Acoustic-Velocity Signatures Associated With Rock-Deformation Processes

Abstract

Distinguished Author Series articles are general, descriptive representations that summarize the state of the art in an area of technology by describing recent developments for readers who are not specialists in the topics discussed. Written by individuals recognized as experts in the area, these articles provide key references to more definitive work and present specific details only to illustrate the technology. Purpose: to inform the general readership of recent advances in various areas of petroleum engineering. Introduction The petroleum industry is increasingly involved in producing oil and gas from problem reservoirs, which typically are those reservoirs susceptible to large deformations and compaction during recovery operations. These problem formations have high porosities, high pore pressures, are poorly consolidated, or are highly fractured. In many cases, production operations can lead to drastic, irreversible damage to these reservoirs. Rock formations can undergo deformation by a wide variety of mechanisms, including dilatancy (precursory to tensile or shear failure), pore collapse, and normal consolidation. In particular, pore collapse and normal consolidation can lead to significant production problems, such as loss of permeability, surface subsidence, and collapse of boreholes. For example, the Ekofisk Field in the North Sea has undergone significant subsidence (more than14 ft) as a result of the extraction of oil and gas. The Groningen gas reservoir in The Netherlands is another example of subsidence induced by hydrocarbon extraction, and the Lagunillas field along Lake Maricaibo in Venezuela has experienced both compaction and subsidence. Dramatic changes in mechanical rock damage are accompanied by significant changes in acoustic velocities. This suggests that such techniques as four-dimensional (4D) -seismic surveys [i.e., repeated three-dimensional (3D)surveys] or crosswell tomography can be used for detection and time-lapsemonitoring of rock-deformation processes in oil and gas reservoirs. In this paper, we review the various processes that cause rock damage and investigate the acoustic detection of the deformational processes. Two distinctly different types of laboratory experiments have been conducted to examine changes in acoustic velocities in porous carbonate rocks. The first involves recording acoustic-wave velocities in both the axial and lateral directions of samples of both Cordoba Cream limestone and Ekofisk chalk deformed during standard triaxial-compression, uniaxial-strain, and hydrostatic compression tests. The second type of experiment presents a new technique (and technology) for acoustic imaging of rock deformation in a high-pressure triaxial cell. The technique uses compressional-wave velocities to image tomographically the localized pore collapse damage created in a confined-indentation experiment.