A Comprehensive Water Saturation Model for Low Porosity Sandstones
Technical advances on many aspects of producing hydrocarbons from low porosity sandstone have lead to increased exploration in basins around the world. It is critical for commercial success to understand the porosity and hydrocarbon saturation relationships associated with these systems. In many basins where tight sandstones are being evaluated for the potential of commercial hydrocarbon accumulations, prospective intervals range in thickness from a few feet to several thousand feet and may cross stratigraphic horizons with various depositional environments.
Determining formation water resistivity (RW) is a critical component in determining if sandstones may be at or close to irreducible water saturation (SWirr). Often during exploration efforts there is very little information available for RW estimates. Calculating RW from SP curves can be misleading due to low porosity/permeability, sand body thickness and the potential for the presence of variable anions associated with formation water. Pickett plots have been used for RW determination for many years but the technique requires prior knowledge or assumption for the value of the Archie saturation exponent “m” and representative sandstones that are water saturated.
A cornerstone to improved petrophysical understanding of low porosity sandstone was recently published by the DOE/KGS project Regional Petrophysical Properties of the Mesaverde Low Permeability Sandstones (2007) and Evidence for a variable Archie porosity exponent “m” an impact on saturation calculations for Mesaverde tight gas sandstone: Piceance, Uinta, Green River, Wind River and Powder River basins (2008) Byrnes et al. Project results document several physical aspects of low porosity sandstones in the Rocky Mountain region including a core derived relationship indicating that a decrease in porosity and/or water salinity result in a decrease in the Archie saturation exponent “m”.
Pickett plots from productive wells from several basins in Colorado, Utah and Wyoming are presented to demonstrate the application of the variable “m” as described by Byrnes et al. Results indicate the variability of the formation water salinity not only aerially between wells within a particular field but between sand bodies within a single well bore over the potential interval of interest. Pickett plots are also demonstrated to be a good tool for determining the bulk volume of irreducible water and consequently a porosity cutoff for hydrocarbon storage.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009