Determination of Fluid Corrected Porosity in Tight Gas Sands and in Formations Exhibiting Shallow Invasion Profiles

Abstract

ABSTRACT Log-core correlations from four Upper Cretaceous wells in Rocky Mountain basins demonstrate that the axiom "high porosity - shallow invasion, low porosity - deep invasion" is not valid in tight gas sands. The data suggests that the zone of flushing is typically much shallower than the density tool depth of investigation. This results in too high density interpreted porosity in low permeability gas sands. The discrepancy is generally 3-4% porosity in gas sands having 8-12% porosity as determined from core analysis. Rigorous log quality control was maintained in the field. This included monitoring of the before and after density calibration record, multiple repeat runs over the cored intervals, slow logging speed, monitoring of tool stability, and digital logging. Whole core Boyle's Law helium porosity and grain density measurements were made on every foot of potential reservoir sand. Core measurements are depth shifted to log data using a combination of core gamma, lithologic description, and a plotted core data overlay. A crossplot of core porosity versus density log calculated porosity is presented for each well. Log calculated porosities are based upon bulk density, the grain density value of each correlated core measurement, and the customary assumed fluid density of 1.00 g/cc. By combining plots of all four wells, a non-linear trend is established for conversion of apparent density porosity to true porosity. The trend has the characteristic of closure to unity for the higher and very low porosity values, with greatest divergence from unity in the middle porosity range. Formations having higher water saturations show less divergence in the middle porosity range. An iterative mathematical model is developed for calculation of "Sxo" and effective porosity. The "Sxo" is actually the average water saturation of the zone investigated by the PGTK density tool and the CNTA thermal neutron tool. This system provides more reliable porosities and may have application as an indicator of potential formation producibility. The technique also has application in other shallow-invasion type borehole environments, e.g., air-gas drilled holes, oil base systems, and underbalanced drilling systems.