Combining Proven Anti-Sag Technologies for HPHT North Sea Applications: Clay-Free Oil-Based Fluid and Synthetic, Sub-Micron Weight Material

Abstract

North Sea wells drilled in high-pressure, high-temperature (HPHT) areas are known for presenting barite sag challenges. Fluid densities in the 17.5-18.0 lbm/gal range require a high percentage of barite, and 400°F temperatures can adversely impact the rheological properties needed for reliable suspension. Further, horizontal and extended reach drilling (ERD) wells can require specific hydraulic parameters for hole cleaning that may entail comparatively high pump rates which can contribute to excessive equivalent circulating densities (ECD).Drilling fluid companies initially responded to these challenges by utilizing modified barite and alternative weighting agents. Micronized barite has been used to help lower ECD values for years. However, a non-barite manganese tetraoxide weighting agent has proven to provide equivalent anti-sag at a lower cost. The sub-micron sized material has a specific gravity (sg) of 4.8, which is higher than barite. Per Stokes' law, small particles settle more slowly so that sag resistance improves which is important during extended static periods like during extensive coring operations. Manganese tetraoxide is also acid-soluble. These parameters provide more operational flexibility.The manganese tetraoxide weight material has now been tested successfully in a clay-free and economical paraffin/mineral oil-based fluid (OBF) for use in North Sea HPHT applications. The low-solids, clay-free system has a long history of reducing ECD and minimizing downhole losses while drilling, running casing and cementing. The 17.9 lbm/gal fluid weighted with manganese tetraoxide material exhibits effective hole cleaning properties at lower viscosities than conventional OBFs and shows no detectable sag tendency. Clay-free fluids also demonstrate excellent return permeability values in a wide range of well types. This paper details the design and testing of the clay-free ERD system, including extreme HPHT static aging and sag testing and modifications to the emulsifier package. The economics of implementing the commercial system are also discussed.

Bingham’s model in the oil and gas industry

“…Clay-free synthetic-based drilling fluids were first developed in 2001, formulated using a synthetic ester-internal olefin blend (Burrows et al 2004). In a recent development, a clay-free oil-based drilling fluid formulation was introduced with combined chemical and particulate stabilization (Carbajal et al 2009). Thermal and baric stability in yielding characteristics is complemented by non-progressive gel strengths.…”

Formation Damage Caused by Improper Mn3O4-Based Filter-Cake-Cleanup Treatments

“…The recommended cleaning fluid to remove the filter cake was 10 wt% citric acid (Donovan and Jones 1995;Svendsen et al 1995;Downs et al 2005). Mn 3 O 4 provides low rheological properties and equivalent antisag behaviour (Kleverlaan and Lawless 2004;Carbajal et al 2009). Tehrani et al (2007) noticed that Mn 3 O 4 -based drilling fluid might undergo a degree of flocculation and produce higher plastic viscosity values upon heat aging.…”

Low Equivalent Circulating Density Organoclay-Free Invert Emulsion Drilling Fluids

“…The use of polymeric additives in the organoclay-free invert emulsion fluids (OCIEFs) would help to form a gel structure required to achieve the necessary viscosity control. This viscosity control would help to increase the sag resistance and the cuttingscarrying capacity of the fluid [19][20][21]. Another advantage of the organoclay-free invert emulsion drilling fluids is the fragile gel structure that they provide [22].…”