Anthony J. Park

Sedimentary basin deformation: an incremental stress approach

Abstract A key component of sedimentary basin evolution is the spatial distribution and temporal variation of stress and deformation. The many deformation processes (poroelasticity, fracturing, irreversible nonlinear viscosity, and pressure solution) are inextricably bound in a tightly coupled network which, in turn, is coupled to a myriad of basin diagenetic, thermal and hydrologic processes. In the approach presented here, the various deformation processes are integrated through an incremental stress approach. Together with mass, momentum and energy conservation, this approach yields a complete, fully coupled basin model that captures basin and fault phenomena that are beyond the scope of simpler or decoupled models. Many of the most interesting basin phenomena are not only dependent on multiple, coupled processes but also are fundamentally three-dimensional. To address this three-dimensional complexity, we have developed a numerical simulator using a moving, adapting, accreting finite element grid which is allowed to deform and to grow and adapt with the addition of sediment to capture smaller sedimentary features. As a result, our fully coupled, comprehensive model allows one to solve a number of key problems in basin and fault dynamics. These include compaction, fractured reservoir and compartment genesis and dynamics. Examples illustrating these applications are presented for idealized systems and the Piceance Basin (Colorado) and the Permian Basin (West Texas). The incremental stress rheology is found to be a powerful formalism for integrating basin hydrology, diagenesis and mechanics.

A forward model of three‐dimensional fracture orientation and characteristics

A new forward modeling approach to simulate the extension/closure and orientation statistics of evolving fracture networks is presented. The model is fully dynamical and couples fracturing to other processes. Thus, fracturing affects hydrology and, in turn, its development is affected by fluid pressure. In this way, highly pressured fluids can enhance their own migration while low-pressured ones may become trapped. Fracturing affects the stress tensor through rock volumetric changes and stress affects fracture dynamics. Thus, to predict fracture dynamics, we coevolve a set of fracture variables simultaneously with mass, momentum, and energy conservation equations for the solid and multiple fluid phases. In our computational approach, a representative set of putative fractures of a range of orientations is introduced. The time-dependent properties of each realized fracture is calculated by using the stress component normal to its fracture plane, pressure, and rock properties. The volumetric strain caused by fracturing allows for a self-limiting feedback that we account for using a nonlinear incremental stress rheology. The anisotropic fracture permeability is obtained using the predicted fracture network statistics. Thus, the coupling between rock deformation, notably fracturing, and hydrology is accounted for as is that between fracturing and stress. The statistical aspect of fracture network dynamics is described by assuming a probability distribution characterizing variations in rock strength within a nominally uniform lithology. The dependence of fracture density and length on the rate of fluid pressure or stress variation is thereby captured. Embedding the model in a three-dimensional basin finite element simulator, we illustrate the dynamical nature of the location and character of fracture zones in a sedimentary basin.

A Reaction-Transport-Mechanical Approach to Modeling the Interrelationships Among Gas Generation, Overpressuring, and Fracturing: Implications for the Upper Cretaceous Natural Gas Reservoirs of the Piceance Basin, Colorado

Predicting reservoir characteristics in tight-gas sandstone reservoirs, such as those of the Upper Cretaceous units of the Piceance basin, is difficult due to the interactions of multiple processes acting on sediments during basin development. To better understand the dynamics of these systems, a forward numerical model, which accounts for compaction, fracturing, hydrocarbon generation, and multiphase flow (BasinRTM) is used in a one-dimensional simulation of the U.S. Department of Energys Multiwell Experiment (MWX) site in the Piceance basin. Of particular interest is the effect of gas generation on the dynamics of the system.

Cellular and oscillatory self-induced methane migration

Abstract Methane genesis from kerogen maturation can induce kilometer-scale flow in a porous medium due to the fluid mass density dependence on methane concentration. The decrease in density with increasing methane content in solution may destabilize a water column, and initiate convective overturns. Conditions favoring these buoyancy-driven flows include a narrow thermal-depth window for methanogenesis and a large initial organic matter content. The self-organizing aspects of methanogenesis-induced flow are explored via the numerical simulation of reaction-transport equations. Cellular convection is demonstrated, and under certain conditions, the flow velocities are found to oscillate in time. For systems with initially random kerogen distribution the convection cells organize into one of a discrete set of well ordered convection cell patterns despite the initial disorder. Simulations initialized with heterogeneous porosity and permeability show that varying sizes and patterns of convection flows may develop which reflect the interplay between the inherent tendency toward self-organization and the nonuniformities in hydraulic properties.

WRIS.TEQ: multi-mineralic water-rock interaction, mass-transfer and textural dynamics simulator

Water-rock interactions in sediments are driven by the state of disequilibrium that persists among solids and solutes due to changing temperature and stress conditions, and advective and diffusive influx and efflux of solutes. Water-rock interactions bring about changes to sediment composition and texture through a complex chemical reaction network. These reactions can be divided into two types: solid-solute and solute-solute.Reactions of solids and solute are kinetic, i.e., they depend on compositions of solids and water, temperature, pore water pressure, and stress. Speciation among solutes are described by thermodynamic relations that depend on water composition and temperature. Both reaction mechanisms, mediated by pore water, are strongly interdependent.WRIS.TEQ is a comprehensive Reaction-Transport-Mechanical (RTM) simulator that accounts for multi-mineralic water-rock interaction mechanisms of kinetics and thermodynamics, and mass transfer due to advection and diffusion. Moreover, the simulators dynamic compositional and textural model based on a composite-media approach allows self-consistent evolution of sediment composition and texture due to water-rock interactions. Thus, the program can be used to make reliable predictions of sediment alteration due to water-rock interactions at the level it was previously not possible.This article describes the fundamentals of water-rock interaction and composite medium models used in the simulator WRIS.TEQ, and how the program is constructed. The utility of the program is demonstrated by simulated diagenetic alteration of sediments composed of complex mineralogy and heterogeneity.

3D fracture network dynamics in reservoirs, faults and salt tectonic systems

Abstract A unique 3D computer simulator is used to predict natural fracture network characteristics in the subsurface. The model is based on the numerical solution of rock deformation processes coupled to the myriad of other basin reaction, transport and mechanical (RTM) processes. The model integrates seismic, well log and surface geological data to arrive at a quantitative picture of the distribution of fractures, stress, petroleum and porosity, grain size and other textural information. An important component of the model is an incremental stress rheology that accounts for poroelasticity, non-linear viscosity with yield/faulting, pressure solution and fracturing. It couples mechanics to multi-phase flow and diagenesis (through their influence on effective stress and rock rheological properties, respectively). The model is fully 3D in terms of the full range of fracture orientations and the tensorial nature of stress, deformation and permeability. All rock properties (rheologic, multi-phase fluid transport, grain shape, etc.) are coevolved with the other variables. Examples illustrate the relative importance of various overpressuring mechanisms, lithology and flexure on the location and characteristics of a fracture network.