Rock Typing In Geothermal Reservoirs, Challenging The Complexity

Abstract

In order to simulate the behaviour of a geothermal system for field management purposes, a numerical model is initialised based on conceptual models that capture the initial state of the reservoir, integrating the current knowledge of the system and its dynamics. Geothermal reservoirs are dynamic systems, where continuous fluid and heat flow affects the reservoir chemical and stress equilibria leading to precipitation or dissolution of minerals and changes in the pore geometry over the lifetime of geothermal activity. The speed and intensity of these changes depend on the rock’s capacity to store and transfer fluids coupled with the physicochemical properties of the fluids and the pressure and temperature of the system. The rocks resulting from these processes are characterized by a wide range of petrophysical properties, which are seldom represented by traditional classification of rocks based on individual geological parameters of genesis, lithology or composition. As a consequence, translating these properties into quantitative inputs for numerical models remains a challenge. This paper presents the conventional approach to petrophysical characterisation of geothermal reservoirs in New Zealand, and proposes the use of textural descriptors as part of a rock typing technique aimed at facilitating the quantification and use of measured petrophysical properties in the reservoir modelling workflow.