ABSTRACT

Incompatibilities between drilling fluid filtrates and formation connate fluids are believed to cause changes in rock wettability and hence impair production for a given reservoir rock.

This paper summarizes test results and analysis of an experimental study to systematically assess the effect of incompatibilities on rock wettability and in turn on its permeability. The rocks considered are dolomite, limestone and sandstone. The parameters that were studied are the ph, monovalent to divalent cation ratio, total cation concentration (salinity) and the drilling fluid surfactants. The Dynamic Wilhelmy Plate Technique was used in the measurement of wettability changes. The analysis of test data was conducted using a defined deviation number that allowed quantitative evaluation of the various effects that were considered in the study. Results have shown that the rnost significant in rock wettability changes are the surfactants in the drilling fluids.

Dolomite rock had the maximum reduction in water wetness when surfactants are absent, while the reduction is about the same for all rocks when surfactants are present. Also, among all five water-base muds studied, the calcium treated gypsum mud had the least water wetness.

INTRODUCTION

In terms of ionic activities the formation fluids and clays are in a state of equilibrium. However when an external fluid such as drilling mud invades the formation, this state of equilibrium may be disturbed depending on contents and properties of drilling fluid. This happens because of the ionic incompatibilities between invading fluids and formation fluids. The main parameters causing incompatibilities are pH, ionic valencies, salinity and presence of surfactants. The incompatibility can alter the permeability mainly through its effects on wettability and, clay swelling and migration. The reduction of permeability near the wellbore restricts the oil production. This paper addresses the issue of rock property alterations due to the interactions of water-base drilling fluid filtrates with formation rocks. The main focus is on wettability alteration.

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