EXPERIMENTS WITH WAIWERA, A NEW GEOTHERMAL SIMULATOR

Abstract

In a companion paper we discuss the development of our new geothermal simulator, called Waiwera, and present results of tests on benchmark problems. In this paper we discuss the application of Waiwera to natural state simulations with models of Ngawha (New Zealand) and System B. These two models were chosen because they both involve mixtures of water and a non-condensible gas (carbon dioxide for Ngawha and air for System B) and they both have large contrasts in permeability. As a result the models run badly with a standard version of TOUGH2.
Recently we have made changes in Waiwera and AUTOUGH2 (our version of TOUGH2) to the way some aspects of two-phase flow are handled. We found that these changes greatly improved the performance of the Ngawha and System B models.
TOUGH2 and Waiwera solve the nonlinear mass and energy balance equations by using the Newton-Raphson (N-R) iterative procedure. At each N-R iteration a large system of linear equations has to be solved. Typically in a difficult natural state simulation as the time step increases the condition number of the linear equations degrades and the N-R process may struggle to converge. With Waiwera we have found, by experiment, that careful selection of linear solvers and preconditioners can greatly improve numerical performance. Results for the Ngawha and System B models using different linear solvers and preconditioners are discussed below.