Oliver Gaede

Improved estimates of percolation and anisotropic permeability from 3-D X-ray microtomography using stochastic analyses and visualization.

X-ray microtomography (micro-CT) with micron resolution enables new ways of characterizing microstructures and opens pathways for forward calculations of multiscale rock properties. A quantitative characterization of the microstructure is the first step in this challenge. We developed a new approach to extract scale-dependent characteristics of porosity, percolation, and anisotropic permeability from 3-D microstructural models of rocks. The Hoshen-Kopelman algorithm of percolation theory is employed for a standard percolation analysis. The anisotropy of permeability is calculated by means of the star volume distribution approach. The local porosity distribution and local percolation probability are obtained by using the local porosity theory. Additionally, the local anisotropy distribution is defined and analyzed through two empirical probability density functions, the isotropy index and the elongation index. For such a high-resolution data set, the typical data sizes of the CT images are on the order of gigabytes to tens of gigabytes; thus an extremely large number of calculations are required. To resolve this large memory problem parallelization in OpenMP was used to optimally harness the shared memory infrastructure on cache coherent Non-Uniform Memory Access architecture machines such as the iVEC SGI Altix 3700Bx2 Supercomputer. We see adequate visualization of the results as an important element in this first pioneering study.

Deep geothermal: The ‘Moon Landing’ mission in the unconventional energy and minerals space

Deep geothermal from the hot crystalline basement has remained an unsolved frontier for the geothermal industry for the past 30 years. This poses the challenge for developing a new unconventional geomechanics approach to stimulate such reservoirs. While a number of new unconventional brittle techniques are still available to improve stimulation on short time scales, the astonishing richness of failure modes of longer time scales in hot rocks has so far been overlooked. These failure modes represent a series of microscopic processes: brittle microfracturing prevails at low temperatures and fairly high deviatoric stresses, while upon increasing temperature and decreasing applied stress or longer time scales, the failure modes switch to transgranular and intergranular creep fractures. Accordingly, fluids play an active role and create their own pathways through facilitating shear localization by a process of time-dependent dissolution and precipitation creep, rather than being a passive constituent by simply following brittle fractures that are generated inside a shear zone caused by other localization mechanisms. We lay out a new theoretical approach for the design of new strategies to utilize, enhance and maintain the natural permeability in the deeper and hotter domain of geothermal reservoirs. The advantage of the approach is that, rather than engineering an entirely new EGS reservoir, we acknowledge a suite of creep-assisted geological processes that are driven by the current tectonic stress field. Such processes are particularly supported by higher temperatures potentially allowing in the future to target commercially viable combinations of temperatures and flow rates.

First Steps Towards Modeling a Multi-Scale Earth System

Recent advances in computational geodynamics are applied to explore the link between Earth’s heat, its chemistry and its mechanical behavior. Computational thermal-mechanical solutions are now allowing us to understand Earth patterns by solving the basic physics of heat transfer. This approach is currently used to solve basic convection patterns of terrestrial planets. Applying the same methodology to smaller scales delivers promising similarities between observed and predicted structures which are often the site of mineral deposits. The new approach involves a fully coupled solution to the energy, momentum and continuity equations of the system at all scales, allowing the prediction of fractures, shear zones and other typical geological patterns out of a randomly perturbed initial state. The results of this approach are linking a global geodynamic mechanical framework over regional-scale mineral deposits down to the underlying micro-scale processes. Ongoing work includes the challenge of incorporating chemistry into the formulation.

Entropic Bounds for Multi-Scale and Multi-Physics Coupling in Earth Sciences

The ability to understand and predict how thermal, hydrological,mechanical and chemical (THMC) processes interact is fundamental to many research initiatives and industrial applications. We present (1) a new Thermal– Hydrological–Mechanical–Chemical (THMC) coupling formulation, based on non-equilibrium thermodynamics; (2) show how THMC feedback is incorporated in the thermodynamic approach; (3) suggest a unifying thermodynamic framework for multi-scaling; and (4) formulate a new rationale for assessing upper and lower bounds of dissipation for THMC processes. The technique is based on deducing time and length scales suitable for separating processes using a macroscopic finite time thermodynamic approach. We show that if the time and length scales are suitably chosen, the calculation of entropic bounds can be used to describe three different types of material and process uncertainties: geometric uncertainties,stemming from the microstructure; process uncertainty, stemming from the correct derivation of the constitutive behavior; and uncertainties in time evolution, stemming from the path dependence of the time integration of the irreversible entropy production. Although the approach is specifically formulated here for THMC coupling we suggest that it has a much broader applicability. In a general sense it consists of finding the entropic bounds of the dissipation defined by the product of thermodynamic force times thermodynamic flux which in material sciences corresponds to generalized stress and generalized strain rates, respectively.

Using Time-Dependent Borehole Failure to Understand Diffusion-Driven Weakening and Strain in Reservoir Rocks and Seals

Understanding the coupling between thermo-hydro-mechanical-chemical (THMC) processes in reservoir rocks and seals will enhance our ability to characterize and predict reservoir behaviour. The coupling between these processes inside a reservoir occurs at various length- and time-scales. However, the coupling is not equally tight at every scale and depends on lithology as well. At the borehole-scale coupled THMC processes manifest as time-dependent borehole failure. Although intrinsic mechanical processes (i.e. creep) can explain some time-dependent failure, it is widely recognised that pressure, temperature and chemical gradients introduced by the drilling mud into the formation play an important role. In these instances equilibration of the gradients is achieved via a diffusion process and the coupling to the mechanical behaviour of the rock either results from a strain (i.e. “swelling” and “shrinking”) or a change in rock strength. The problem is that the exact interaction between these thermodynamic gradients and their associated mechanical failure processes are not well known, because (a) the impact of thermodynamic gradients is not routinely considered in rock mechanical testing and (b) repeat or “time-lapse” runs of borehole imaging tools over the same formation interval are rare. In this contribution we present a fully coupled material model that allows for diffusion-driven weakening and strain. The material model is based on non-equilibrium thermodynamics and the material weakening is achieved via a damage tensor. This material model is implemented into a Finite Element simulation of near-well bore processes. Apart from improving borehole stability predictions (the forward problem), such numerical simulations enable us to study the influence of the individual coupling parameters. We therefore conducted a parameter study that explores the feasibility of using in-situ observations (e.g. time-dependent widening of borehole breakouts in repeat image logs) for the inversion of THMC coupling parameters. We will show that a complete inversion for the most general formulation is not possible due to non-uniqueness of the problem. However we can constrain the type and tightness of coupling mechanisms and thereby improve modelling of wellbore stability.

Microstructural Influences on the Stress Sensitivity of Crystalline and Sedimentary Rocks

P002 Microstructural Influences on the Stress Sensitivity of Crystalline and Sedimentary Rocks O. Gaede* (Freie Universitaet Berlin) S. Shapiro (Freie Universitaet Berlin) D. Dewhurst (CSIRO Petroleum) S. Stanchits (GeoForschungsZentrum Potsdam) & A. Siggins (CSIRO Petroleum) SUMMARY Understanding the stress dependence of elastic rock properties is an essential tool for various applications in the earth sciences and the resource industries. The stress sensitivity approach presented here derives the nonlinear stress-strain relationship for a porous medium by considering the stress induced changes of the pore geometry. We compare the stress dependence of ultrasonic velocity measurements for three rock samples that were specifically