Deriving Reservoir Net Pay Cutoffs from Mercury Injection Tests and Routine Core Analysis

The net pay cutoffs are the critical parameters in calculating the total pay thickness and estimating hydrocarbon reserves. To determine these parameters, the industry has been largely relying on drill stem tests (DST) or downhole fluid samplings, which provide information on the productive capacity of different pay intervals. However, due to the high operational cost, these measurements are typically quite scarce, particularly in offshore oilfields. In complicated reservoirs where hydrocarbon viscosity shows vertical variation, the net pay cutoffs change as well and hence even more measurements are required to accurately characterize the changing cutoffs.

In this paper, we propose a novel workflow to derive the porosity and permeability net pay cutoffs by integrating core Mercury Injection for Capillary Pressure (MICP) test and core porosity and permeability, therefore reducing the need for costly DST and sampling. Our core dataset consists of 1024 rock samples, among which 529 have MICP test data. These samples come from 14 oilfields in Bohai Bay where the operator has some local knowledge of the porosity and permeability cutoffs, as well as the oil viscosity. In general, the porosity and permeability cutoffs discovered through testing have a poor correlation with oil viscosity.

Using our core dataset, we built a localized Winland equation relating the apex pore throat aperture (R_apex) to core porosity and permeability. With this equation, we translated porosity and permeability cutoffs into R_apex cutoff. Our study shows that R_apex cutoff has a much better correlation with viscosity than porosity cutoff and oil density. Based on this finding, we developed the following workflow to accurately predict porosity and permeability net pay cutoffs from oil viscosity using R_apex as a guide:

1. For all rock samples, graphically find the apex aperture radii on the corresponding MICP curves.

2. Using regression, find customized Winland equation relating R_apex to core porosity and permeability.

3. For oilfields where the porosity and permeability cutoffs are well defined, calculate R_apex cutoff corresponding to porosity and permeability cutoffs using the equation in step 2. Plot oil viscosity against R_apex cutoffs in a chart and fit an exponential function to the sample points.

4. For new oilfields with known oil viscosity but unknown net pay cutoffs, we enter the oil viscosity into the chart in the previous step and read off the R_apex cutoff value from the regression line.

5. Draw the porosity and permeability values corresponding to the R_apex cutoff in a poroperm plot using the equation in step 2, and draw the core porosity and permeability points from the field under study as overlay. The porosity and permeability net pay cutoffs can be found at the intersection of the core points and the R_apex line.

The porosity and permeability cutoffs found using R_apex cutoff are highly consistent with those found by actual DST or samplings.