Mercury injection capillary pressure (MICP) measurements are frequently considered as part of routine core analysis (RCA) programs, and a fair number of MICP samples are then taken from each core brought to surface. MICP data in combination with log data are used in building saturation-height functions that in turn are necessary to calculate STOIIP and initialize reservoir simulation models.
From a physics point of view, MICP has a resemblance to porous-plate (PP) measurements: at a preselected number of pressure steps the amount of injected fluid is measured after equilibration, that is, when fluid movement has come to a standstill. It is well known that PP experiments, just like other special core analysis (SCAL) measurement methods are subject to interference between capillary forces and viscous forces due to relative permeability. For that reason, state-of-the-art SCAL data are extracted from experiments through interpretation-by-simulation. In SCAL relative permeability experiments, the impact of such interpretation is often significant, e.g., residual oil saturations may be reduced by 10 to 15 units.
Some 40 samples have been investigated by MICP under various measurement protocols to study a possible saturation shift due to an interference with relative permeability similar to what is observed in other SCAL experiments. The measurements were designed using an adapted version of the license-free SCAL simulator SCORES. In line with the simulations, the experiments show that mainly recordings of the saturations of the plateau of the capillary pressure curve do not reach equilibrium conditions in conventional MICP measurements. The plateau values should be shifted to lower wetting-phase saturations by 10 to 15 units. In hydrocarbon reservoirs with an extensive transition zone, the transition zone may be significantly reduced and a sizeable effect on STOIIP, and therefore on reserves, can be expected.
The paper presents detailed information on an improved MICP measurement protocol.