Development and Evaluation of Various Drilling Fluids for Slim Hole Wells
Authors: Enilari M.G. , Osisanya S.O. , Ayeni K.B. 
Published: March 2010

Abstract
Slim hole drilling has gained wide acceptance, however, it presents a new set of challenges and problems. One such challenge is reducing annular pressure losses (APLs). Due to the reduced annular clearance in slim hole wells, the APL is usually high, translating to a high equivalent circulating density which can lead to fracturing the formation and lost circulation. Fifty-three fluids were evaluated for their APLs using three different wellbore configurations under atmospheric conditions and at 150?F. Each fluid was characterized using the Bingham Plastic and Power Law rheological models. It was observed that the partially hydrolyzed polyacrylamide (PHPA) viscosified brines gave the lowest frictional pressure losses of all fluids tested. However, the xanthan viscosified brines exhibited better carrying capacity due to their high viscosities. As expected, APL gradients were reduced at high temperatures due to reduced viscosities which led to reduced carrying capacities of the fluids.

Introduction
Drilling cost optimization has always been, and will continue to be, the most important issue in the petroleum drilling industry. Using slim hole technology is one of the sure ways to achieve substantial cost savings. The basic difference between conventional wells and slim hole wells is the wellbore geometry. While the hole diameter of the production interval in a conventional well ranges from 6? in to 95/8 in, that of a slim hole well ranges from 3? in to less than 6 in. Recent advances(1–3) in slim hole technology include the development of drilling fluids to reduce the APLs because 60% of the pressure loss occurs in the annulus of slim hole wells(1).
Some developments in slim hole drilling fluids design have included the works of Downs et al.(2) They formulated formate brine-based fluid with xanthan gum as a viscosifier. Randolph et al.(3) developed a unique weighting agent for slim hole drilling. In general, low solids-fluids are used to prevent the plating out of solids inside the drill string, and the fluids should have low viscosity to avoid an excessively large equivalent circulating density (ECD). These systems are very costly, thereby reducing the cost savings significantly. The purpose of this study was to develop a low-cost, low viscosity fluid that will reduce the annular pressure loss significantly while maintaining a gauged hole.

Study Methodology
his study involved the development of several fluids and their rheological characterizations with Bingham Plastic and Power Law models. The data were used for calculating APL gradients for three slim hole geometries and carrying capacities of the fluids. The fluids developed were water-based because they are cheap, environmentally accepted and they require little effort when detecting gas kicks. Two main groups of fluids were tested: bentonite (clay)-based fluids and viscosified brines. Salts were added to the fluids for wellbore stability. The two types of salts used in this study are potassium chloride (KCl) and potassium formate (KF). Xanthan and PHPA (partially hydrolyzed polyacrylamide) polymer were used as viscosifiers in the brines in order to investigate their effects on annular pressure losses.