A Users Guide to Leak-off Test Procedures and Interpretation for Geothermal Wells

Abstract

Leak-off Tests (LOT) and Extended Leak-off Tests (XLOT) are conducted during drilling to verify the competence of the cement around a casing shoe and determine the maximum allowable mud weight for the next hole section. (X)LOT interpretation yields the fracture gradient, which is a key parameter in well control procedures, mud program design and cement operations. These data are also one of the minimum requirements for any geomechanical study that seeks to understand the role fractures play in reservoir permeability. Despite its utility for successful well completion and studies investigating reservoir permeability, (X)LOT procedures are often poorly implemented and the interpretation methods used are not always appropriate for geothermal conditions. Using a case study, we describe the test procedures and interpretation process for typical geothermal conditions. We address key issues that set geothermal apart from oil and gas, such as the impact of high temperatures on test results, the high frequency of naturally occurring fractures, and the variable physical properties of hydrothermally altered volcanic rocks. We illustrate the impact of quality checking an existing (X)LOT dataset and review methods for estimating the fracture gradient prior to drilling in extensional environments. While our paper addresses a number of detailed technical issues related to interpretation, it aims to be a pragmatic and geothermally-relevant guide for those who aspire to improve the quality of these tests.