New methods for qualitative and quantitative determination of wettability from relative permeability curves: Revisiting Craig's rules of thumb and introducing Lak wettability index

Craig's rules of thumb have been widely used for qualitative determination of wettability from relative permeability curves. According to the first rule, in water-wet rocks the water relative permeability at residual oil saturation (ROS) is generally less than 30%, whereas in oil-wet systems this is greater than 50%. The second rule considers a system as water-wet, if saturation at the crossover point of relative permeability curves is greater than water saturation of 50%, otherwise oil-wet. The third rule states a relationship between the interstitial water saturation (IWS) and wettability. In this study, Craig's second rule is modified. A Reference Crossover Saturation (RCS) is defined as RCS = 1/2+S-wc-S-cr/2(S-wc and S-or stands for IWS and ROS, respectively). For water-wet rocks the crossover point of relative permeability curves lies to the right of RCS, whereas this point lies to the left of the RCS for oil-wet systems. Furthermore, a RCS definition is derived for characterization of wettability in gas-liquid systems. In addition, it is explained that Craig's third rule should be discarded from the analysis. A new index, called Lak wettability index, is presented by combining Craig's first rule and modified Craig's second rule. Lak index, which varies from -1 to +1, describes wettability quantitatively from a single pair of relative permeability curves. Twenty water-oil relative permeability experiments were performed and an agreement was observed between the predictions made by Craig's first rule, modified Craig's second rule, and Lak index, whereas Craig's second and third rules failed in characterizing wettability.

Automated summary

Craig's rules are widely used for qualitative determination of wettability. The first rule describes characteristics of water-wet rocks and oil-wet systems, the second rule judges water-wet and oil-wet systems based on the crossover point of relative permeability curves, and the third rule explains the relationship between interstitial water saturation and wettability. A modified version of Craig's second rule is introduced in the study, along with the new Lak wettability index.