Petrophysical and Geomechanical Analysis of Knox Group Reservoirs and Caprocks Across the Northern Appalachian Basin as a Function of Depth
Authors: Raziperchikolaee S. , Miller J. F. , Sminchak J. 
Published: October 2015

Growth in unconventional shale development and its associated flow-back water disposal needs have led to significant demands for brine disposal wells in the Appalachian Basin. The Ordovician-Cambrian age Knox Group Carbonates and Clastics includes several formations utilized for brine disposal in the Northern Appalachian Basin. The depth of the Knox Group injection intervals changes significantly in the basin, from outcroppings to potentially over 15,000 feet. The significant depth change causes variation in porosity, permeability, and elastic properties in the Knox Group and caprocks that should be analyzed for estimating injection capacity across the region.

To study petrophysical and rock mechanical behavior with increasing depth, we selected eight wells whose depths of injection intervals in the Knox Group and the overlying caprock formations are progressively deeper to the east. The first step is building scatter plots to understand how petrophysical characteristics, including sonic wave velocity, density-neutron porosity, and gamma ray data, are related to each other. Then, multivariate linear regression is used to determine compressional and shear wave velocity as a function of other petrophysical parameters. The t-statistic for each coefficient in the correlation tests the null hypothesis to show significance of coefficients. The dynamic mechanical properties are also determined using dipole sonic and density logs across the region.

We investigate the effect of depth on the relationship between petrophysical parameters, and we study the variations in trends that may be caused by geological events. While the large variation of petrophysical parameters in the Rose Run Sandstone shows the important effect of formation cementation and consolidation by depth, the limited variability of petrophysical parameters in the Copper Ridge Dolomite injection interval proves that its characteristics are not dependent on depth. The effect of petrophysical variation shows its direct effect on rock mechanical properties that its distribution is limited in Copper Ridge Dolomite in comparison to Rose-Run Sandstone. In addition, where the Knox unconformity is present, there exists a completely different relationship between petrophysical parameters for the injection intervals below it and the main caprocks above it. While the content of gamma ray affects formation characteristics above the unconformity, such a trend was not seen in formations below it.

With focus on injection into sub-Knox unconformity formations, understanding petrophysical parameter variations in the injection and caprock intervals seems necessary. Our results indicate that using statistical plots and regression provides an opportunity to understand the relationship between multiple numbers of petrophysical parameters with significant amounts of data. The derived correlations could also be used to determine mechanical properties for the geomechanical analysis across the region, especially when dipole sonic logs are not available.