Polymer Replacement for Organophilic Clay Benefits Mineral-Oil-Based Drilling Fluids
Authors: Wilson Adam 
Published: October 2018

This article, written by Special Publications Editor Adam Wilson, contains highlights of paper SPE 191776, “Successful Replacement of Conventional Organophilic Clay With Novel Polymer as Viscosifier and Filtration-Control Agent in Mineral-Oil-Based Drilling Fluids,” by Jing Zhou, SPE, Jairo Cortes, SPE, and Hisham Nasr-El-Din, SPE, Texas A&M University, prepared for the 2018 SPE Liquids-Rich Basins Conference—North America, Midland, Texas, USA, 5–6 September. The paper has not been peer reviewed.
Organophilic clays mixed in oil-based drilling fluids (OBDFs) do not exhibit the same viscosity or suspension characteristics as they do in water-based drilling fluids because electrical interaction between particles is minimal, creating difficulty in building viscosity and gel strength under high-pressure and high-temperature (HP/HT) conditions. To address that problem, a new mineral-oil-based drilling fluid (MOBDF) was created by replacing the conventional organophilic clay with a novel polymer.
Introduction
Organophilic clays are widely used as primary viscosifiers for OBDFs and synthetic-based drilling fluids. These chemicals are prone to stratification in certain conditions, slow chemical-reaction times, high pressure spikes, and high equivalent circulating densities (ECDs). These mud systems also have temperature limitations. At high temperatures, thermal thinning and thermal degradation of their organophilic colloids permanently destroy their viscosification capabilities. At low temperatures, excessive and rapidly increasing viscosity is experienced.
A need, therefore, exists for a fluid that produces higher gel strengths initially and maintains a relatively flat response to temperature throughout the complete temperature cycle of a drilling operation. Organophilic clays mixed in OBDFs do not exhibit the same viscosity or suspension characteristics as they do in water-based drilling fluids because electrical interaction between particles is minimal, creating difficulty in building viscosity and gel strength under HP/HT conditions.
By switching from a standard OBDF containing organophilic-clay additives to a clay-free OBDF, the drilling operation will benefit from a drilling fluid capable of performing under the desired conditions planned, an overall reduction in logistics, a reduction in volume of fluid and material needed, higher rates of penetration because of lower ECDs, and the formation of a thin nondamaging filter cake that would eliminate the need for costly and time-consuming cleanup treatments.
This paper studies the feasibility of developing an organophilic-clay-free drilling fluid capable of performing similarly to an already-tested drilling fluid containing organophilic-clay additives.
Experimental Studies
Two MOBDF formulas were used for the experiments. The first, MOBDF A, was prepared with a formula containing organophilic-clay additives. The second, a clay-free drilling fluid, MOBDF B, was prepared with a formula containing the replacement polymer viscosifier and filtration-control agent.
Calcium carbonate was chosen as the weighting material for the two MOBDFs used for the analysis.
High-permeability Indiana limestone cores with a thickness of 0.25 in. and a diameter of 2.5 in. were used to perform the filter-cake-formation and filtration analysis.
Berea sandstone cores with a thickness of 6 in. and a diameter of 2.5 in. were used to perform the rock-wettability and emulsion-breakdown analysis.
Both MOBDFs were heated to 150°F for 16 hours under hot rolling, and no phase separation was observed.