Land Subsidence Caused by Fluid Removal: Oil and Geothermal Operations
Dennis R. Allen
Subsidence caused by the removal of both fluids and solids is a worldwide environmental problem. The extraction of coal, natural gas, oil, fresh water, brine, etc, have all caused subsidence at various times and locations. Although subsidence related to oil and geothermal field operations is relatively rare, it has caused severe problems in the past and currently.
In porous media, the surface is held stable by underground support from the matrix rocks and the pore fluids. Upon the production of reservoir fluids, pore fluid pressure drops and the portion of geostatic loading borne by the pore fluids is transferred to the rock matrix. This increase in load causes both elastic and inelastic deformation of the skeletal structure of the reservoir being produced. Pore volume is lost, the reservoir decreases in the vertical dimension, and subsidence might occur at the surface.
As soon as subsidence is detected, most observers try to estimate the maximum that might occur. This prediction has proved to be a very difficult proposition owing to geologic complexities and unknown petrophysical data.
Seven examples of subsidence in oil and geothermal operations show many similarities, such as high porosity, large fluid removal, limited arching in the structure, large width to depth ratios, usually poor cementation, grabens and normal faulting, and coincidence of time and place of fluid removal and subsidence. None of these are critical, but the result in a field that has most of these properties often is surface subsidence.
Reservoir compaction in situ has been measured by two methods: (1) casing joint length changes and (2) the location of radioactive bullets shot into the formation. Casing joint measurements, although having accuracy limitations, usually furnish data as to the degree of compaction without previous planning.
AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.