The time-dependent creep compaction of porous rock is being studied in the laboratory at simulated in-situ conditions. Of primary concern is rock from subsurface reservoirs containing oil, gas, or geothermal fluids. To simulate the effect of reservoir production, compaction is triggered by pore pressure reduction. Typical measured rates of porosity loss are about 1 × 10 -4 /day, although extremes vary greatly from this Theories of inelastic, time-dependent rock compaction are being developed or extended to allow the extrapolation of laboratory measurements to very long times.
To evaluate production potential for a subsurface fluid-producing reservoir, it is important to have a knowledge of porosity, compressibility and permeability. It is equally important to know if changes in these parameters will occur as functions of time during reservoir production. Well testing can provide valuable reservoir engineering data, but it cannot predict future changes in reservoir parameters or provide a complete understanding of the mechanisms of reservoir production. Cases where prediction of behavior or understanding of mechanisms of reservoir production are deemed useful usually require laboratory experiments and theory.
In production of subsurface fluid reservoirs, large volumes of fluid production over long periods of time can result in significant reduction of pore pressures. The resulting increase ofeffective stress bears on the rock and causes it to compact. It may also lose permeability. The questions raised here are: when and how are these effects triggered, what is their magnitude, and to what extent are they irreversible? To answer these questions, sedimentary reservoir rock has been selected from cores taken at two geothermal fields, East Mesa in southern California, and Cerro Prieto in northern Mexico, one geopressured reservoir in Louisiana, and several hydrocarbon-bearing reservoirs. Laboratory tests are being conducted at simulated in-situ conditions and results are being analyzed. Particular emphasis is being placed on long-term creep compaction of the rock, since it is hypothesized that a considerable part (often most) of the changes in values of parameters caused by pore pressure reduction occur slowly over long periods of time. These changes would not be observed in a conventional test (typically less than 1 hour duration). A major challenge then, is to learn how to extrapolate data gathered during laboratory tests of relatively short duration to field production of many years duration.
Most experiments are done using a system designed and built at Terra Tek for high-temperature creep measurements. The system is shown by photograph in Figure 1 and by schematic drawing in Figure 2. Capabilities of the system in its current configuration are confining pressure of 30,000 psi, axial load of 100,000 lbs, pore pressure of 10,000 psi, and temperature of 300°C. Axial and transverse sample displacements
(Figure in full paper)
(strains) can bemeasured. The axial system utilizes linear variable differential transformers (LVDTs) which are extremely stable and precise for creep measurement purposes. The pore pressure system can be used for flow (permeability) and measurements of ejected fluid volume (pore volume change). It can handle low pH brines.
