CHOPS

Cold Heavy Oil Production with Sand (CHOPS) is a primary recovery technique for extra-heavy oil in unconsolidated Cretaceous Mannville Group sands in the Western Canada Sedimentary Basin, in which sand grains are deliberately allowed to flow into the wellbore together with oil rather than being controlled by gravel packs or screens, because the sand influx destabilizes the near-wellbore formation and initiates a network of wormholes that dramatically increases the productivity of reservoirs whose matrix permeability and fluid viscosity alone would not support economic flow rates; CHOPS was developed commercially in the Lloydminster heavy oil area of the Alberta-Saskatchewan border beginning in the 1980s and has since become the dominant production mechanism for 8 to 14 API gravity extra-heavy oil in the Sparky, McLaren, Waseca, General Petroleum, and Colony sands at depths of 350 to 900 m across a 40,000 km2 area of the WCSB fairway from Lloydminster northwest to Cold Lake and north to Pelican Lake. The wormhole network that CHOPS creates is the critical production mechanism: wormholes are centimetre-to-metre-diameter conduits of mobilized sand that extend 50 to 500 m radially from the wellbore through the unconsolidated Mannville sand, dramatically reducing the flow resistance between the reservoir and the producing well by creating a high-conductivity network that allows heavy oil viscosities of 1,000 to 100,000 cP at reservoir temperature to flow at economic rates without steam or diluent injection. Surface production rates in WCSB CHOPS wells range from 5 to 80 m3/d of oil-sand slurry, with produced sand cuts of 3 to 30 percent by volume of total fluid; progressive cavity pumps (PCPs) rated at 30 to 200 m3/d fluid capacity are the standard artificial lift method because their helical rotor-stator geometry tolerates produced sand without the valve plugging and wear that would disable rod pump equipment. CHOPS ultimate recovery is typically 5 to 15 percent of original oil in place, significantly lower than steam-assisted recovery (SAGD: 50 to 70 percent OOIP; CSS: 25 to 40 percent OOIP), but CHOPS requires no steam generation infrastructure and achieves lifting costs of $8 to $20 per barrel in mature fields, making it economically viable at lower oil price environments than thermal recovery projects with capital costs of $30,000 to $80,000 per barrel per day of capacity.

• Wormhole network development and geomechanics of CHOPS sand production in WCSB Mannville sands: Wormhole initiation in CHOPS wells occurs when the bottomhole flowing pressure drops below the sand grain cohesive strength of the unconsolidated Mannville sands (cohesion typically 0 to 50 kPa, versus consolidated sandstone at 1,000 to 10,000 kPa); once cohesive failure occurs, viscous flow forces carry sand grains through the formation toward the wellbore in a process analogous to piping failure in earthworks. In WCSB Lloydminster Sparky sands with reservoir pressure of 3 to 5 MPa and viscosity of 5,000 to 30,000 cP, wormhole growth rates are 0.1 to 1.0 m/day during the early production period (first 6 to 18 months), creating a dendritic network of high-conductivity channels that accounts for 80 to 95 percent of the well's total productivity. Wormhole networks mapped by 4D time-lapse seismic in WCSB fields show drainage areas 3 to 10 times larger than would be predicted by Darcy flow in the unfractured matrix, explaining why CHOPS wells produce at rates 5 to 50 times their theoretical matrix productivity index before wormholing.
• Progressive cavity pump selection and sand management surface facilities for WCSB CHOPS operations: Surface facilities for WCSB CHOPS wells must handle a three-phase slurry of heavy oil, produced water, and sand at temperatures of 15 to 35 degrees Celsius (below thermal EOR temperatures) without blocking processing equipment. The PCP string (downhole pump, sucker rod drive, surface drive head) is selected for a total fluid rate 1.5 to 3 times the anticipated oil rate to accommodate produced water and sand; elastomer stator selection (Nitrile NBR for oil gravities above 12 API, Hydrogenated Nitrile HNBR for higher aromatic content) is critical because heavy oil aromatic fractions swell inferior elastomers and cause stator failure within weeks. At surface, hydrocyclones or desanding cones remove produced sand from the oil-water emulsion before the emulsion enters the treater, with sand slurry pumped to a disposal well or temporarily stockpiled in lined sand pits; AER Directive 023 governs produced sand management in Alberta, requiring that sand disposal wells inject below the base of groundwater protection and that cumulative sand injection volumes be reported quarterly.
• CHOPS production decline and wormhole stabilization in WCSB Lloydminster and Pelican Lake fields: CHOPS wells follow a characteristic production profile: a ramp-up period of 3 to 18 months during wormhole growth (increasing oil rate and increasing sand cut), a plateau period of 1 to 3 years (near-maximum productivity, sand cut stabilizing at 5 to 20 percent), and a long decline period of 5 to 15 years (declining oil rate as reservoir pressure depletes within the drainage radius, sand cut often dropping as wormholes stabilize). Production decline is managed in WCSB CHOPS fields by infill drilling on 200 to 400 m spacing to drain interwell areas not accessed by wormhole networks from existing wells, by re-perforating to new sand intervals within the Mannville stack when the primary zone is depleted, and by transitioning depleted CHOPS patterns to polymer flooding (as at Pelican Lake, where Cenovus operates the world's largest polymer flood with 400+ producers and injectors recovering incremental oil from post-CHOPS drainage areas).
• CHOPS applicability criteria and comparison to thermal recovery in WCSB extra-heavy oil evaluations: CHOPS is the preferred primary recovery method in WCSB Mannville sands that meet specific criteria: oil viscosity below 100,000 cP at reservoir temperature (above this threshold, wormholes do not propagate efficiently because the viscous pressure gradient cannot mobilize sand at economic rates), reservoir pressure above 1.5 MPa (necessary to drive fluid into wormholes), sand that is unconsolidated enough to fail at low drawdown (unconfined compressive strength below 200 kPa), and net pay thickness of at least 3 m (below which wormhole height is too constrained to achieve adequate drainage volume). Reservoirs that fail CHOPS criteria due to high viscosity (greater than 100,000 cP) or thin pay (below 3 m) at Mannville depths are candidates for Cyclic Steam Stimulation (CSS) or SAGD if the formation thickness supports steam chamber development. In the WCSB Cold Lake area, the Clearwater Formation at 450 to 600 m is too viscous for CHOPS (viscosity above 500,000 cP at 12 degrees Celsius) and is produced by Imperial Oil CSS with surface steam generation, while the shallower General Petroleum sand at the same location is produced by CHOPS in interleaved well patterns.
• Environmental and regulatory aspects of CHOPS in WCSB Alberta and Saskatchewan operations: CHOPS operations in Alberta are regulated under AER Directives 023 (produced sand disposal), 056 (well event reporting), and 083 (primary heavy oil production monitoring), which require operators to track cumulative sand production volumes, demonstrate that disposal wells inject below BGWP at all times, and report unusual sand breakthrough events (sudden sand cut increases above 40 percent by volume) that indicate wormhole intersection with a disposal well or water-bearing formation. In Saskatchewan, the SMARTER regulatory system requires similar sand disposal well approval and cumulative injection reporting under SaskEnergy Upstream Petroleum Industry Guidelines. Greenhouse gas emissions from CHOPS are significantly lower than thermal recovery because no steam is generated; however, solution gas venting and flaring from CHOPS tank batteries at Lloydminster and Pelican Lake has been a compliance focus under AER Directive 060 and Alberta's methane emission reduction regulations, requiring gas conservation or flaring minimization plans for batteries producing above 500 e3m3/month of solution gas.

