John B. Comer¹, Anthony J. Park², Kagan Tuncay², Peter J. Ortoleva²

(1) Indiana Geological Survey, Bloomington, IN
(2) Lab. for Computational Geodynamics, Indiana University, Bloomington, IN

ABSTRACT: Predicting Fractured Gas Reservoirs Using a 3-D Finite Element Diagenetic, Hydrologic, Rock Mechanics Model, Rullison Field, Piceance Basin, Colorado

Subtle structural curvature of Upper Cretaceous strata in the Rulison Field is cited as causing sufficient permeability enhancement to allow commercial gas production from tight gas sands in the Mesaverde Group. This concept has been evaluated using Basin RTM, a comprehensive code that models natural chemical reactions, fluid transport, and mechanical processes in three dimensions using fully coupled algorithms based on fundamental laws of physics and chemistry. The simulations show that multiple episodes of compartmentation caused by pressure build up, fracturing, and pressure release began around 50 Ma and that development of a gas phase began around 40 Ma. Maximum development of gas charged compartments coincided with the period of deepest burial during the Eocene and Oligocene between 40 and 25 Ma. The simulations indicate that fractured, gas charged compartments developed locally in regressive marine sandstones of the Iles Formation, fluvial and coastal sandstones of the Williams Fork Formation, and parts of the Wasatch Formation. Large pressure contrasts between compartments and surrounding strata that existed before 20 Ma in the Wasatch and Williams Fork Formations mostly have dissipated during subsequent uplift and erosion. Current producing reservoirs are the remnants of more extensive and highly charged compartments that formed during mid-Tertiary deep burial. Modeling results show that even subtle bends in the strata affect internal stress distribution sufficiently to modify the porosity, fracture permeability, fluid flow direction, and pressure distribution, and thus exercise an important control on the location and evolution of gas charged compartments through time.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado