Uncertainty analysis of Archie's parameters determination techniques in carbonate reservoirs

Reservoir evaluation is one of the critical tasks of any reservoir exploration and field development plan. Water saturation calculated from open-hole resistivity measurements is a primary input to hydrocarbon reserves evaluation. Archie's equation is the water saturation model for the determination of water saturation. Application of Archie equation in carbonate reservoir is not easy due to high dependency of its parameters on carbonate characteristics. Determination techniques of Archie's parameters are relatively well known and validated for sandstone reservoirs, while carbonates are heterogeneous and a correct estimation of Archie' parameter is important in their evaluation. In the case of carbonate rocks, there are considerable variations in texture and pore type, so, Archie's parameters become more sensitive to pores pattern distribution, lithofacies properties and wettability. Uncertainty in Archie's parameters will lead to non-acceptable errors in the water saturation values. Uncertainty analysis has shown that in calculating water saturation and initial oil in place, the Archie's parameters (a, m, n) have the largest influence and R_t and R_w are the least important. The main objective of this study was to measure Archie's parameters on 29 natural carbonate core plugs at reservoir conditions, using live oil, these core samples were taken from three wells. For this purpose, three techniques were implemented to determine Archie's parameters; conventional technique, core Archie's parameters estimate technique and three-dimensional technique. Water saturation profiles were generated using the different Archie parameters determined by the three techniques. These profiles have shown a significant difference in water saturation values and such difference could be mainly attributed to the uncertainty level for the calculated Archie parameters. These results highlight the importance of having accurate core analysis's measurements performed on core samples that yield representative a, m and n values that highly influence the water saturation values.