PETROPHYSICAL PARAMETER MEASUREMENTS: COMPARISON OF SEMI-DYNAMIC AND CENTRIFUGE METHODS FOR WATER-WET AND OIL-WET LIMESTONE SAMPLES

Reservoir simulations require relative permeability and capillary pressure curves as representative as possible of the studied case, especially in terms of reservoir conditions. Several methods can be used to determine these parameters and each one has its own advantages but also its own limitations, whatever the experimental conditions, ambient or reservoir. Within a SCAL study, the selection of the most appropriate method is thus very difficult for the reservoir engineer. The main objective of this paper is to compare two methods for capillary pressure measurement: the centrifuge technique, which is widely used under ambient conditions, and the semi-dynamic method (SDM), first presented in 1993, which can operate under reservoir conditions. The principle of the experimental and interpretation methods is recalled in the first part of the paper (Forbes' interpretation method for the centrifuge technique and a recently patented interpretation method for the semi-dynamic technique that allows determining the impact of the small scale sample heterogeneity on the petrophysical parameters). The second part is dedicated to the acquisition of experimental data. Although some results are available in the literature, the comparison is difficult since the experiments were performed on sister plugs and not exactly on the same sample. A new experimental data set was collected with the two methods, applied on the same sample, a homogeneous Lavoux limestone. In the first set of experiments, first drainage was performed with each method on the water-wet sample, at ambient conditions. Both methods lead to comparable results for this homogeneous sample. The sample was then rendered oil-wet and the forced imbibition was studied with both techniques. In that case, due to a nonuniform initial Swi-profile, the sample behaved as a heterogeneous one: the SDM and centrifuge Pc-curves derived from the homogeneous approach are not superimposed and the bandwidth delimited by the local SDM Pc-curves is wide. The interpretation of this apparent disagreement is given in the third part of the paper, based on the numerical experiment of forced centrifuge imbibition on the La6 sample at non-uniform Swi conditions. The "homogeneous" interpretation of this numerical forced imbibition provides a Pc-curve very close to the Pc-curve derived from the real centrifuge experiment. This demonstrates the importance of the sub-plug size heterogeneities in the determination of the Pc-curve with the centrifuge method, while its interpretation is based