Abstract:
Pressure transient analysis (PTA) of geothermal wells requires numerical models rather than conventional analytical models due to high temperatures and reservoir complexity. For this reason a framework has been recently developed for using numerical simulation for PTA, utilising the TOUGH2 simulator and automated with the PyTOUGH scripting code. The basic radial grid design has the flexibility to include reservoir boundaries by modification of the block volumes and surface areas. In this case study a channel boundary is implemented as two linear impermeable boundaries. The model is then matched to field data from an injection-falloff test in a New Zealand geothermal well. This inverse modelling process utilises software for parameter estimation and uncertainty analysis (PEST). A good match indicates that Well-X lies within a reservoir channel of approximately 130m width. In the process the numerical PTA framework is demonstrated as a viable alternative to analytical PTA. Conventional analytical PTA is included for comparison, utilising the software SAPHIRTM. Both numerical and analytical models require weighting of the intermediate- and late-time field data in order for the model to match during this period. The numerical and analytical model matches appear very similar in the derivative plot and history plot, however the estimated parameters are different. The numerical results are more reliable as they account for the injectate temperature effect.