within the reservoir. Production-induced stressing may promote frictional sliding on pre-existing faults. Within the reservoir itself, normal faulting is promoted if the regional stress is extensional and the Biot coefficient is sufficiently large, α>0.85 for reasonable coefficients of friction. On the other hand, dilatant fracturing and normal faulting are always promoted, in extensional environments, near the edge of the reservoir or in regions of high pore-pressure gradient. It is suggested that such fracturing could enhance fracture permeability in tight rocks adjacent to portions of the reservoir that experience large reductions in pore pressure due to production. In regional compressional environments, production modestly favors reverse faulting above and below the reservoir. The ratio of thermal to poroelastic stress can be quite large in geothermal reservoirs such as The Geysers. Reservoir-wide energy balance considerations suggest that the average temperature has declined at The Geysers by 6°C during the past 20 years. Reservoir average stress changes are thus on the order of ∼2 MPa, and are certainly much larger near injection wells and steam-producing fractures.