Traditionally, completion equipment has been run in clear fluids, brines, or base oils, to minimize the potential for solids to plug equipment, preventing tools from being set correctly or setting prematurely. However, both brines and base oils have limitations. Base oils typically have densities around 0.8 sg which has well control implications and brines can become prohibitively expensive where high densities are required.
In the North Sea, many wells are either highly deviated or require extended reach to access compartmentalised reservoirs. Consequently, the reservoir often comprises a significant proportion of shales. The use of brine as a completion fluid can therefore lead to significant wellbore instability and reservoir impairment issues.
A novel approach has been employed by a UK operator to address these problems by compromising between the shale stability provided by oil and the density achievable with brine. The fluid employed is a low-solids, invert emulsion formulated with a high-density internal phase brine. Due to the density limit of the base solids-free brine, additional density is achieved through the use of manganese tetraoxide (Mn3O4). Manganese tetraoxide has a very small, sub-micron, particle size which allows a degree of self suspension thus allowing a fluid with low viscosities to be utilized. The use of this type of formulation provides a fluid which gives good wellbore stability while running completions screens but allows the completion fluid to be produced back in the testing phase of the well without plugging the screens due to the extremely fine particle size of the manganese tetraoxide. This fluid was designed to exhibit excellent sag performance while minimizing the gellation potential of the fluid. The physical characteristics of manganese tetraoxide (small spherical particles) were an important factor in the design of the fluid due to the very tight clearances expected when setting downhole tools.
This paper presents laboratory design data and case histories from the UK sector of the North Sea and in Kazakhstan where this type of high-density fluid has been used to run the lower completions while successfully maintaining wellbore stability.