Abstract: Many mercury-injection capillary pressure curves, which inferred from the samples of compacted and intense secondary carbonate rocks in Luojiazhai gas field, northeastern Sichuan, China, have not very good horizontal line segments. Thus, it is not easy to get the data of drainage capillary pressure. A new way to answer this problem is put forward. That is using cubic spline interpolating data to match the capil-lary pressure and its corresponding mercury saturation, enabling determination of the drainage capillary pressure to become getting the first turning point of mercury-injection capillary pressure curve. The method is not only accordant with the definition of capillary pressure curves, easy io use. but also fits the calculation of any morph ologic capillary oressure curve.

Key words: mercury injection experiment, threshold pressure (drainage capillary pressure), interpolation