A NOTE ON INDUCED STRESS CHANGES IN HYDROCARBON AND GEOTHERMAL-RESERVOIRS

Earthquakes have been induced by oil and gas production, where pore pr essures have decreased, in some cases by several tens of MPa. It has p reviously been suggested that such earthquakes are caused by poroelast ic stressing of crust surrounding the reservoir. Induced earthquakes a re 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 decad es. Stress measurements within hydrocarbon reservoirs show that the le ast horizontal stress decreases with declining reservoir pressure, as predicted by poroelasticity. For circular disk-shaped reservoirs, isot hermal reduction in pore pressure induces a relative horizontal tensio n within the reservoir. Production-induced stressing may promote frict ional sliding on pre-existing faults. Within the reservoir itself, nor mal faulting is promoted if the regional stress is extensional and the Blot coefficient is sufficiently large, alpha > 0.85 for reasonable c oefficients of friction. On the other hand, dilatant fracturing and no rmal faulting are always promoted, in extensional environments, near t he edge of the reservoir or in regions of high pen-pressure gradient. It is suggested that such fracturing could enhance fracture permeabili ty in tight rocks adjacent to portions of the reservoir that experienc e large reductions in pore pressure due to production. In regional com pressional environments, production modestly favors reverse faulting a bove and below the reservoir. The ratio of thermal to poroelastic stre ss can be quite large in geothermal reservoirs such as The Geysers. Re servoir-wide energy balance considerations suggest that the average te mperature has declined at The Geysers by 6 degrees C during the past 2 0 years. Reservoir average stress changes are thus on the order of sim ilar to 2 MPa, and are certainly much larger near injection wells and steam-producing fractures. (C) 1998 Elsevier Science B.V. All rights r eserved.