Re-Birth of a Mature Carbonate Gas Pool
- K.C. Carr (BP Canada Energy Company) | M. Fiori (BP Canada Energy Company)
- Document ID
- Society of Petroleum Engineers
- Abu Dhabi International Conference and Exhibition, 10-13 October, Abu Dhabi, United Arab Emirates
- Publication Date
- Document Type
- Conference Paper
- 2004. Society of Petroleum Engineers
- 4.1.5 Processing Equipment, 5.8.7 Carbonate Reservoir, 3 Production and Well Operations, 4.6 Natural Gas, 5.8.8 Gas-condensate reservoirs, 3.2.2 Downhole intervention and remediation (including wireline and coiled tubing), 1.2.1 Wellbore integrity, 5.4.3 Gas Cycling, 5.2 Reservoir Fluid Dynamics, 5.8.3 Coal Seam Gas, 4.3.4 Scale, 3.3 Well & Reservoir Surveillance and Monitoring, 4.1.6 Compressors, Engines and Turbines, 5.7.5 Economic Evaluations, 5.2.1 Phase Behavior and PVT Measurements, 5.4.2 Gas Injection Methods, 3.3.1 Production Logging, 2.2.2 Perforating, 4.2.3 Materials and Corrosion, 3.1.6 Gas Lift, 1.6 Drilling Operations
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Operation of a mature pool requires increasing cost control and infrastructure optimization and consolidation to maintain profitability. The decision to initiate this type of reservoir management plan, sometimes described as harvest, is driven in part by reservoir performance analysis and in part by commodity prices at the time the decision is made. The impact is not limited to operating cost; it can affect reservoir performance itself and the ultimate recovery of reserves.
This paper presents the account of a 3.7 TCF sour gas pool in Alberta, Canada and the technical impact of harvest on field operations, reservoir performance and ultimate recovery. The combination of new production decline analysis, well workover with surprising results, and key field observations led to a complete change in reservoir interpretation. A new model emerged where near wellbore regions are deeply damaged and the gas-water contact (GWC) is much lower than expected.
Management of mature pools is not a new challenge. Numerous papers have been written over the years, addressing various technical aspects and aimed at increasing pool longevity1,2,3,4. In the Western Canadian Basin, the surge of exploration and development activity in the 1950's and 1960's has left a legacy of very mature pools with many of these 40 to 50 years old pools still producing today. Alberta Energy and Utilities Board (EUB) records show that of the 30,000 gas pools discovered in Alberta, 2000 or 7% were discovered before 1970. These pools represent in the aggregate 15 TCF of remaining recoverable reserves, as of January 1, 2002, or nearly 40% of the total. Each of these pools was at some point declared mature based on the science of the day and treated as a harvest candidate, i.e. a pool with very limited upside where the best strategy is to control cost and produce what is left without further investment.
In some cases this approach becomes a self-fulfilling prophecy. This paper uses the example of the Kaybob South Beaverhill Lake A pool in Alberta, Canada to demonstrate how a large carbonate reservoir can be pulled back from harvest mode to sustained development. First, a history of the main operational activities will be summarized. A description of pool geology and reservoir fluids will be presented next. Operational changes brought about by the perception of the pool as mature will be reviewed and their impact on reservoir surveillance and operation itself will be detailed. Events leading to the change in perception of remaining potential of this pool will be related. The latest drilling activity resulting from this paradigm shift and subsequent production performance will conclude this paper.
The Beaverhill Lake A Pool is a large sour retrograde gas condensate reservoir located in the Kaybob South Field, near the town of Fox Creek, 250 miles northwest of Calgary, Alberta, Canada (Figure 1). It was discovered in 1961 at a depth of 10,500 ft. Initial reservoir pressure was 4722 psia, 1107 psia above dew point pressure. The reservoir is underlain by a large aquifer that has provided pressure support to the Beaverhill Lake A pool and to other hydrocarbon pools in the area.
The Beaverhill Lake A pool was divided into three operational units. Kaybob South Beaverhill Lake Units No. 1 and No. 2 (BHL #1 and BHL #2) are operated by one company while another operates Kaybob South Beaverhill Lake Unit No. 3 (BHL #3). Figure 2 illustrates the three production units.
Production started in 1968 in BHL #1. Secondary recovery in the form of lean sweet gas cycling was applied immediately to increase recovery of natural gas liquids (NGL) and condensate. Secondary recovery ended with gas blowdown in BHL #1 in 1983. Production started in 1970 and 1972 in BHL #2 and BHL #3 respectively; both switched to gas blowdown in 1990. The decision to proceed with gas blowdown was based upon the maximizing energy recovery given the projections of the time. The action coincided with the onset of low North American natural gas prices, below $2/mcf and an operational switch from sustained development to harvest mode.
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