CHOPS sand management
Cold heavy oil production with sand (CHOPS) recovery processes generate large volumes of sand that must be managed. In Canada in 1997, approximately 330,000 m3 of sand (approximately 45% porosity sand at surface) were produced from CHOPS wells. Individual wells may produce as much as 10 to 20 m3/d of sand in the first days of production and may diminish to values of 0.25 to 5 m3/d when steady state is achieved. Sand grain size reflects most of the reservoir. There is little sorting or segregation in the slurry transport to the well; however, not all zones in the reservoir may be contributing equally at all times.
Environmental issues and waste definition
Sand separated from the production stream contains 1 to 6% oil by weight: the more fine-grained the sand, the higher the residual oil content. Separated sand also contains large amounts of chlorides-rich formation water, generally approximately 30,000 to 50,000 ppm NaCl. This means that water-saturated waste sand of 40% porosity contains more than 3,000 ppm chlorides. In Canada, this changes the environmental classification and disposal methods.
Produced sand is classified as nonhazardous oilfield waste. Other wastes that must be disposed of include produced water (usually cleaned and reinjected), as well as liquid wastes that contain various amounts of oil and suspended fine-grained mineral matter, generically called "slops." A particularly difficult material to dispose of is stable emulsion, which is a mixture of water, 20 to 50% oil (enriched in polar asphaltenes), and fine-grained mineral matter. Emulsions are generated during production and tank cleaning when high shear occurs. Attempts to break the emulsion are costly because the oil recovered does not pay for the treatment. This troublesome material represents a difficult challenge for the CHOPS industry.
Separation and stockpiling produced sand
Insulated vertical separators (stock tanks) of 100 to 200 m3 capacity, 6 to 8 m high, and heated to 60 to 80°C receive oil directly (usually one tank per well). Well rates are generally less than 30 m3/d; therefore, residence time is sufficient for heating and effective gravitational segregation. Without interrupting production, oil, water and sand are withdrawn from stock tanks periodically to keep levels within certain ranges.
Tank are cleaned in several ways. Fig. 1 illustrates tank designs and cleaning methods. The most common method is to introduce "stingers" (high-pressure hoses) from pressure-treatment trucks to slurry the sand, which is then aspirated into vacuum trucks attached to other ports. This process generates additional emulsion because of intense shear, which creates another treatment and disposal problem, but this tank-cleaning process is the most widely used.
Auger systems have been developed to remove sand without introducing additional water. Through a specially designed port, a robust auger is screwed into the bottom of the tank, and the almost-solid sand slurry is withdrawn to a sealed tub truck. This method reduces the amount of waste generated.
Vacuum trucks and load-haul-dump units (tub trucks) transport the sand either directly to a disposal site or to a site where excess liquid is withdrawn. Sand is dumped into managed stockpiles separated by membranes from underlying surficial strata with run-off capture trenches and with groundwater quality monitoring for environmental control. If produced sand is left in a stockpile to drain before disposal, Cl– content usually decreases to less than 3,000 ppm, which is the limit for landfill placement.
Sand and fluids disposal
Land spreading, road spreading, road encapsulation, and reuse
Land spreading (land farming) and road spreading are becoming less acceptable. No new sites for land spreading have been permitted in Canada since 1990, and Canadian regulatory agencies have indicated that road spreading will be phased out. Direct road spreading is acceptable on local nonpaved roads, but the uniform and fine-grained sand is quite unstable, leading to greater road maintenance needs.
Manufacturing high-quality asphalt concrete with produced sand is difficult because of the uniform grain size and the strong dilution effect of the remnant oil. Encapsulation involves mixing dry produced sand with asphalt concrete mix (approximately 50:50) to generate a low-grade material suitable for roadbed enhancement. It is used as a base course to underlie high-quality asphalt concrete used as a surface course. Approximately 1 m3 of sand per meter length of two-lane roadway can be disposed of in this manner; therefore, it is a limited means of disposal.
Other uses (addition to cement kilns, sand-blasting sand, feedstock for manufacturing processes) involve only small amounts of the total sand produced, and the sand cannot be cleaned economically to meet specifications for use as fiberglass sand or sand blasting. These methods cannot be used as primary disposal approaches for the volumes of sand produced.
Hot water and surfactant separation has been used to wash sand either for secondary use or for local disposal; however, since 1990, three commercial plants in Alberta have failed financially because of the high cost of dealing with three waste streams (dirty water, dirty oil, and sand) created from a single waste (oily sand). Sand cannot be washed sufficiently clean of oil for use in sand blasting or industrial feedstocks. Despite its superficial attractiveness, sand washing is not advised.
Class II landfills for nonhazardous oilfield waste are required for disposal of solids that do not contain draining water. Definitions, guidelines, and other information exist on regulatory agency websites. Landfills are the cheapest of the three disposal methods, but obtaining a license and complying with regulations has not always proved easy. Also, the long-term security of landfills and their proximity to groundwater remain serious concerns, particularly given the difficulty of guaranteeing that all wastes meet guidelines.
