A new method for determination of wettability of reservoir rocks.
On the basis of previous work, a new method for the determination of wettability of reservoir rocks, which can be designated as the modified method of spontaneous imbibition, has been established. The method is provided based on the principle that the wettability of rock surfaces is a key factor affecting the spontaneous imbibition rate as the rock contacts fluids. The influence of oil/water interfacial tension, liquid viscosity, pore geometry, as well as the shape and size of core samples, can be eliminated by introducing a dimensionless time, tD, instead of the imbibition time. The wettability index of rock surfaces under a given condition, WR, was determined quantitatively from the pseudo capillary pressure curve measured in the process of imbibition. The larger the value of WR is, the stronger the water wetness will be. The magnitude of WR depends on the rate of spontaneous imbibition and the total volume of the imbibition liquid. The imbibition tests were conducted by suspending a core from an electronic balance and immersing it in brine. The constitution of brine used in this test should be the same as the brine used for establishing the initial water saturation. The change in weight of the suspended core versus time was monitored for the period of time from core immersing to the weight of the core keeping constant (usually 72h). The oil recovery by spontaneous imbibition, Rim, at a given time was calculated and the curve of Rim versus tD, was plotted from the change in core weight and the density difference between oil and brine. The pseudo capillary pressures, pc,ps, and water saturation, Sw, corresponding to various values of tD were calculated and the curve of pc,ps versus Sw was plotted. Finally, the wettability index, WR, for each core sample was determined from the area under this curve.The experimental results indicate that the wettability indices determined using this method is considerably comparable with the wettability indices, Iw-o， determined using the traditional Amott method. Compared with the Amott method, the accuracy of the results determined using the new method is higher, and the experimental procedures are simpler. It is therefore convenient for applying more widely. However, the method is only suitable for the determination of wettability ranging from neutral wet to strongly water-wet condition. In general, the resolving power of WR is higher for the strongly water-wet rocks, while the resolving power of Iw-o is higher for the weakly water-wet rocks.