Visit to CO2 EOR Project in Weyburn Oilfield, Saskatchewan, Canada

Following the IEA collaborative project on EOR annual workshop and seminar held in September 2003 in Regina, Canada, delegates were invited to visit the nearby Weyburn CO2 miscible flood project. Here one of the UK delegates, David Hughes, principal reservoir engineer with Senergy Ltd in Aberdeen, recounts the visit. This article contains predictions made available by EnCana - click here to view advisory statement.

Along with around 25 fellow delegates (including fellow UK delegates Tissa Jayasekera and Eugene Balbinski) I boarded the bus to Weyburn with eager anticipation. The use of CO2 to enhance recovery at the EnCana operated Weyburn field has been hailed as the best example of a project of its kind in the world.

After an 80 mile journey through the plains of southern Saskatchewan we arrived at the town of Weyburn and picked up our guide for the day Dave Craigen who works for EnCana in Weyburn Operations. As we entered the field it was apparent that the operations covered a very large area (around 70 square miles) with wellheads stretching as far as the eye could see.

Weyburn lies on the northwestern rim of the Williston Basin . The reservoir is located at a depth of 4400-5000 ft in the Midale beds of the Mississippian Charles formation. The Midale beds comprise two layers: the Marly (low permeability chalk dolomite) with the Vuggy (highly fractured and permeable limestone) below. Payzone thickness is 10-90ft. The oil is 25-34° API gravity sour crude with a GOR of 190 scf/stb and an original estimated in place volume of 1.4 billion stb. The field was developed in the 1960s by a pattern waterflood (currently there are 946 wells of which 199 are horizontal). This has resulted in a good sweep of the Vuggy but a much lower recovery from the Marly. The estimated recovery from waterflooding alone is 30% STOIIP.

Following detailed assessments, EnCana announced in 1997 that it was implementing a miscible CO2 flood in Weyburn. The source of the CO2 is the Great Plains Synfuels Plant in North Dakota which produces 160 MMscf/d of natural gas from the gasification of lignite coal. The CO2 is a by-product of this process and up to 95 MMscf/d (of the 240 MMscf/d produced) is transported to Weyburn as a liquid along a 200 mile pipeline. This gas was previously vented to the atmosphere so injecting it gives a considerable environmental benefit.

Great Plains Synfuels Plant and Where CO2 Emerges at Weyburn 200 Miles Away

CO2 injection is being implemented in phases at Weyburn. The target area comprises 75 inverted nine-spot patterns with a STOIIP of around 810 MMstb. 19 patterns were converted to CO2 injection in September 2000 (Phase 1A), four were added in 2002 (Phase 1B) and nine have been added in 2003 (Phase 1C). The CO2 flood is expected to increase the recovery from 30% (using waterflooding alone) to 46% of STOIIP over the target area.

Schematic Map of Weyburn CO2 Project

It was at the site of two horizontal producer well heads in Phase 1A that we stopped to take our first detailed look. All producers are equipped with artificial lift mainly sucker rod pumps although these are shut-in if the well can continue to flow naturally. Two types of sucker rod pumps were in use at this site. One a more conventional "nodding donkey" and the other a vertical pump with a much longer stroke.

Pumps at a Phase 1A Wellhead Site

Dave Craigen of EnCana Answers Questions From Inquisitive Visitors

Next we visited a satellite gathering station. There are a number of these stations each receiving production from a local cluster of wells. The first stage of separation occurs here with all hydrocarbon gas (plus any produced CO2) and the liquid production streams being sent via separate flowlines to the central separation facility. Also on each site is a distribution point for the CO2. The CO2 mixed with recycle gas is supplied to the site from the central distribution point and then via separate pipes from a manifold to the individual gas injectors. There is also a water injection distribution system; make up water is obtained from an aquifer. Noticeable was the noise level in the separation building and the odour of mercaptains (light hydrocarbons where a single hydrogen atom is replaced by a sulphur atom originating from the lignite) in the gas distribution building. Courtesy of EnCana we were provided with hard hats, and eye and ear protection but the odour was still noticeable!

Satellite Gathering Station and CO2 Injection Distribution Point

The Weyburn field is particularly suited to CO2 flooding. The reservoir is continuous and the geological sequences promote gravity segregation. The minimum miscibility pressure is easily achieved and the swelling of the crude by the addition of CO2 reduces the viscosity significantly. The successful water flood has led to a good understanding of the reservoir and the horizontal development gives a very efficient CO2 displacement. In some areas water flooding is continued into the Vuggy (vertical injectors) while the Marly is targeted by the CO2 (horizontal injectors). In others both the Marly and the Vuggy receive both water and gas through vertical injectors in a water-alternating-gas (WAG) scheme.

Following our stop at the satellite gathering station we travelled to the main office complex where an enjoyable lunch was served. After lunch we visited the control room which is located within the complex. This was a neat room with an array of computer screens manned by a single operator. From here via a series of schematic displays of the facilities hardware and circuitry it is possible to monitor and control all aspects of the field's operations.

Adjacent to the office complex are the central facilities. Before the introduction of CO2, the facilities were able to handle oil and water production and a small amount of solution gas. These have been augmented with the introduction of a free water knock-out vessel and a battery gas and recycle compressor package. Also installed is a flare system. Medium pressure gas gathering lines bring the produced gas from the production satellites to the central recycle compression station. Here it is mixed with the fresh CO2 arriving from Dakota and distributed through the injection network.

As of September 2003, 31 patterns are on CO2 injection with a cumulative injection to date of 70 billion scf. The current injection rate is 95 MMscf/d of CO2 with an additional 23 MMscf/d of hydrocarbon gas and CO2 being recycled. Greater than 5000 stb/d of incremental oil is being produced. Over the lifetime of the field and estimated incremental recovery from CO2 injection is 130 million stb giving a net storage of 7 to 8 Mscf of CO2 per incremental stb.

Historical Production and Predicted CO2 Response

Further information on the Weyburn CO2 miscible flood project can be obtained on the following websites:

• http://www.encana.com/operations_and_projects/weyburn.shtml
• http://www.encana.com/legal.shtml

Weyburn CO2 injection is also the subject of a scientific monitoring project in which the IEA participates through its Greenhouse Gas collaborative project (GHG). CO2 sequestration in oil reservoirs is seen as an important future technology so this multinational monitoring project is designed to assess in detail the short and long term fate of the CO2 in the field. Further information can be found on these websites:

• http://www.netl.doe.gov/publications/proceedings/01/carbon_seq_wksp/3aEOR-IEA.PDF
• http://www.ieagreen.org.uk/weyburn1.htm
• http://www.ptrc.ca/projects/weyburn.htm ,
• www.co2captureproject.org/reports/documents/OutReach/sept2002/CCP NGO FG SMV Weyburn.ppt

Following the visit to the central facility we returned to the office complex and a light hearted discussion ensued between our host Dave Craigen and József Pápay from MOL ( Hungary ) over the expected outcome from the project in terms of the incremental recovery. We then boarded our bus in amiable mood for the return trip to Regina.

On behalf of the IEA delegates, thanks go to Dave Craigen and his EnCana colleagues for a very interesting and enjoyable day out.

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