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ABSTRACT: Earthquakes Previous HitInducedNext Hit by Hydrocarbon Production

Paul Segall

Previous HitSeismicityNext Hit has been associated with oil and gas production in a number of areas. Earthquakes related to fluid injection for secondary oil recovery were documented over 20 yr ago. These earthquakes are understood to be caused by decreasing effective normal stress resulting from increased pore-fluid pressures. The reduced frictional resistance allows faults to slip in response to preexisting tectonic shear stress.

Seismic activity has also been associated with hydrocarbon fields in which no fluid injection has taken place and in which measured pore-fluid pressures have declined. It is now understood that, in some circumstances, pore-fluid production itself can induce earthquakes. Production decreases pore pressures in the reservoir, and this causes the reservoir rocks to contract slightly. This volumetric contraction leads to marked subsidence in some fields. Strata above and below the reservoir do not experience the same pore-pressure decline and thus do not contract. The difference in volumetric strain between the reservoir rocks and the surrounding rocks leads to a strain mismatch that generates elastic stresses. These stresses may cause earthquakes, depending on the preexisting tectonic str ss state and the mechanical properties of the rocks.

The theory used to calculate stress changes resulting from hydrocarbon production can be readily tested. The mathematical models used to compute stress change predict horizontal as well as vertical deformation at the earth's surface. These deformations can be measured using a variety of geodetic techniques. The degree to which the theoretical predictions match observed surface strains and displacements serves as a check on the theory.

These results have significant implications for petroleum production. The importance of the state of stress on fracture permeability is widely recognized. Production alters the state of stress in the reservoir and its surroundings. These changes can be predicted and checked. An interesting result is that Previous HitinducedNext Hit Previous HitseismicityTop itself can provide important insights into the state of stress in the reservoir and surroundings.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990