Some Examples of Fluid Flow Mechanism in Limestone Reservoirs

The properties of limestone reservoir rocks such as the distribution anddegree of continuity of the pore systems, and the relative volumes andpermeabilities of the systems making up the complex cause large variationsbetween performance of individual limestone reservoirs and their susceptibilityto secondary recovery methods. The effects of these factors on the mechanism offluid flow cannot be adequately evaluated with presently developed concepts, laboratory data, and geological information. The observance and interpretationof the performance of individual limestone reservoirs provides, at present, themost adequate approach for evaluating the integrated effects upon performanceof the many, now immeasurable variations in the properties of limestonereservoirs. Introduction The development and application of techniques for increasing the efficiencyof oil recovery from natural reservoirs is a problem of primary importance tothe industry. During the past several years, a great deal of effort has beenexpended by the technical personnel of the industry toward the improvement ofpresent known methods of oil recovery, and evaluating the factors which controlthe susceptibility of particular reservoir types to economic application ofsecondary recovery methods. Providing adequate and accurate laboratory data onthe properties of the reservoir rock and its contained fluids, together withgood production statistics, are available, methods have been evolved forestimating with reasonable accuracy the performance of an oil reservoir undereither continued natural depletion or conditions imposed by introducingadditional displacing fluid into the reservoir from an extraneous sourcethrough the injection of gas and/or water. The susceptibility of limestone reservoirs to secondary recovery by gasinjection is very much dependent upon gas-oil relative permeabilityrelationships for the reservoir in question. In a rock formation in which theporosity is of the intergranular type such as is found in sand stones and somenonfractured dolomites, a representative sample of the pore structure can beobtained in a small core plug. In this type of reservoir, it has been shownthat relative permeability data obtained from laboratory experiments can beproperly applied to evaluate the flow relationships actually observed duringthe depletion of a reservoir. In addition, a good idea of the fraction of thereservoir which will be swept by the injected gas may be obtained from thespacing pattern used and the permeability profile of cored wells. From thesedata, it has been demonstrated that reliable estimates of performance andrecoveries under gas injection operations can be made for reservoirs in whichthe porosity is of the intergranular type. T.P. 2638