EFFECTIVE Qv BY NMR CORE TESTS

摘要

Qv, the cation exchange site concentration per unit pore volume, is a rock property that is used to quantify the conductivity contribution of pore-lining clays in rocks. Qv is a key term in resistivity-based water saturation models for shaly sands. The combined effects of Qv, salinity, and total porosity control the volume of clay-bound water (CBW) in water-saturated shaly sands and influence most estimates of effective porosity and hydrocarbon storage. Common experiments to quantify Qv include wet chemistry cation exchange capacity tests (CEC) and multiple salinity core conductivity tests (Co-Cw). CEC tests are determined on disaggregated material and data application includes the assumption that the resulting Qv is uniformly distributed in the porosity and that it is 'effective' Qv. Results of the Co-Cw test method, derived from tests at net confining stress on intact core plugs, include the Qv distribution within the porosity and represent effective Qv. However, the method requires flow-through of multiple brines and can be time consuming and impractical for very low permeability samples. We investigated identification of effective Qv by a method that has neither the assumptions nor limitations of CEC or Co-Cw methods. This method uses stressed helium total porosity and NMR T2 measurements on confined or unconfined brine-saturated core plugs to determine clay-bound water porosity. Knowledge of CBW porosity, total porosity, and saturating brine concentration provides the input to determine effective Qv by this method. This test is rapid, non-destructive, repeatable, and relatively simple, yet retains the effect of clay distribution within the porosity and the effect of net stress on total porosity while not requiring extensive brine flow-through. Previous investigators have approximated this method and have found different T2 cutoffs to represent CBW porosity. We further examined the process and defined both the method and a recommended T2 cutoff. In doing so we also evaluated how this method may be affected by salinity, non-conductive clays, clean (Qv-free) microporosity, internal field gradients, and net confining stress. We provide validation for the method by calculation of CBW porosity from Co-Cw Qv tests on a large, diverse group of sandstone core samples. In turn, these CBW porosities were compared to cumulative porosities from NMR T2 distributions and matching T2 cutoffs were determined. Review of those results indicates a T2 cutoff for CBW of 2.8 milliseconds will provide a representative value for input to Qv calculations.