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Cold heavy oil production with sand
Cold heavy oil production with sand (CHOPS) involves the deliberate initiation of sand influx during the completion procedure, maintenance of sand influx during the productive life of the well, and implementation of methods to separate the sand from the oil for disposal. No sand exclusion devices (screens, liners, gravel packs, etc.) are used. The sand is produced along with oil, water, and gas and separated from the oil before upgrading to a synthetic crude.
To date, deliberate massive sand influx has been used only in unconsolidated sandstone (UCSS) reservoirs (φ ≈ 30%) containing viscous oil (μ > 500 cp). It has been used almost exclusively in the Canadian heavy-oil belt and in shallow (< 800 m), low-production-rate wells (up to 100 to 125 m3/d). Fig. 1 shows Canadian heavy oil and extra-heavy oil deposits. Because of the economic success of CHOPS for these conditions, the concepts behind sand influx management are being tried in other oil production processes. The cavity completion approach developed for coalbed methane exploitation is a similar process carried out for similar goals: to increase well productivity by enhancing fluid flow in the near-wellbore region.
History of sand production in Canadian heavy oil reservoirs
The first discoveries in the Canadian heavy-oil belt were made in the Lloydminster area in the late 1920s. High asphaltene-content heavy crude, an ideal feedstock for asphalt products, has been produced since that time. Typically, 10- to 12-mm diameter perforations were used, and pump jacks were limited by slow rod-fall velocity in the viscous oil to a maximum of 8 to 10 m3/d of production, usually less. Operators had to cope with small amounts of sand, approximately 1% in more viscous oils. Small local operators learned empirically that wells that continued to produce sand tended to be better producers, and efforts to exclude sand with screens usually led to total loss of production. Operators spread the waste sand on local gravel roads and, in some areas, the roadbeds are now up to 1.5 m higher because of repeated sand spreading.
The sharp oil price increases in the 1970s and 1980s led to great interest in heavy-oil-belt resources (approximately 10 × 109m3). Many international companies arrived and introduced the latest screen and gravel-pack technology but, in all cases, greatly impaired productivity or total failure to bring the well on production was the result. To this day, there are hundreds of inactive wells with expensive screens and gravel packs.
The advent of progressing cavity (PC) pumps in the 1980s changed the nonthermal heavy-oil industry in Canada. The first PC pumps had low lifespans and were not particularly cost-effective, but better quality control and continued advances led to longer life and fewer problems. The rate limits of beam pumps were no longer a barrier and, between 1990 and 1995, operators changed their view of well management. Sand became an asset because more sand clearly meant more oil. Individual well productivity began to rise higher than the 4 to 5 m3/d average. The goal of completion and workover strategies gradually became clear: initiate and maintain sand influx. Old, inactive fields that had produced only 4 to 6% of original oil in place (OOIP) could be rehabilitated profitably with large-diameter perforations and PC pumps. More highly integrated sand separation, transportation, and disposal methods were developed.
CHOPS is a new and rapidly developing production technology. Optimal workover strategies, sand-disposal practices, and improved recovery methods (waterflooding, pressure pulsing) are advancing quickly. Given the moderate operating costs and no need for thermal energy, interest in CHOPS as a primary production method is substantial. In 2002, the only serious limitation on the amount of oil in the heavy-oil belt produced by CHOPS is the lack of refinery upgrading capacity. Heavy oil is rich in carbon, heavy metals, and sulfur; therefore, conventional refineries cannot accept it as feedstock. Specialized and costly refineries called upgraders use coking and hydrogenation to produce synthetic crude oil, which then can be refined in a conventional refinery.
CHOPS status worldwide
CHOPS has been widely used only in Canada; however, anecdotal evidence suggests that heavy-oil operators in California traditionally took no steps to exclude sand, understanding that screens and sand packs would become blinded and production would cease. In the Duri field in Sumatra, Indonesia, heavy oil is produced by thermal methods, and large amounts of sand accompany the oil. In China, CHOPS was tried with some success in the Nanyang oil field, Hebei, between 1997 and 2000 but was not adopted permanently. In the Liaohe oil field, Liaoning Province, trials were conducted under challenging conditions. Jilin oil field has limited CHOPS production from a 300-m-deep UCSS.
Broad-ranging acceptance of sand influx as a viable production enhancement mechanism has not yet happened despite Canadian production levels of more than 70,000 m3/d in 2000. The reasons for the lack of acceptance include the fear of sand in a producing asset, the nontraditional nature of the production mechanisms, difficulty in production predictions, complexity in properly implementing CHOPS, and the need for sand management and disposal strategies.
