Compressibility of Reservoir Rocks

The compressibility of reservoir rock is a factor which is generallyneglected in reservoir engineering calculations. This is due in part to thefact that there is little published information on rock compressibility valuesfor limestones and sandstones. Omission of rock compressibility is undoubtedlyjustified in calculations for saturated reservoirs; however, in undersaturatedreservoirs, expansion of the rock accompanying decline in the reservoirpressure may be of such magnitude as to affect materially the prediction ofreservoir performance. The effect of rock compressibility will be of mostimportance in: (1) calculation of oil in place by pressure decline data inundersaturated volumetric reservoirs when the limits of the field are unknownor indefinite, and (2) studies of natural water drive performance. To estimateits importance in such cases, a series of laboratory tests were made to obtainusable values for reservoir rock compressibility.

The total, or effective, compressibility of any reservoir rock is a resultof two separate factors, namely, expansion of the individual rock grains, asthe surrounding fluid pressure decreases, and the additional formationcompaction brought about because the reservoir fluids become less effective inopposing the weight of the overburden as reservoir pressure declines. Both ofthese factors tend to decrease porosity. The laboratory tests were conducted ina manner that would give compressibility values representing a combination ofthe two factors, and would, as nearly as possible, duplicate the behavior ofthe rock under reservoir conditions. Fig. 1 shows a diagram of the equipmentused to determine rock compressibility.

The rocks used in these tests were cores cut from producing limestone andsandstone formations. They were from 2 to 3 1/2 in. in diameter, and 5 to 6 in.long. A core to be tested was completely surrounded by a Lucite jacket with theexception of the connection at one end to allow production of liquid duringtesting. To represent the effect of the overburden, a constant externalhydrostatic pressure of 3,000 psig was applied to the core throughout the test.(The amount of external pressure was varied in a series of tests and no changewas evidenced in the measured compressibility values.) Starting atapproximately 1,500 psi static fluid pressure in the core, the core pressurewas reduced by steps of 100 to 200 psi. Precise measurement of liquidproduction was made for each decrease in pressure.

T.N. 149