Paper: Rock Properties And Their Effect On Gas Flow And Recovery

Voids within the rock matrix form the container in which gas accumulates; therefore, knowledge of the quantity and distribution of these voids (pore space) is essential in assessing the quantity and distribution of gas present. The shapes, variety of sizes, and distribution of the pore space on a microscopic scale are referred to as pore geometry. This geometry differs widely because of the varying depositional environment of reservoir rocks, and subsequent diagenesis. Pore geometry is related to the quantity and distribution of storage space (porosity), and through capillary forces it influences the reservoir distribution of gas and water. The ability of a formation to transmit fluids (permeability) is also related to pore geometry. The native ability to flow can be decreased during completion operations by reaction of the rock with the completion fluids. In addition, an increase in effective overburden pressure typically occurs during production, and this also results in decreased flow capacity. Low permeability gas reservoirs are particularly sensitive to both these effects. The introduction of filtrate into a gas formation reduces gas flow capacity-even when no rock-fluid reaction occurs. This permeability reduction is controlled by relative permeability characteristics of the rock. This loss of flow capacity occurs whether the extraneous liquids are introduced in the zone from filtrate invasion during completion or workover operations, or by retrograde condensation. Influx of water into a gas reservoir traps a quantity of gas behind the water-gas front. This trapped gas is not recoverable and varies with rock type, and in some cases with permeability and porosity within a given formation. The magnitude of this trapped gas must be known and accounted for in order to estimate gas recoverable reserves in water-drive and gas storage reservoirs. Gas storage projects serve as accumulators of gas near the area of need. Storage capacity, capillarity, transmissibility, relative permeability characteristics, and trapped gas quantities are necessary in evaluating the potential of the storage zone. In addition, threshold pressure tests are required to evaluate the suitability of the caprock matrix that overlies the storage zone.