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).
Benefits and Risks of Use of Newtonian Fluids in DW Cementing
Benefits. Reduction of Hydrostatic Pressure in the Annulus. Although the density of drilling fluids typically varies from approximately 10 to 16 lbm/gal in DW wells, in general, simple Newtonian -fluids vary from fresh water at 8.34 lbm/gal to saturated NaCl brine at 10.0 lbm/gal and exhibit lower density than the drilling fluid with which the section is drilled. Thus, the use of Newtonian fluid is aimed at reducing the total hydrostatic pressure of the fluid column in the annulus and maintaining it within the PP and FG. Newtonian fluids are also very thin and will allow mud removal and cement placement at higher pump rates while maintaining the ECD below the FG.
Turbulent-Flow Regime. The turbulent-flow regime is very difficult to achieve at rates executable in DW applications for common weighted fluids exhibiting a yield stress. Therefore, we must turn to Newtonian fluids to achieve mud removal with the turbulent-flow regime.
Contamination Effects on Cement Slurry Are Less Severe. As a general rule, water-based Newtonian fluid has a less detrimental contamination effect on cement slurry than NAFs or the weighted spacer. Newtonian fluids pumped at a rate sufficient to be in turbulence all around the pipe will have a flatter and cleaner interface than weighted fluids pumped in laminar flow.
Risks. More-Severe Channeling If Effective Turbulent Flow Is Not Achieved. Flow becomes turbulent when the critical velocity is achieved. Historically, the most commonly used method is to calculate the critical Reynolds number assuming 100% standoff. An eccentric annulus will feature laminar flow on the narrow side and turbulent flow on the wide side if this method is used. In the past, a correction factor has been used to account for eccentricity. This can result in a flow path through the mud on the wide side that subsequent fluids, such as weighted spacer and cement slurries, will preferentially follow.
Direct Contact of Mud and Cement. If an insufficient volume of Newtonian fluid is used or effective turbulent flow is not achieved throughout the entire annular cross section, then direct contact of mud and cement will likely occur.
Loss of Hydrostatic Overbalance in the Annulus. If the job is designed in a way that allows the hydrostatic pressure of the fluid column in the annulus to drop below the PP of the formation, then the formation fluids can invade the wellbore. This would affect the cement job adversely and would reduce the probability of achieving zonal isolation.
Guidelines and Considerations for Designing DW Cement Jobs by Use of Newtonian Fluids
The authors of the original paper have developed a set of guidelines to be followed when designing cement jobs using Newtonian fluid and the turbulent-flow regime. Most of these guidelines are included here, although a complete list can be found in the complete paper.
Use Sufficient Volume for Effective Turbulent-Flow Cleaning. In DW wells, the distances that fluids have to travel inside the pipe are often much longer than they are in shelf and land wells. The amount of contamination occurring because of fluid intermixing will therefore be greater and should be accounted for by using additional volume. It is accepted in the industry that larger volumes of wash and spacer are required for bulk mud displacement and water-wetting when performing cementing operations in NAF-drilling-fluid environments.
Ensure Turbulent Flow Is Achieved Around the Entire Annulus in an Eccentric Pipe Annulus. In experimental studies, turbulent flow achieved around the entire annulus was more effective at mud-filter-cake removal than any other method except for scratchers. Eccentricity adversely affects the ability to create the turbulent flow around the entire annulus. The Reynolds number required for a Newtonian fluid to achieve turbulent flow around the entire annulus increases with an increasing annular eccentricity. If effective turbulent flow cannot be achieved, then a Newtonian fluid should not be used. Using a Newtonian fluid when effective turbulent flow is not achievable will increase the probability of channeling and providing a preferential flow path for the remaining fluids to follow.
Use Newtonian-Fluid Preflushes With Caution in Wells With High Deviation. As previously mentioned, if the Newtonian fluid is not in turbulence around the entire annulus, then it is probable that the fluid will be in turbulence on the wide side of the annulus and remain in laminar flow, or worse, have no flow, on the narrow side of the annulus, resulting in extreme channeling with no mud displacement on the narrow side of the annulus and a failed cement placement.
If this occurs, the mud remaining on the narrow side of the annulus will come in contact with the next fluid pumped, which is cement in most cases. This allows direct contact with incompatible fluids. It is important to ensure that the centralization program provides sufficient standoff to allow effective turbulent flow to be achieved at pump rates the well can withstand. In high-deviation wells, well control must be managed carefully when using Newtonian fluids because they will often have a lower density than the drilling fluid.
Stay Within the PP/FG Window. One of the benefits of using Newtonian fluid is the ability to lower the friction pressure generated during placement and the hydrostatic pressure of the column in the annulus during cement-job placement and when cement is in place. However, it is important that the hydrostatic pressure never fall below the PP during or after the cement placement. Well control must be maintained at all times.
Deviated wellbores can complicate the design, and PP/FG uncertainty can add additional risk. It is important to have reliable data on formation properties, especially in the DW environment where drilling margins tend to be narrower, depths are greater, and the use of NAF drilling fluid often makes it more difficult to determine influxes in a timely manner.
Appropriate use of Newtonian fluids can be advantageous for cement placement where low FGs exist or where it is necessary to place a long cement column. Effective turbulent flow enables cement to be placed without meeting laminar-flow-design criteria for the spacer fluid and the cement slurry.
Design for Chemical Compatibility. Newtonian fluids are often easier to design for compatibility with the formation, drilling fluids, and cement slurries than weighted spacers. Because Newtonian fluids rarely contain insoluble weighting agents, it is only necessary to obtain the correct surfactant blend for breaking the NAF-drilling-fluid emulsion and the correct salinity to address formation compatibility where drilling has occurred with NAFs.
Consider Fluid Trains Ahead of Cement Slurry. The use of weighted spacers in effective laminar flow and Newtonian fluids in effective turbulent flow in combination has proved to be successful in achieving effective isolation and target cement coverage while maintaining well security. There are two options: (1) Newtonian fluid followed by weighted spacer or (2) a weighted spacer followed by Newtonian fluid. Both options were used with success.
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.