Elastic Fracture Analysis in the Tight Gas Sands of the Mesaverde Formation, Piceance Basin, Colorado
Jay R. Scheevel
Scheevel Geo Technologies, LLC, Grand Junction, CO
I present theory and supporting mechanical data demonstrating that elevated pore pressures (and consequent elastic grain shrinkage) are capable of inducing tensional framework stresses in the Mesaverde rocks of the Piceance Basin of northwestern Colorado. Elevated pore pressures in the basin are the result of gas generation in Upper Cretaceous Mesaverde coals. Tensional framework stress conditions inevitably result in the spontaneous creation and opening of extension fractures in these clastic lithologies.
This treatment makes use of linear elastic theory. I solve explicitly for the contributions of both framework- and grain-volume deformations resulting from specific overburden-, tectonic-, pore-pressure- and temperature-induced strains. Fracturing is enabled by the presence of a zero lateral strain boundary condition. This boundary condition is imposed because of the semi-infinite lateral extent of layered rocks in the subsurface basinal setting.
Data from the Multi-Well Experiment (MWX) illustrates my conclusions. The MWX was a multi-year, multi-million dollar Department of Energy (DOE) sponsored experimental study of the Mesaverde tight gas sands and has proven to be a wealth of both data and insight on the Mesaverde lithologies of the Piceance Basin.
Rock mechanics charts illustrate the results of the elastic analysis, showing the onset and opening tendency of fractures in rocks over a spectrum of rock-mechanical properties and pore-pressure gradients. To reveal how the Mesaverde lithologies may respond to subsurface conditions, rocks from the MWX test wells are depicted on these charts, predicting that open fractures will result from pore-pressure gradients in the 0.5 to 0.8 psi/ft range.