Applying The Pressure Derivative Method To Identify Geothermal Well Responses

Abstract

The pressure derivative method is one of the most significant developments in the history of pressure transient analysis (PTA). PTA results that would otherwise be difficult to interpret can have a clear and characteristic pressure derivative. The application of this technique in groundwater, coalbed methane, mineral exploration and oil and gas wells has been widespread. However their application in the well test analysis of geothermal wells is still very limited. The usefulness of the pressure derivative technique is demonstrated in this study. Despite the challenges of geothermal wells it remains a key diagnostic tool to determine the processes taking place during the test and hence the models which might be applicable. Two common issues with applying PTA to geothermal wells are 1) downflows and 2) non-zero flow early in the pressure falloff (PFO) test. Downflows of fluid from a higher permeable zone to a deeper one of lower pressure can occur during the test. Non-zero flow can occur due to the delay inevitable when manually closing a master valve. For each issue of downflow and early non-zero flow some real examples are given. The characteristic anomalies these effects produce in a derivative plot are presented along with a discussion of the extent to which these issues affect the results obtained by treating these as standard analyses.