Mercury Capillary Pressure Curves and the Intepretation of Pore Structure and Capillary Behaviour in Reservoir Rocks

Mercury injection and withdrawal capillary pressure curves provide information regarding the efficiency with which a nonwetting phase can be withdrawn from a pore system. Withdrawal (recovery) efficiency varies among samples of differing lithology and can be explained in terms of the geometric aspects of the pore systems, some of which can be interpreted from the capillary pressure curves themselves and others only by direct observation of rocks and resin pore casts. Withdrawal efficiency is defined as the ratio of the volume of mercury withdrawn from a sample at minimum pressure to the volume injected before pressure was reduced. Some features of capillary pressure curves result from experimental procedure and the influences of sample size and shape in conjunction with effects that are peculiar to the external boundary region of the sample. Features which are not typical of an infinite pore system must be recognized before interpretations are made. From experiments on rock samples as well as from theoretical consideration of artificial models it is found that withdrawal efficiency tends to increase with increase in initial saturation. Withdrawal efficiency also increases as throat to pore diameter and volume ratios increase, as the homogeneity of throat and pore sizes increases, and as the number of connections (throats) per pore increases. For a particular sample, withdrawal efficiency is also dependent on saturation history. The initial injection curve combined with the final withdrawal and final reinjection curves for a sample is sufficient to predict withdrawal efficiency from any initial saturation and to construct any desired pair of withdrawal and reinjection curves for intermediate saturations. Experimental determination of the dependency of mercury withdrawal efficiency on saturation history, and the ability to predict withdrawal efficiency for any initial saturation, may be useful in understanding nonwetting phase movements due to capillary effects in hydrocarbon reservoirs.