A new method to acquire m exponent and tortuosity factor for microscopically heterogeneous carbonates

Many factors control accurate determination of water saturation (Sw). Cementation exponent (m) and tortuosity factor (a) are from those that have been focus of many studies. Log-log plot of porosity (φ{symbol}) versus formation factor (F) is used to determine m and a. The cementation exponent is determined from the negative slope of the least square fit straight line of the plotted points, while the tortuosity factor is the intercept of the line where φ{symbol} = 1. In heterogeneous carbonate reservoirs where pores and pore throat networks are complex due to various diagenetic processes, F and φ{symbol} scatter significantly on the φ{symbol} - F plot. This will cause a small coefficient of determination between F and φ{symbol} and thus less reliable m and a. Although classification of data based on petrofacies and/or permeability may improve the correlation to some extent, data still show significant scatter. This study has established a new approach to classify φ{symbol} and F data based on current zone indicator (CZI) and electrical flow unit (EFU). The approach can obtain more accurate m and a and thus more realistic calculation of Sw. This study also shows forcing a to any fixed value, will lead to both optimistic and pessimistic estimation of Sw for a reservoir.