Compaction Measurements in Oil-Bearing Formations of the San Joaquin Valley

PEMPER, RICHARD, Baker Atlas

The effects of subsurface rock compaction and the accompanying phenomenon of surface subsidence must be taken into consideration when extracting hydrocarbons from producing formations. This is an important issue in California, where compaction sometimes occurs in unconsolidated formations. During the period from 1926-1967, for example, thirty feet of compaction was observed near Long Beach in one of the oil fields within the Los Angeles basin.

The problem of subsidence can be minimized by injecting water into the formation to replace hydrocarbons in an effort to restore original pressures. The effectiveness of this process can be enhanced with better knowledge of the zones surrounding production. Satellite data can monitor surface movements, however, it cannot provide detailed pictures of expansion or contraction thousands of feet below the earth's surface. Such information can be provided with the Compaction Monitoring Instrument (CMI), a device capable of measuring subsurface shifts to an accuracy of about one millimeter.

The CMI measures the distance between radioactive markers placed into the formation or onto casing at intervals of about 35 feet. Changes in the distance between the markers corresponds to compression or expansion of the casing or formation into which the markers are embedded. The CMI system contains four gamma ray detectors, a tri-axial accelerometer, and pressure and temperature sensors. The spacings of these detectors are calibrated using an invar rod which houses four Cesium-137 sources, the lengths between which have been measured by the National Institute of Standards and Technology (NIST) to an accuracy of 0.0015 inches. Compaction measurements have been obtained in several wells within the San Joaquin Valley. These measurements, when combined with satellite subsidence data, have been used to refine reservoir subsidence models, adjust water flood programs, and ultimately minimize future compaction and subsidence.

AAPG Search and Discovery Article #90911©2000 AAPG Pacific Section and Western Region Society of Petroleum Engineers, Long Beach, California