Methylene Blue Adsorption: a Real Time Rig Geologist Tool for Estimating Geothermal Reservoir Temperatures and Forecasting Drillhole Stability Problems

Abstract

Smectite and smectite-illite clays are the predominant rock alteration products that form in the 50" to 200?C zone of most high temperature volcanic-hosted, neutral pH geothermal systems. Methylene Blue is an organic dye which shows a high selectivity for adsorption by smectitic (swelling) clays in the interlayer site. The dye is also adsorbed by the smectite component of mixed-layer clays such as illite-smectites and chlorite-smectites. This selectivity permits it to be used to estimate, on a semi-quantitative basis, the amount of smectite and/or mixedlayer clays in alteration mineral assemblages in hydrothermal systems.

The technique is rapid, requires very little sample and can readily be carried out by the rig geologist, as drilling proceeds, under relatively unsophisticated field conditions. The method cannot be considered sufficiently accurate to replace the more traditional analytical techniques of X-ray deaction studies. Its relevance is that immediate information can be obtained on the clay mineral composition and likely reservoir temperatures, since X-ray dieaction studies carried out in central laboratories may not be completed until much later after the well has been completed and perhaps discharged. It has been successfully applied in several geothermal fields in Indonesia.

In high clay content formations, routine methylene blue analysis may also immediately identifj high clay zones which may cause wall collapse and blockages. In addition, if the rig geologist regularly analyses cuttings by methylene blue, good correlations have been found between methylene blue adsorption and resistivity data.