Expert Viewpoint—Sand Screens
An overview of heavy oil sand control issues and options by Ezio Toffanin.
In common with several other heavy oil production challenges, high viscosity is the root cause of several sand management issues. In the unconsolidated sand environments common to many heavy oil reservoirs, high viscosity means that there are strong forces dragging sand grains and destabilizing the rock formation. Viscosity also raises oil mobility issues—water flows more easily and once water breaks through to the well, it becomes even harder to make heavy oil flow. Stopping sand is easy, but stopping oil is even easier!
A further challenge is that asphaltenes and wax deposits can block the spaces in sand screens and other parts of the completion system. This is particularly important offshore, where cold production is the most common method. In addition, cost control plays a major role in the selection of sand management strategies—low production rates and lower quality (and price) oil impact economics, particularly when there are many wells. Corrosion, such as from CO2 or H2S, is always a consideration, but is rarely a limiting factor.
Barefoot completion is the cheapest downhole option, using progressing cavity pump (PCP) systems to produce the sand-laden oil and then managing the produced sand at the surface. Perforated liner is often used to maintain borehole stability in cold heavy oil with sand (CHOPS) operations.
Slotted liner is the most common sand screen solution used for steam assisted gravity drainage (SAGD) wells, typically with 2–3-in (5–8 cm) long slots each 8–10 gauge (.08–010-in, 0.2–0.25 mm). Slotted liner is cost-effective and very robust, but the high temperatures and rapid changes in temperature experienced in cyclic steam stimulation (CSS) and SAGD operations can induce stress and deformation
Wire-wrapped screens are used very successfully in some thermal applications. Schlumberger has developed fit-for-purpose wire-wrapped screen systems for thermal applications which have proven to be simple to install and very effective during operation. Direct wire-wrapped screens can also be used for applications where robustness is key to the success of the installation.
Premium mesh screens have superior sand retention capabilities and can provide better mechanical and thermal integrity properties. However, to date their use in heavy oil reservoirs has been limited.
Offshore, where steam generation is difficult, cold production is the norm, but sand management at the surface can be a problem. Good results in such situations have been achieved by frac-packing, in which the hole is cased, perforated and screened, then fracturing gels are pumped into the formation.
In land operations, SAGD is fast becoming the production method of choice, where reservoir conditions permit. Most SAGD wells are intended to last a long time—perhaps 40 years—so longevity is a key design consideration that impacts the choice of equipment and ease of its replacement (workover). If wells can be made to last longer, they can be economically drilled longer and more can be invested in completions. Schlumberger has provided MeshRITE screens for installation in several heavy oil fields around the world. MeshRite screens were initially used in the Boscan field, operated by Chevron in the Maracaibo Basin, Venezuela. The crude oil here has 10 API gravity and a viscosity that ranges from 300 to 2000 cps. Since 1996, Chevron has evaluated and tested a variety of alternative completions strategies. The Boscan sands have a wide range of grain sizes, and the finer grains cause blockages in many systems. The MeshRite system is designed to produce the fine grains while holding back larger grains. Results to date and laboratory testing show that it is delivering high production rates and indicate that the system is a robust long-term solution. For more information, see Underdown, D., and Sanclemente, J.: “Producing Sand for Sand Control: A Novel Approach,“ SPE paper 74394 presented at the SPE International Petroleum Conference and Exhibition in Mexico (10–12 February 2002, Villahermosa, Mexico).
The Schlumberger approach to designing completions strategies is firmly based on reservoir characteristics and uses all available data, including logs, core analysis and particle size distribution (PSD) from sand sieve analysis. Once the production environment is understood, technical options can be identified and then the economics and risks can be evaluated. Injection profiles need similar analysis.
Today, only a small proportion of heavy oil fields have sand control solutions more sophisticated than slotted liner. Based on heavy oil experience around the world and supported by a global network of research and engineering facilities, Schlumberger is developing new technologies to deliver cost-effective improvements in productivity. A key focus is the development of new sand control solutions that combine the technical benefits of premium mesh sand screens with the economies of slotted liner.
