Manganese Tetraoxide Weighted Invert Emulsions as Completion Fluids

Abstract

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 112313, "Manganese-Tetraoxide-Weighted Invert Emulsions as Completion Fluids," by L.P. Moroni, SPE, J.R. Fraser, and R. Somerset, Baker Hughes; A. Jones, Oilexco North Sea; and A. Guarneri, Eni AGIP SpA, originally prepared for the 2008 SPE International Symposium and Exhibition on Formation Damage Control, Lafayette, Louisiana, 13-15 February. The paper has not been peer reviewed. Traditionally, completion equipment is run in clear fluids, brines, or base oils to minimize the potential for solids to plug equipment. Base oils typically have approximately 0.8 specific gravity (SG), which has well-control implications, and brines can be prohibitively expensive where high densities are required. A novel approach was used by a UK operator to address these problems by compromising between the shale stability provided by oil and the density achievable with brine. Introduction Once the drilling phase is complete, most operators clean up the well before running the completion string. With non-aqueous fluids (NAFs), cleanup can be quite complicated, requiring a sequence of surfactant pills to remove mud residue from the wellbore and casing. The objective is to leave the well with a clean fluid that has very-low solids content (typically less than 0.05%) or meets a clarity specification. This process can result in large volumes of fluid that are considered to be contaminated to the point of requiring disposal. Where an NAF is used, environmental limitations usually require the contaminated fluid to be contained for special disposal. Manganese tetraoxide (Mn3O4) has been used as an alternative weighting agent in both water-based and nonaqueous drilling fluids where equivalent circulating density and sag performance have precluded use of barite-weighted fluids. The combination of small particle size, spherical shape (Fig. 1), and high SG (4.8 g/cm3) makes Mn3O4an ideal weighting agent for fluids in which a low viscosity profile and low gel strength are required. Case History 1 A temporary completion fluid with 1.95 SG was required while running the completion string to provide a secondary well-control barrier in addition to the cemented liner while the blowout-preventer (BOP) stack was replaced with the Christmas tree. Use of cesium formate was one alternative explored by the operator, but the cost was considered prohibitively high. A previous application that used a water-based mud containing ultrafine barite failed on three issues: The fluid was prone to barite sag, the fluid left a coating on tubulars that was virtually impossible to remove, and the fine barite particles plugged surface well-test-line valves. One of the major concerns was the tight clearance (0.1 in.) of the hydraulically set packer and the risk of setting prematurely because of pressure surges while displacing to the oil-based packer fluid. The wellbore geometry consists of a long tangent between 40 and 50º, so sag prevention is a key requirement.