Abstract

An objective of slim-hole drilling for exploration is to use the smallest hole size that permits the acquisition of sufficient information to evaluate the potential of a prospect. Slim-hole wells are often continuously cored, and this core information is used to generate a geological and petrophysical description of the prospect.

Wireline well-logs are used to supplement this information in several ways. Their main advantages are that they generate a continuous record of formation parameters, such as porosity, rather more easily and cheaply than core analysis. Continuous records are important for well-to-well correlation, and for formation evaluation if the prospect proves commercial.

We describe the evolution of a well-logging programme adapted to slim-hole drilling in Sumatra. Originally sonic transit time logs were used to estimate porosity and electrical logs (short and long normals and spontaneous potential) to estimate formation resistivity and hence water and hydrocarbon saturations. Since formation waters are of low and variable salinity and saline (potassium chloride) drilling fluids are favoured to control well problems, this logging suit is not ideal. Results, comparable with conventional logging tools, can be obtained with slim-hole variants of density and neutron logging tools and focused deep resistivity devices.