In the 100+ years since the first offshore well was drilled in California’s shallow waters during the late 1800s, offshore drilling technology in the US has advanced dramatically. The first offshore rigs were built on wooden piers that extended from land, but progressive development and investment has resulted in deepwater platforms and drillships that can operate more than 160 km (100 miles) offshore in water depths below 2,438 m (8,000 ft).
In the near future, the industry is expected to drive exploration of this previously undrillable ultra-deep area. As developments in drilling technology increase the depth, reach, and overall complexity of deepwater wells, advances in the capabilities of completion tools, materials, and simulation software also are needed.
Deepwater technology development
Deep water is by far the most challenging and expensive area in which to operate. Operating in these complex conditions requires a significant investment in technology with a strong emphasis on innovation regarding safety, efficiency, and environmental protection. One potentially beneficial innovation is an alternative placement technique: reverse-circulation primary cementing (RCPC).
Onshore applications of this placement technique have brought benefits such as reduced equivalent circulating densities (ECD), reduced placement time, and reduced use of cementing additives. A current government-sponsored study is investigating if this placement technique can be transferred from its previous onshore applications to a deepwater environment, if similar benefits would be seen in deep water, and what additional technology needs to be developed before RCPC can be used in deep water.
Deepwater RCPC tool, modeling challenges
Reverse cementing of deepwater casing and liner systems will require that tools be available to switch fluid flow paths from what is conventionally used to run and cement casing and liners. Conventionally, fluid is pumped down the work string through the inside diameter (ID) of the casing, and then it returns to the surface through the annular area between the open hole and the outside diameter (OD) of the casing. Reverse cementing these systems will require that fluid be pumped down the work string, switched at the top of the casing to flow down the annulus between the OD of the casing and the open hole, return to the surface through the casing ID, and then be switched at the top of the casing back to the annular area between the work string and the previously run casing. Figure 1 compares a typical deepwater conventional cementing flowpath to the anticipated RCPC flowpath.
Other tool challenges exist for fluid separation and float equipment to retain the cement in the casing annulus after it has been placed. Wiper plugs, darts, and balls only separate fluids while in the work string and could prove to be a barrier for displacing a mechanical device to set a liner hanger. Additionally, conventional float equipment is designed to allow flow in one direction, while reverse cementing requires flow in both directions.
Premature shutoff of flow through specialized float equipment while reverse-cementing could be problematic. Float equipment is needed to allow free flow of fluid in either direction and not close until the cement has been placed and a liner hanger has been set. All mechanical devices of the system should be designed to minimize ECD while running and cementing casing and liner systems.
Modeling and simulations of the completion of oil and gas wells, particularly offshore wells, are vital parts of the planning and design process. Predicting downhole pressures and ECD is an important step in avoiding both lost circulation and fluid influx, while predicting downhole temperatures is important for slurry design. Both of these are necessary to achieve a good cement job and zonal isolation.
The methods for predicting these parameters have grown more complex in conjunction with the complexity of the wells themselves. Techniques have advanced far beyond the rudimentary American Petroleum Institute correlations used in the past to 1-D and 2-D finite-difference models. But in spite of the sophistication of modern simulators, these are still unable to model the RCPC process because they cannot incorporate the crossover valve configuration used in reverse-circulation systems.
Path forward for development
A key component to enable reverse-cementing systems in deepwater applications is clearly a switchable crossover flow tool that has the ability to allow flow in the conventional direction while running into the hole and then switch on demand to a reverse flow direction for the cementing operation. The primary mechanical challenges of reverse-cementing in deepwater applications are the development of a switchable crossover flow tool, fluid separators, and floating equipment.
At a minimum, the crossover tool should allow switching from conventional flow to reverse flow and then back to conventional. Fluid separators are needed that will not interfere with the switchable crossover tool system and will not prevent a mechanical device for setting a liner hanger from reaching its seat.
Current commercial simulators are unable to model a deepwater RCPC flowpath. To overcome this obstacle, a finite-element analysis (FEA) is being developed that will solve the Navier-Stokes equations and the convective heat equation simultaneously.
In addition to being able to handle the RCPC configuration, the finite element model has a number of other benefits (Figure 2). FEA has become commonly used in a wide range of applications, including structural analysis, electromagnetic modeling, and fluid dynamics and heat transfer, because it is flexible, accurate, and computationally efficient. This model is fully dimensional and therefore can accurately model complex heat transfer.
Other challenges include fluid design, fluid separation, mud removal efficiency, and the need to form contingency plans for this new operation. Overall, the most crucial potential benefit of RCPC for cementing technically challenging deepwater wells would be the reduction of ECD. If this is possible, the benefit to the industry would be significant. This technique could be used on wells where conventional placement is undesirable due to formations with narrow pore and fracture gradients or well architecture with narrow annuli.
From the beginning of offshore operations, technology has been pushed to extend development farther offshore to previously unreachable areas. A shift in thinking is needed for tomorrow’s technical challenges in operations, planning, design, tools, modeling, and materials.
If RCPC is to be implemented in deep water, tool reliability and overall well safety need to be, at a minimum, equivalent to conventional operations. The government-sponsored study to investigate feasibility of deepwater RCPC will continue analysis and investigation, and a full technical report will be completed in June 2014.
Funding for the project is provided through the “Ultra-Deepwater and Unconventional Natural Gas and Other Petroleum Resources Research and Development Program” authorized by the Energy Policy Act of 2005 and administered by the Research Partnership to Secure Energy for America under contract with the US Department of Energy’s National Energy Technology Laboratory. Learn more at rpsea.org.
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