Oil muds offer many advantages over water based muds. The high initial cost of the oil based mud can be a factor in not selecting this type of mud system. However, if the overall drilling costs are considered, the costs accompanying the use of an oil mud are usually less than that for a water mud. Some of the applications of oil-based muds will be described below.
Shale Stability - Oil muds are most suited for drilling water sensitive shales. Formulated with the proper salinity, oil muds can prevent water movement from the mud into the shale. In some cases, water can actually be drawn from the shale and could result in strengthening. However, it is also possible to draw too much water from the shale (with too high a salinity) and cause a shale to be less stable. It is desirable to have enough salinity to prevent water migration into the shale but not to allow dehydration of the shale. This is the "balanced activity" concept. The required salinity is usually determined through field experience. Shale cores that have not been altered by the oil mud are necessary to accurately determine the salinity requirements.
Penetration Rates - Oil-mud formulations can usually drill faster than water muds and still provide excellent shale stability. Relaxed filtrate invert emulsions usually have a high oil to water content and some of the additives used to control fluid loss are omitted. These systems do not use the primary emulsifiers, which have been shown to reduce drilling rate, and they do not have the same temperature stability as conventional invert oil muds. The relaxed type oil muds are especially suited to drilling with PCD bits.
High Temperatures - Oil muds have the ability to drill formations where bottom hole temperatures exceed water base mud tolerances, especially in the presence of contaminants. Oil muds have been used at temperatures approaching 550°F. Oil muds can be formulated to withstand high temperatures over long periods of time, unlike water muds, which can break down and lead to loss of viscosity and fluid loss control, as well as corrosion.
Drilling Salts - Invert oil muds will provide gauge hole and do not leach out salt. The addition of salt to the water phase will prevent the salt from dissolving into the emulsified water phase. Water-based mud, even up to saturation and over saturation does not assure that the salts will not be leached out.
Coring Fluids - Special oil muds provide a native state coring fluid with minimum wettability changes. These fluids are usually water-free and thus require only a minimal content of emulsifiers. Oil mud emulsifiers are very strong oil-wetting agents and can cause oil-wetting of the formation. Oil-based coring fluids will not introduce any water into the core, so determination of water saturation can be more accurately determined.
Packer Fluids - Oil mud packer fluids are designed to be stable over long periods of time and when exposed to high temperatures. Oil muds provide long term stable packer fluids under conditions of high temperature since the additives are extremely temperature stable. Since oil is the continuous phase, corrosion is almost negligible compared to water muds under the same conditions. Properly formulated, oil mud packer fluids can suspend weighting material over long periods of time.
Lubricity - The high lubricity offered by oil muds makes them especially suited for highly deviated and horizontal wells. Along with the higher lubricity, the risk of differential sticking is minimized when using oil muds. An oil mud has a thin filter cake and the friction between the pipe and the wellbore is minimized, thus reducing the risk of differential sticking.
Low Pore Pressure Formations -The ability to drill low pore pressure formations is easily accomplished with oil muds since the mud weight can be maintained at a weight less than that of water. Mud weights as low as 7.5 lb/gal can be achieved.
Corrosion Control - Corrosion of pipe is controlled since oil is the external phase and coats the pipe. Oil muds offer exceptional corrosion protection due to the non-conductive nature of the oil, and corrosion cells cannot develop since the metal surfaces are oil wet. The products used in oil mud are very thermally stable and do not degrade to produce corrosive products. Also, bacteria do not thrive in oil muds.
Re-Use - Oil muds are well suited to be used over and over again. The oil mud can be stored for long periods of time, since bacterial growth is suppressed. The oil mud can be conditioned before being
used again by reducing the drill solids content with mechanical removal equipment instead of relying on dilution.
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