CHOPS Wormhole Expansion Driving Infill Drilling Optimization at WCSB Lloydminster Sparky Unit

A Lloydminster area Sparky Formation CHOPS unit with 45 producers on 320 m spacing showed average well production declines from 18 m3/d at peak (month 8 post-completion) to 6 m3/d after 4 years, with sand cuts stabilizing at 8 to 12 percent. A 4D seismic survey comparing baseline and 4-year surveys mapped wormhole drainage halos of 80 to 160 m radius around each producer; interwell areas beyond 180 m from any producer retained 75 to 85 percent of original oil saturation. An infill program drilled 22 wells on 160 m infill spacing, each targeting interwell drainage areas confirmed by 4D seismic to have residual oil saturation above 70 percent. Initial infill well rates averaged 22 m3/d (versus 18 m3/d for original producers), confirming that the infill targets were in undrained rock. The infill program recovered an estimated 8 percent additional OOIP from the unit at a lifting cost of $14/bbl, extending unit economic life by 6 years beyond the decline curve predicted from primary producers alone.

Related Terms

Wormhole is the near-wellbore flow conduit that CHOPS relies on for productivity; wormhole networks 50 to 500 m long extending from WCSB Lloydminster and Pelican Lake CHOPS wells are responsible for 80 to 95 percent of total well productivity and determine the drainage area for infill spacing decisions. Progressive cavity pump (PCP) is the universal artificial lift method for WCSB CHOPS wells; its helical rotor-stator design tolerates produced sand and heavy oil emulsions that would plug and seize rod pump equipment within days of startup in Mannville sand production. Heavy oil (8 to 14 API gravity, viscosity 100 to 100,000 cP at reservoir conditions) is the target resource for CHOPS in the WCSB Lloydminster fairway; viscosity below 100,000 cP is the key CHOPS applicability criterion that determines whether primary sand production can achieve economic wormhole propagation. Polymer flooding is the secondary recovery successor to CHOPS in WCSB Pelican Lake and Lloydminster fields; post-CHOPS areas with 5 to 15 percent OOIP primary recovery and residual oil saturation of 70 to 85 percent in interwell areas are the target for Cenovus and other operators' polymer injection to recover incremental oil. Steam-assisted gravity drainage (SAGD) is the thermal recovery alternative to CHOPS for WCSB extra-heavy oil too viscous or too deep for cold primary production; SAGD achieves 50 to 70 percent OOIP recovery at higher capital cost and greenhouse gas intensity than CHOPS primary production in the Lloydminster and Cold Lake areas.