Abstract

The use of potassium hydroxide (KOH) lignosulfanate drilling muds is becoming popular in Indonesia for the control of shales and hydratable clays. This mud system has produced major improvements in hole gauge, decreased fill on trips, reduced caving and helped in the control of rheological properties in shaly sequences of different Indonesian basins. However, complications of log analysis and interpretation can occur with the use of this mud system.

Potassium hydroxide is significantly more chemically reactive and conductive than sodium chloride. Thus, any increase in the proportion of potassium cations in the mud filtrate creates a different electrochemical potential between formation fluid and mud filtrate.

The effect on wireline logs can be substantial. Most log interpretations assume the presence of sodium chloride as the predominant salt in the mud filtrate and formation water. Currently no commercial log analysis corrects for the use of potassium hydroxide-based muds. Thus significant errors may ensue if these corrections are not made.

A previously unpublished method for interpretation of Spontaneous Potential logs is presented. A further discussion of responses is also undertaken. Field comparisons of SP analysis using the new relationship with the published correlations are shown to produce large discrepancies in Rw determination when KOH-based drilling muds are used.