Improved capillary pressure model considering dual-pore geometry system in carbonate reservoirs

In carbonate rocks, it is difficult to precisely represent the relationship between capillary pressure and water saturation due to the high degree of heterogeneity in pore geometry. Carbonate rocks have various types of pore systems such as fractures and vugs, and this heterogeneity should be considered for more realistic prediction of oil production in carbonate reservoirs. However, the previously developed capillary pressure models like Leverett J-function and Thomeer method cannot accurately describe the carbonate rocks having great heterogeneity in its pore system. These models consider the pore as a single pore system. Therefore, we present a capillary pressure model that can handle the pore system as macropore and micropore, separately. We compare the result of this model against experimental data measured for special core analysis of six different cores of carbonate rocks. As a result, it was found that this model favorably matches with experimental results in the entire range of water saturation. The applicability of this model for the greater heterogeneous pore system was also investigated. From the comparison with experimental results for three core samples with different degrees of heterogeneity, one can realize that the error is much greater in Thomeer method and Leverett J-function than this model especially in heterogeneous system. When the developed model is applied to the case without capillary pressure data, the model firstly quantifies the matching parameters, i.e. pore geometrical factor, displacement pressure and residual saturation for each macropore and micropore. And then, the applicability of this model was tested with these matching parameters by comparing to experimental data. The result showed excellent matches in cases of both macropore and micropore systems with correlation coefficient value of 0.97. Therefore, it can be concluded that the capillary pressure model proposed in this study could apply appropriately to the greater heterogeneous pore system like carbonate rocks.