Fast prediction method of Archie's cementation exponent

Archie's cementation exponent (m) is the most important parameter in calculating water saturation. Using core measurements to obtain m is often expensive and cannot be carried out in all reservoirs. Besides, because of rare uncontaminated connate water samples in most cases, it is difficult to use connate water resistivity (Rw) to calculate accurate m directly by using Archie's equation in water zone. In this study, based on Archie's equation in an interconnected water layer, a prediction method of cementation exponent is proposed. The method includes two main steps. The first step is to propose a numerical method and the corresponding program to obtain the fixed Rw and variable m in a water layer. The program calculates formation resistivity values (Rt_cal) at each Rw (a total of 50 values) and m (a total of 161 values) in an interval, and then it consists of two comparisons: (1) the program compares the 161 Rt_cal values for each Rw value with the measured formation resistivity (Rt) at each depth to obtain the Rt_cal of the minimum error; (2) 50 Rt_cal curves of all Rw values are collected in the interval and are used to compare with the Rt curves using the minimum error and smallest standard deviation to obtain the best Rt_cal curve. Therefore, the fixed Rw and variable m can be obtained corresponding to the best Rt_cal curve. The second step includes several steps as follows: (1) establishing the porosity estimation model by using the core scale logging method, porosity curve can be calculated from conventional logs by using it; (2) choosing a representative water layer, the fixed Rw and variable m can be obtained by using the program; (3) calibrating the values of x and y by using the cross-plot method, the regression relationship between φ and m can be established; (4) considering the same geologic setting, the regression relationship can be used to predict variable m in adjacent reservoirs. The synthetic test example can validate the reliability of the proposed numerical method and program, and two field examples from different basins can confirm the availability and applicability of the prediction method. The method could provide an alternative solution in the petrophysical assessment (Rw and m) of exploratory and appraisal wells which are usually lack of rock resistivity measurements.