Analysis of Smectite Clays in Geothermal Drill Cuttings by the Methylene Blue Method: For Well Site Geothermometry and Resistivity Sounding Correlation

Abstract

The simple, quick and inexpensive methylene-blue (MeB) dye technique long used in drilling mud analyses has been adapted to provide estimates of smectite clay content in geothermal well cuttings. Case histories from Unocalís Namora-I-Langit and Awibengkok geothermal fields in Indonesia illustrate applications of this new method to geothermal exploration and development. MeB smectite measurements can be used to constrain subsurface temperatures since smectite is unstable at temperatures over 70?C and mixed layer smectite-illite typically becomes undetectable in porous rocks at temperatures over 200?C. Therefore, many well casing decisions that depend on formation temperatures can be supported by well-site MeB estimates. Well-site smectite detection at formation temperatures over 200?C can also help characterize the unstable dehydrated smectite zones that are common drilling hazards in volcanic geothermal reservoirs. Since resistivity patterns are largely controlled by the smectite distribution over and adjacent to geothermal reservoirs, MeB results can facilitate revisions of conceptual models and well targeting plans that were based on resistivity surveys. The semi-quantitative MeB smectite estimation method is applicable to both geothermal well-site geology and detailed analysis of hydrothermal alteration patterns that may be missed by the coarser sampling of well cuttings typical of more precise and expensive approaches. Since it can be carried out by the wellsite geologist during drilling, it offers immediate (real time) information regarding potential drilling hazards, mineral geothermometer temperatures, and (at least locally) changes in permeability.