Geocrack2D

The Geocrack2D program solves the fully coupled fluid flow/heat transfer/rock deformation problem, including discrete fractures.  This means that the fracture opening is a function of fluid pressure, nonlinear contact stiffness, and local deformation of the rock due to elastic stresses and thermal contraction.

The objective of this work is to provide a tool that can be used in the design and operation of fractured reservoirs. Some of the specific questions that need to be addressed include:

• The lifetime performance of the reservoir, including thermal drawdown and the change in reservoir permeability during operation.
• Design of the reservoir, including the number or wells and spacing of wells.
• Optimum operation of the reservoir, including the goal of improving heat mining efficiency and reducing the potential for short circuiting.

It is, of course, impossible to exactly represent a reservoir in a model.  We do not even know the geometry two miles below the surface.  However, there is considerable indirect data (i.e.: micro-seismic data, surface pressures and flow rates under steady state and transient operation, and tracer data) that provides a benchmark for analyses.  Although greatly simplified, we believe our analyses have provided useful insights into the reservoirs we have worked on.

Graduate students have included Brian Hardeman, Tim Sprecker, Mel Beikmann, Rick Martineau, Mark James, and Shun-Lung Su.  Bob DuTeaux significantly guided the development as he used the code to simulate data he was taking during operation of experiments at Fenton Hill.

A short description of our analyses is given below.

Finite elements (11 KB file) are used to develop the model. The analysis is a fully coupled structure/fluid/heat transfer model. A formulation was used where the structure and fluid equations are solved simultaneously, introducing coupling terms between the fluid and structure models that speed convergence. The coupling between fluid/structure and heat transfer models is less strong, so a staggered-step solution method is used to reach a solution.

Flow paths (14 KB file) are predicted as part of the analysis. This shows a typical flow distribution in an analysis.

Temperatures (138 KB file) in the reservoir can be calculated to find the lifetime thermal performance of a reservoir. This shows the temperatures as they change during 20 years of operation (animated gif).

Displacements (65 KB file) due to thermal contraction are also predicted (animated gif).

Some of the conclusions of the study are:

• Thermal reservoir lifetimes of 20 years or more are possible at commercial energy production rates.
• Reservoir permeability increases with heat production.
• Spacing of wells can be increased without significantly increasing pumping costs.
• Flow rates should be controlled to match heat removal rate to rate of heat conduction.