Fresh Formation Water in Tight Reservoir Rocks Creates Contention between Petrophysics and Geoscience

Abstract

Evaluation of tight reservoir rocks, having low porosity and low permeability, is difficult enough. It becomes even more challenging when the formation water in these reservoirs is either fresh or has low salinity. The combination of fresh formation water and tight nature of the reservoir rocks causes the resistivity logs to read high, resulting in a low contrast in resistivity between fresh water bearing formation and hydrocarbon bearing one. This creates an uncertainty on the type of reservoir fluid and computed water saturation, if the formation water salinity is not known. Consequently, this becomes a source of contention between geoscientists who estimate hydrocarbon resource volumes and petrophysicists who provide the input parameters used in hydrocarbon resource assessment. This problem is becoming more common as wells are being drilled deeper into tighter rocks. Several case histories are presented in this paper, highlighting the difficulties encountered in evaluating fresh water bearing tight formations. Based on all available data, all of these wells have been evaluated as fresh water bearing tight reservoirs, with very small amount of dissolved or residual gas. The challenge here is to perform a conclusive petrophysical evaluation technically acceptable to all parties concerned.