A New Approach to Oil-Base Muds for Lower-Cost Drilling
Authors: Simpson Jay P. 
Published: December 2007

Oil-base muds now are being used for borehole stability and corrosion protection. They are achieving drilling rates equivalent to good water-base protection. They are achieving drilling rates equivalent to good water-base muds. This paper describes the laboratory studies and field development of the low-colloid oil-base mud system. Field examples are presented to show savings in drilling costs of about $200,000 per well.

Introduction
Field performance throughout the world has proven the effectiveness of oil-base muds in combating drilling problems caused by shale hydration, differential pressure problems caused by shale hydration, differential pressure sticking, corrosion, and high formation temperatures. Oil-base muds also have served to protect against damage to productive formations. In the past, however, the advantage of this superior performance often has been offset by high drilling costs caused by slow drilling rates - especially when using oil-base muds for drilling carbonate rock.When considering factors that might affect drilling rate, there are two aspects of oil-base muds that are probably significant. Generally, oil-base muds have had probably significant. Generally, oil-base muds have had higher viscosities than water-base muds of equal density, and oil-base muds have exhibited extremely low filtration rates. There is a reason viscosities would tend to be higher, but the low filtrates may have been mostly a matter of tradition.High viscosity in an oil-base mud usually can be related to attempts to improve hole cleaning (lifting capacity) or to suspend weighting material. In oil-base muds, asphaltic materials are relied on to increase viscosity both for suspending solids and for lowering filtration. "Invert emulsion" oil-base muds tend to rely on a relatively high concentration of emulsified water for these purposes. Either a highly viscous asphaltic oil phase or a high concentration of emulsified water can phase or a high concentration of emulsified water can retard significantly the rate of penetration in carbonate rock, as shown in the laboratory drilling rate studies below. For faster drilling, an oil-base mud system must be versatile enough to provide hole cleaning and suspension of weighting material, while retaining a low-viscosity oil phase and low water content. Attention then can be given to selection of filtration properties for optimum performance. The following discussion describes the laboratory and field development of such a system.

Laboratory Studies of Drilling Rate
Drilling rates for various fluids were studied using laboratory equipment designed to simulate downhole conditions. The equipment has been described elsewhere. The primary components consist of a drilling chamber, heater, rotary drive, and a circulating pump. Auxiliary pumps supply pressure boosts for the pump. Auxiliary pumps supply pressure boosts for the drilling fluid, terrastatic, and formation pressures. All data reported here were obtained under the following conditions:
Drilling fluid pressure, psi (kPa) 5,000 (34 500)Formation pressure, psi (kPa) 4,000 (28 000)Terrastatic pressure, psi (kPa) 5,000 (34 500)Weight on bit, lbf (kg) 1,000 (454)Rotary speed, rpm 60Flow rate, gal/min (m /h)Rock bit 7 (1.6)Diamond bit 10 (2.3)Drilling fluid temperature, deg. F (deg. C) 150 to 200 (66 to 93)Rock type Leuders limestone
JPT
P. 643