Pressure Transient Analysis of Geothermal Wells: a Numerical Modelling Approach

Abstract

Geothermal wells are the lifeblood of a geothermal development, warranting extensive testing and monitoring. The general term well testing is used in relation to a wide array of testing programmes and techniques, including pressure transient analysis (PTA), a technique which is under-utilised by the geothermal industry due to the fact that conventional PTA analytical models often cannot match eld datasets from real geothermal wells, or give results that do not make sense. This is primarily due to the high temperatures and two-phase conditions of geothermal systems. These can only be adequately represented by numerical modelling, however there has been a lack of numerical tools or software for geothermal PTA. This research divides naturally into two parts: practical issues related to collection of geothermal PTA eld datasets, and numerical techniques for analysis of those datasets. The majority of the practical issues can be avoided or minimised with careful test design and adequate data logging, and a set of recommendations forms part of this thesis. A core component of this research is the numerical PTA framework, a range of guidelines for setting up the numerical simulations. It has been implemented in Python, using PyTOUGH to control the numerical simulator TOUGH2, though it is applicable to other simulators. It has been demonstrated on a wide variety of geothermal eld PTA datasets, proving superior to analytical models. It can account for a variety of e ects, including non-isothermal conditions during injection, and can model well behaviours beyond the capability of conventional analytical models, including sudden changes in permeability during the pressure transient. The numerical PTA framework has been applied to a variety of eld datasets from geothermal wells, including assessment of stimulation due to de agration and thermal stimulation during injection. A range of reservoir-scale structural features have been identi ed, including a single linear impermeable boundary, channel boundaries of various widths, and reservoirs with di erent fractional dimensions. The reservoir-scale structural features can be highly signi cant to the conceptual model of a eld, and this is also explored.