Typical CHOPS well behavior
CHOPS wells display wide variations in their production histories. CHOPS production depends on the range of factors discussed in CHOPS production rate increase mechanisms, CHOPS physical mechanisms, and CHOPS operational and monitoring issues; however, the major aspects of a “typical” CHOPS well include the following factors:
- When a new well is completed, initial sand influx is large: 10 to 40% of the volume of the (gas-free) produced liquids and solids.
- Over a period of a few days to several months, the sand rate gradually decays toward a steady-state influx rate (0.5 to 10%), depending on oil viscosity.
- The oil production rate increases to a maximum several months or more after placing the well on production and then slowly declines as reservoir-depletion effects begin to dominate.
- All CHOPS production is accompanied by substantial gas production, and gas/oil ratio (GOR) values tend to remain relatively consistent over many years.
- Short-term sand influx rates and oil production rates fluctuate chaotically about the mean value.
- A successful workover can partly re-establish oil and sand rate but generally not to levels as high as the first cycle.
Fig. 2 shows a production profile for a typical CHOPS well, and Fig. 3 shows CHOPS well behavior over three production cycles.
The liquid flux pattern is different from that of conventional well behavior. Because there is a peak in the oil-rate curve, there must be at least two counteracting physical mechanisms with different characteristic effects. The well productivity increases because of enhanced fluid conductivity around the wellbore with continued sand production and diminishes as a result of reservoir energy depletion. These two effects combine to give a peak in the production history, followed by a gradual decline as depletion effects begin to dominate. Fig. 4 illustrates this behavior.
Summary of CHOPS technology
There are several important facts about this heavy oil production method:
- In UCSS reservoirs with good solution-gas quantities, no cohesion, and viscous oil, sand influx is initiated and sustained to take advantage of huge increases (3-fold to 20-fold) in "primary" production rate, as compared with cold production with sand exclusion.
- Rate increases are associated with sand liquefaction, permeability increase, foamy oil mechanisms, and elimination of near-wellbore fines and asphaltene blockage.
- Ultimate recovery factors have improved from 2 to 6% to 12 to 20% of OOIP in many heavy oil fields with the use of CHOPS.
- Between 1985 and 2002, great improvements have been seen in handling sand, maintaining wells on sand production, lifting efficiency, and other aspects of CHOPS.
- Operating expenses have been cut almost in half between 1989 and 2002, while the total CHOPS production rate has more than tripled. CHOPS concepts are beginning to affect conventional oil and gas production approaches in suitable reservoirs.
- Given the vast heavy-oil resources and the inevitable depletion of conventional oil, technologies such as CHOPS will steadily become more important.
- Palmer, I.D., Mavor, M.J., Seidle, J.P. et al. 1993. Openhole Cavity Completions in Coalbed Methane Wells in the San Juan Basin. J Pet Technol 45 (11): 1072-1080. SPE-24906-PA. http://dx.doi.org/10.2118/24906-PA.
- “OTS Heavy Oil Science Centre,” Lloydminster Heavy Oil, www.lloydminsterheavyoil.com/narrativ.htm.
- “Conventional Heavy Oil Resources of the Western Canadian Sedimentary Basin,” Natl. Energy Board of Canada.
- “Canada’s Oil Sands: A Supply and Market Outlook to 2015,” National Energy Board, http://www.nebone.gc.ca/energy/EnergyReports/EMAOilSandsSupplyMarket2015Canada2000_e.pdf.
- Hu, C., Liu, X., Wang, J. et al. 1998. Cold Production of Thin-bedded Heavy Oil Reservoir in Henan Oilfield. Presented at the SPE International Oil and Gas Conference and Exhibition in China, Beijing, China, 2-6 November 1998. SPE-50885-MS. http://dx.doi.org/10.2118/50885-MS.
Noteworthy papers in OnePetro
Istchenko, C. M., & Gates, I. D. 2013. Well/Wormhole Model of Cold Heavy-Oil Production With Sand. Society of Petroleum Engineers. http://dx.doi.org/10.2118/150633-PA
Rangriz Shokri, A., & Babadagli, T. 2012. Evaluation of Thermal/Solvent Applications With And Without Cold Heavy Oil Production with Sand (CHOPS). Society of Petroleum Engineers. http://dx.doi.org/10.2118/158934-MS
Rangriz Shokri, A., & Babadagli, T. 2012. An Approach To Model CHOPS (Cold Heavy Oil Production with Sand) and Post-CHOPS Applications. Society of Petroleum Engineers. http://dx.doi.org/10.2118/159437-MS
Alshmakhy, A., & Maini, B. B. 2012. A Follow-Up Recovery Method After Cold Heavy Oil Production. Society of Petroleum Engineers. http://dx.doi.org/10.2118/157823-MS
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