Deep injection of sand and fluid wastes
Large volumes of waste sand can be slurried with dirty produced water that must be disposed of by conventional well injection and fractured at high rates into oil-free zones or depleted reservoirs. This technique has been used in the US, Canada, and Indonesia. The target zone can be a depleted reservoir or a new, oil-free zone. The zone must have adequate flow properties and reservoir capacity to accept 200,000 to 400,000 m3 of slurry of density 1.15 to 1.25 g/cm3 injected over a period of 1 to 3 years. Injection is normally episodic on a daily basis, allowing time for pressure dissipation before another 8 to 12 hours of 0.8 to 1.6 m3/min slurry placement is undertaken. Fig. 2 shows a typical pressure-time response.
In addition to high environmental security, another advantage of injecting a slurry is that dirty liquids, sludges, and even some emulsion can be added to the mix and co-disposed. A disposal well approved for nonhazardous oilfield waste allows more flexibility in handling the various waste steams. Because disposal costs for produced water may exceed Canadian $7.00/m3, the approximately 4 to 6 m3 of produced water used to slurry each cubic meter of sand represents a cost savings.
Because of cost, some operators have used massive deep injection of pure emulsions. However, this impairs the performance of fracture injection wells, leading to premature casing distress unless carefully executed in conjunction with large volumes of sand.
Salt cavern placement
Solution caverns in salt are used in Canada for CHOPS wastes, as well as for other oil industry wastes (e.g., refining sludges from the synthetic crude plants at Ft. McMurray). Trucks transporting wastes dump directly into a hopper, and slurry pumps place the materials into caverns at a depth of 900 to 1200 m. In the cavern, solids (ρ ~ 2.65) drop to the bottom, and oils and emulsions (ρ ~ 1.0) float to the top of the brine (ρ = 1.2) and are removed through the annulus. The cavern acts as a huge gravitational separator for solids and oils. The advantages of salt cavern disposal are similar to those for slurry injection, except that total costs are somewhat higher (~10 to 20%), and excess brine must be disposed in a brine-injection well.
Emulsion, slops, and oil treatment
Slops and emulsion generally are dewatered and sent to caverns, deep-placement injection sites, or special treating facilities that remove water, separate oil and solids, and dispose of the streams. Requirements for emulsion breaking, centrifuge separation, and heat treatment make this processing expensive.
Before shipment to the upgrader or transport by pipeline, produced oil must be heat treated and stripped of remnant solids and water by chemical treatment with surfactants or emulsion breakers. Many companies send intermediate waste streams to permanent disposal facilities rather than pay the high costs of additional treatment or of recycling wastes through local treatment facilities.
- Alberta Energy Utilities Board, http://www.eub.gov.ab.ca/.
- Saskatchewan Energy and Mines, http://www.gov.sk.ca/enermine/.
- Wan, R.G. and Wang, J. 2001. Analysis of Sand Production in Unconsolidated Oil Sand Using a Coupled Erosional-Stress-Deformation Model. Proc., CIM Petroleum Society 52nd Annual Technical Meeting, Calgary, paper 2001-049.
- Bilak, R.A. and Dusseault, M.B. 1997. Regulatory Controls And Slurry Fracture Injection. Presented at the Technical Meeting / Petroleum Conference Of The South Saskatchewan Section, Regina, Oct 19 - 22, 1997 1997. PETSOC-97-152. http://dx.doi.org/10.2118/97-152.
- Dusseault, M.B., Bilak, R.A., and Rodwell, L.G. 1997. Disposal of Dirty Liquids Using Slurry Fracture Injection. Presented at the SPE/EPA Exploration and Production Environmental Conference, Dallas, Texas, 3-5 March 1997. SPE-37907-MS. http://dx.doi.org/10.2118/37907-MS.
- Dusseault, M.B. et al. 1994. Disposal of Granular Solid Wastes in the Western Canadian Sedimentary Basin by Slurry Fracture Injection. Proc., 1994 Symposium on Deep Injection Disposal of Hazardous and Industrial Waste, Berkeley, California, 725–742.
- Campbell, C.J. and Laherrère, J.H. 1998. The End of Cheap Oil. Scientific American 278 (3): 78.
- Dusseault, M.B., Bruno, M.S., and Barrera, J. 2001. Casing Shear: Causes, Cases, Cures. SPE Drill & Compl 16 (2): 98–107. SPE-72060-PA. http://dx.doi.org/10.2118/72060-PA.
Noteworthy papers in OnePetro
Wagg, B. T., & Fang, Y. 2007. CHOPS Without Sand. Petroleum Society of Canada. http://dx.doi.org/10.2118/09-03-52
Wang, R., Yuan, X., Tang, X., Wu, X., Zhang, X., Wang, L., & Yi, X. 2011. Successful Cold Heavy Oil Production with Sand (CHOPS) Application in Massive Heavy Oil Reservoir in Sudan: A Case Study. Society of Petroleum Engineers. http://dx.doi.org/10.2118/150540-MS
Young, J. P., Mathews, W. L., & Hulm, E. 2010. Alaskan Heavy Oil: First CHOPS at a vast, untapped arctic resource. Society of Petroleum Engineers. http://dx.doi.org/10.2118/133592-MS
Yinghong, Z., Mingming, Q., Min, Z., Benjing, D., Cunzhi, C., & Zuokun, Z. 2008. Questions and comprehensive economic evaluation model for sand management. American Rock Mechanics Association. https://www.onepetro.org/conference-paper/ARMA-08-195
Servant, G., Marchina, P., & Nauroy, J.-F. 2007. Near Wellbore Modeling: Sand Production Issues. Society of Petroleum Engineers. http://dx.doi.org/10.2118/109894-MS
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