Newtonian Fluids in Cementing Operations in Deepwater Wells

Abstract

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 166456, ’Newtonian Fluid in Cementing Operations in Deepwater Wells: Friend or Foe?,’ by Polina Khalilova, SPE, Schlumberger; Brian Koons, SPE, Chevron; and Don Woody Lawrence and Anouar Elhancha, SPE, Schlumberger, prepared for the 2013 SPE Annual Technical Conference and Exhibition, New Orleans, 30 September-2 October. The paper has not been peer reviewed. This paper discusses the factors to be taken into account when designing cement jobs using fluids in turbulent flow and some of the overlooked benefits of using turbulent-flow design for mud removal. It will be demonstrated that, when designed and executed properly, the use of turbulent-flow mud removal with Newtonian fluids can be beneficial in deepwater (DW) cementing operations. Introduction When one considers the risks associated with poor zonal isolation in DW wells, such as release of well fluids to the environment, gas migration to the surface, underground blowouts, and well-killing operations, the importance of proper cementation and sufficient zonal isolation becomes clear. These issues are often difficult and time-consuming to address, a situation further complicated by the daily cost of DW-well operations. One of the complexities in DW wells is that of drilling with oil-based or synthetic-based muds, commonly referred to as nonaqueous fluids (NAFs). NAFs are compressible fluids that dynamically change density and rheological properties as temperature and pressure increase or decrease. This makes modeling flow regimes for proper mud removal very difficult. These types of drilling fluids also oil-wet the surfaces to be cemented, making cement bonding more troublesome. When cement slurries are contaminated with NAF, it can extend thickening times, cause undesirable gel-strength development, and ruin compressive-strength development. Another complexity in drilling DW wells is the narrow margin between pore pressure (PP) and fracture gradient (FG). A given reservoir that is reached on a land location has a wider PP/FG margin as compared with that of the same reservoir reached from a DW location. The available PP/FG margin would be narrower as the amount of overburden available on land is reduced relative to the depth of water. This hinders cementing because it reduces the equivalent circulating density (ECD) under which the cement slurries can be placed. An additional challenge with DW wells is that increasing water depth brings an increasing length for the fluids to travel to reach their placement depth. Unless mechanical separation (wiper balls or darts) is used, the spacers and cement slurries will have an increased mixing interface and will be exposed to additional contamination from mud film remaining on the pipe inner surface while being pumped through the drillpipe or casing before reaching their final placement position in the annulus. Currently, there are two primary flow regimes used when planning a mud-removal design for cementing: turbulent flow and effective laminar flow. Turbulent flow is generally considered the preferred method for efficiently removing drilling fluids, but there are requirements for turbulent flow to be effective that may not always be achievable. Effective laminar flow is the most widely used placement method because of the ease of maintaining hydrostatic control of the well, the ability to achieve effective mud removal at the low velocities typically required during cementing in DW wells, and the ability to maintain ECD below the FG. Drilling fluids, weighted spacers, and cement slurries are all placed by use of the effective-laminar- flow method. Brines, water, and chemical washes are all placed by use of turbulent flow. There are many examples in historical publications and operational case histories in which turbulent flow is used for effective mud removal, but no defined guidelines or discussions were found by the authors of the original paper for the use of Newtonian fluids specifically in DW-cementing operations (although case studies in which that method was used in DW do exist; several are provided in the complete paper).