CALCULATED DETERMINATION OF VARIABLE WETTABILITY IN THE MIDDLE BAKKEN AND THREE FORKS, WILLISTON BASIN, USA

摘要

The determination of oil in place can be problematic informations with varying wettability using resistivitybased saturation models. This is because the Archie nsaturation exponent increases as wettability goes from 1)water wet to 2) mixed wet to 3) oil wet. The MiddleBakken and Three Forks in the Williston Basin are bothin this category and are formations with highly varyingwettability.In this study, we examined a combination of triplecombo, NMR, dielectric, and core data to determinewettability and the Archie n. By examining fluidcomposition as a function of T2 or T1 distribution, wecould examine wettability as a function of mineralogy(including clay), porosity, pore size, and depth. Theadvantage of this technique over using special coreanalysis is 1) reduced cost, 2) a continuous measurementwith depth, and 3) the ability to determine these valuesin-situ. The inherent changes in fluid properties resultingfrom core extraction, the changes in wettability becauseof core cleaning, and the uncertainty in measuring coreat reservoir conditions is effectively eliminated with thistechnique.Our results show that the Middle Bakken is mixed wetwith intervals that are more oil wet. It was determinedthat as a result of the increased carbonate and chloritecontent in the Three Forks, this formation is more oil wetthan the Middle Bakken. In both formations, the cleanestintervals are the most oil wet. Using the calculatedvariable n, a favorable comparison with core SCAL andsaturations was obtained, and will be shown in the paper.CRIM inversion of dielectric data, which gives a totallyindependent determination of bulk volume water,confirms the water volumes calculated with the variablen resistivity model.