Stress- and Chemistry-Mediated Permeability Enhancement/Degradation in Stimulated Critically-Stressed Fractures DE-FG36-04GO14289, M001 October 1, 2004 – December 31, 2007 (3 years) Derek Elsworth, Penn State University, PI Avrami Grader (EGEE, PSU) Chris Marone (Geosciences, PSU) Phillip Halleck (EGEE, PSU), & Peter Rose, EGI, University of Utah

Towards the engineering of “EGS”: –Long-lived –Low-impedance –High heat flow Consistent understanding of the evolution of flow connections resulting from stimulation –Physical (effective stresses) –Chemical (dissolution/precipitation) Critical influences of: –Mechanical Influences [THM] –Chemical Influences [THC] Importance where fractures are “critically stressed” Resolve anomalous observations Purpose THMC

Objectives “… a consistent view of the thermal, hydraulic, mechanical, and chemical processes that influence permeability enhancement….and to be able to apply these principles to EGS reservoir development.” Hydro-MechanicalHydro-Chemical Constitutive Models Modeling/ Upscaling

Plan and Approach Hydro-MechanicalHydro-Chemical Constitutive Models Modeling/ Upscaling

Hydro-Chemical Reactor - Experimental Arrangement

Q mass X-ray CT Q fluid =Constant Apparatus

Typical Response [Polak et al., GRL, 2003]

Experiment Matrix

Hydro-Mechanical Reactor

Slide-Hold-Slide Friction Experiments Hold periods of 30 – 10 4 [sec]Hold periods of 30 – 10 4 degree-C, peak coefficient is independent of hold degree-C, peak value increases with increase of hold period

Experiment Matrix – Similar to Hydro-Chemical Reactor

Results 1.Hydro-Chemical Reactor 2.Hydro-Mechanical Reactor 3.Modeling

Fractured Limestone – Features of Response (predate this project) 0 hr 1462 hr 0 hr1462 hr

1. Hydro-Chemical Reactor

Coso core at 646 ft

Typical slice

Thresholded three- dimensional image of the fracture

Three- dimensional image of the large openings of the fracture

Three- dimensional combined image of the large aperture openings and the linking smaller apertures within the core.

2. Hydro-Mechanical Reactor

Goal: Construct a numerical model to simulate permeability enhancement caused by hydraulic and chemical stimulation – ultimately apply to stimulation at Coso FLAC3D → [High Peclet Number Flows] → ToughReact → Fist step; Focus on a behavior of a single fracture ─ Mass transport within a fracture ─ Solve an advection-dispersion equation, complete with a reactive term ─ FEM, FDM ─ Accommodate a problem with high Peclet number (advection dominant) 3. Transport and Mechanical Modeling

Advection-dispersion equation with high Peclet number A Lagrangian-Eulerian Method Continuous injection

Replicate experimental result (Nova II) 1. Set initial aperture distribution 2. Apply I.C. and B.C. → Obtain velocity distr. in a fracture by solving Reynolds’ equation 3. Dissolution at contact area and free-face (reaction) → Obtain concentration distribution + Modify aperture distribution due to dissolution 4. Lagrangian-Eulerian method (Advection-diffusion) → Obtain concentration distribution within and out of domain Iteration

Replicate experimental result (Nova II) Numerical model is capable of replicating experiment though prescribed multiplier for dissolution rate constant is relatively large. Another mechanism instead of pressure solution may be active (mechanical creep?).

Replicate experimental result (Nova II) CT image Model prediction The model cannot perfectly represent experiment, but predict changes in aperture and contact area distribution with time

Impact/Merit Project recently initiated Providing meager data/information that are not well understood, and linking with improved understanding –Stress- and chemistry-mediated influences are potent –High temperatures where few data exist –Current understanding lacking Linkages and Dissemination –Closely tied to EGI parallel study incl. personnel transfer –Potential isotopic linkages for heat-flow areas (E. Sonnenthal) Products –Elsworth, D., and Yasuhara, H. (2005) Short timescale chemo- mechanical effects and their influence on the transport properties of fractured rock. Submitted for publication. Earth and Planetary Research Letters. 40 pp. –GRC Meeting September

Completion