An Introductory Discussion of the Reservoir Performance of Limestone Formations

Field experience with limestone and sandstone production indicates theexistence of wide differences between the reservoir behavior of these two typesof formation. Little attention appears to have been given to the separate studyof the flow of fluids and the retention of fluids in limestones. This paperpresents data and arguments to demonstrate the existence of a difference?between the two types of formation, and urges the separate intensiveinvestigation of limestone reservoirs. All experimental data presented pertainto dolomitic limestone formations in West Texas and New Mexico. Two kinds of porous media are recognized intergranular and intermediate.Intergranular rocks are those in which the porosity and permeability aredetermined by the geometrical properties and the sorting of the sedimentaryunits; intermediate rocks, those in which there is no direct relationshipbetween grain properties and the porosity and permeability. Limestones ingeneral are intermediate media. The partial relationship between porosity andpermeability of any class of porous media is represented by an area of finiteextent and definite shape on the permeability-porosity plane. The horizontal and vertical variations of porosity and of permeability inlimestone and sandstone formations are discussed and compared. Comparisons are made of connate-water content,. the relativepermeability-saturation relationship, and. capillary phenomena in the two kindsof rocks. A number of tentative conclusions are drawn relative to the reservoirproperties of limestones; in particular,dolomitic limestones are oilbearing and apparently are oil productive in zones of permeability less than0.1 millidarcy, andprimary depletion (by gas expansion) oil saturationsmay be lower in dolomitic limestones than in sandstones. Introduction It is well known that the reservoir performance of limestones displays numerousirregularities when compared with that of sandstones, and that the departuresfrom "normal" behavior of limestones are more frequent and generallymore marked than in sandstones. This is particularly true of fracturedlimestones and those that have undergone considerable development of secondaryporosity. A review of the literature of production research published during the past ISyears reveals a startling absence of theoretical and experimentalinvestigations directed specifically toward explaining and predicting theperformance of limestone reservoirs, even though such problems as theprediction of 'water encroachment and the estimation of ultimate recoveriescontinue at the. "inspired guess" stage. By far the greater part of theexperimental data reported pertain to measurements on sandstone core samples, while the theoretical studies, almost without exception, are developed from theassumption of an ideal porous medium and therefore seldom are applicable tolimestone formations. T.P. 1791