Threshold Displacement Pressure - Considerations for Caprocks of Abnormal Pressure Reservoirs

Abstract

Abstract Threshold displacement pressures were measured for shale caprocks of underground gas storage reservoirs and found to be as high as 2,000 psi or more. These measurements were under approximate simulated reservoir conditions. As a result, some new considerations have been set forth as to the possible mechanisms of sealing of abnormal or geo-pressured reservoirs. The presence of abnormal pressure reservoirs, information gathered while drilling them and some physical properties of rocks under abnormal pressures were reviewed to shed light on these new considerations. Field examples of abnormal pressure reservoirs in the Gulf Coast were analyzed. A new concept based on the surface phenomena in shales has been developed phenomena in shales has been developed explaining the retention of hydrocarbon fluids below caprock of abnormal pressure reservoirs. The process of compaction, its rate and the decrease of permeability under effect of overburden pressure have been suggested as some of the controlling factors leading to the present capillary sealing of abnormal pressure present capillary sealing of abnormal pressure reservoirs. Introduction Before the inception of American Gas Asso. research project PR26-47 on threshold displacement pressure in gas storage, consideration had been pressure in gas storage, consideration had been given to Gulf Coast reservoirs with geo-pressures of as much as 0.8 lb/ft of depth. The caprocks to these reservoirs must have a severe pressure gradient in them similar to the pressure gradient in caprocks submitted to high threshold displacement pressure tests. It was known that these values could be as high as 2,000 psi or more and that study of these abnormal pressure reservoirs in the Gulf Coast might shed some light on the threshold pressure displacement phenomenon. The question may be raised as to what extent this naturally occurring high pressure reservoir can be used to verify that high threshold displacement pressure can be expected for high quality pressure can be expected for high quality caprocks. In looking into the matter, some new considerations are set forth as to the possible mechanism for the abnormal pressure reservoir and its overlying caprock of reaching its present state. There appears to be no doubt present state. There appears to be no doubt that surface phenomena are involved in the forces which retain the hydrocarbon fluids below the caprock; these forces are the same as those which retain natural gas in underground storage reservoirs with positive delta pressures.