Abstract:

The testing data indicate that formation water salinity of low-permeability reservoirs varies in a wide range.What's worse, under the impact of complex wettability, classical methods including spontaneous potential and a combination of reservoir resistivity and porosity fail to predict formation water salinity.This leads to great difficulty in the prediction of reservoir oil saturation.To this end, we proposed a new method to predict the formation water salinity with the results verified by experiments.The new method is shown as follows:assuming the salinity of formation water was approximately equal to that of irreducible water in adjacent mudstones, we carried out resistivity and acoustic logging in the relatively stable parts with high natural gamma ray, without borehole enlargement and low resistivity in adjacent mudstones; then conducted compaction correction of the interval transit time; and at last identified the mudstones of diverse salinity (0-20 g/L, 20-40 g/L, 40-60 g/L and >60 g/L) via cross-plotting the resistivity vs.corrected interval transit time.In practice, a contour map of formation water salinity of the study area was drawn by using the formation water salinity of Chang 8¹ member in 106 wells predicted by this method and in combination with the test results of 69 formation water samples.This is conducive to the selection of formation water salinity and the planar distribution pattern study of salinity.In addition, the newly proposed method in the study serves as a feasible method for the prediction of formation water salinity in low-permeability reservoirs with complex wettability, and is universally applicable.

Key words: wettability, formation water salinity, low-permeability reservoir, Yanchang Formation, Ordos Basin