Capillary Pressure Drainage Curves: Modelling and Prediction of Capillary Entry Pressure

Abstract Capillary pressure (CP) measurements on core samples generally deploy one of three tests: porous plate (PP), centrifuge (CF) and mercury injection (MI). The first two mentioned tests are most often used to directly determine a CP relationship and the PP procedure most closely conforms to reservoir situations. CF test results may be superior for certain types of plugs and testing conditions, not to mention that testing is considerably faster. MICP tests are typically used for determining pore size distributions but may also be used to generate CP curves, with best results obtained by validation against the other two tests. MI tests also have the shortest timeline. CP curves have been modelled (fitted or matched) over the years with many alternative formulations, from the well-known J-function approach (Leverett, 1941) to more recent methods. Some formulations can realistically match the low pressures of CP profiles, including capillary entry pressure, while others do not cater for it at all. When the low-pressure part of CP curves exhibit a "roll-over effect (concave downward profile), entry pressure prediction may require high frequency data from MICP testing to give more definition. This paper examines several CP data sets, comparing PP and MI results, in light of pore throat distributions. Seven formulation are used to match entire CP profiles from PP data and are subsequentlyexaminedfor being able to represent capillary entry pressure. Various types of pore structures are discussed: homogeneous (narrow distribution), broad (poorer sorting) and bimodal (two distinct distributions) where the smaller pores are often related to pore-fill. It is shown that the Modified Carman-Kozeny Purcell (MCKP) formulation (Behrenbruch et al, 2011) is excellent in capturing the level of entry pressure for narrow pore throat size distributions or homogeneous plugs while entry pressure prediction for poorer sorted plugs tends to be less accurate, partially due to variation in samples.The detailed modelling of the roll-over effect requires a separate model. A new and improved correlation for (model) capillary entry pressure has been derived, showing results from Australian and other fields.