Gas Shutoff Evaluation and Implementation, North Slope, Alaska
- D. Hupp (Schlumberger) | A. Frankenburg (BP Exploration Inc.) | P. Bartel (BP Exploration Inc.) | G. Roberts (BP Exploration Inc.)
- Document ID
- Society of Petroleum Engineers
- SPE Production & Facilities
- Publication Date
- February 2002
- Document Type
- Journal Paper
- 4 - 10
- 2002. Society of Petroleum Engineers
- 5.4.2 Gas Injection Methods, 2 Well Completion, 5.6.4 Drillstem/Well Testing, 2.2.2 Perforating, 3.3.1 Production Logging, 4.1.2 Separation and Treating, 1.14 Casing and Cementing, 5.1.2 Faults and Fracture Characterisation, 5.3.2 Multiphase Flow, 4.6 Natural Gas, 4.3.4 Scale, 5.2.1 Phase Behavior and PVT Measurements, 1.6 Drilling Operations, 5.6.1 Open hole/cased hole log analysis, 4.1.5 Processing Equipment
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Oil production on Alaska's North Slope has been limited as a result of gas production and the ensuing gas handling-facility constraints. Diagnosis of gas production problems is complicated by the fact that many of the wells are high-angle or horizontal. This paper presents case studies in the diagnosis and treatment of excess gas production in three horizontal oil wells located on the North Slope of Alaska.
Candidate wells for gas shutoff (GSO) were identified based on gas flow rates and the gas/oil ratio (GOR). Wells that were not commercially producible were selected for evaluation as potential GSO wells. These wells were assessed with new-technology logging tools conveyed on coiled tubing or with a downhole tractor. The new production-logging technology provided direct gas and oil holdup measurements, allowing for interpretation of three-phase production in the horizontal well environments. The goal was to identify dry-gas production (i.e., gas production without associated oil production).
After dry-gas production was identified, GSO procedures were designed based on the location of the gas entry into the wellbore. Two of the case studies involved GSO from the toe of the horizontal section of the wells, which was accomplished by placing a cast-iron bridge plug on coiled tubing above the gas-producing perforations. The third case study identified dry-gas production from the top perforated interval. This GSO involved setting a plug below the gas-producing perforations and squeezing them off. The plug was then drilled out with coiled tubing and pushed to total depth (TD).
These techniques proved to be economical GSO solutions in difficult well environments, and each case resulted in significant reduction of free gas and increased oil production.
Increased gas production can significantly reduce oil production in a mature oil field. Typically, gas production in oil fields increases with time for many reasons, including reduced reservoir pressure, coning from a gas cap, and gas-injection enhanced oil recovery techniques. The ability to handle this increased gas production is critical to efficient field production. Many fields are limited by facility design in their ability to handle produced gas. Once facility limits for gas handling are met, wells that produce more gas than the marginal limit for the facility will be shut in, eliminating the production of oil from that section of the reservoir. Even before a well reaches the marginal gas/oil ratio (MGOR), oil production may decline because of wellbore hydraulics, in which gas is produced preferentially over oil. Therefore, to maximize oil production on a well-by-well basis and to manage field offtake, the ability to shut off or reduce gas production in individual wells becomes critical. The challenge to an operator is to both identify the gas entry point and to design an effective GSO technique.
Traditionally, on the North Slope of Alaska, wellbore gas-entry evaluation has been accomplished through production-logging programs, which include temperature, spinner, pressure, capacitance, and density measurements.1,2 This logging suite is fairly successful in accurately identifying gas entry points in vertical or near-vertical wells. What this tool suite does not do is provide a three-phase split (oil, gas, and water) for each set of perforations in higher deviations. Without knowing how much oil will be lost when shutting off a gas entry point, an accurate financial evaluation of a GSO proposal cannot be made. Horizontal wells provide additional challenges to the evaluation of production logs because of measurement limitations, complex wellbore geometries, and the complex downhole flow regimes. Many times, coiled-tubing-conveyed logging strings create such a significant downhole choke that gas entry points cannot be accurately identified.
Recent advances in production-logging technologies, including a new optical probe tool for measuring gas holdup, have provided the ability to directly measure gas holdup (Yg) in horizontal wellbores. This measurement has greatly enhanced our ability to quantify oil and gas contributions from producing intervals. Once accurate splits of oil, gas, and water contributions have been calculated, decisions can be made concerning shutting off the gas. These decisions are based on how much oil will be shut off with the gas, the technical feasibility of the shutoff, the reservoir feasibility of shutting off the gas, and the cost of the shutoff procedure.
This paper will describe the techniques used to identify and quantify gas entry in a horizontal wellbore and the implementation of GSO procedures in three horizontal wells in a mature North Slope, Alaska, oil field.
The wells in this study are from the mature Prudhoe Bay field on the North Slope of Alaska. The field has been in production for more than 20 years and contains a gas cap and aquifer as well as a remaining light-oil column. Enhanced recovery techniques used in the field include produced water and gas reinjection to maintain reservoir pressure as well as miscible injectant to increase displacement efficiency. Both of the gas-injection techniques, along with natural pressure depletion, have created free-gas entry points within existing well completions.
The number of horizontal wells with small casing sizes has grown in recent years, largely because of advances in through-tubing, coiled-tubing, sidetrack-drilling techniques. This increase in horizontal well stock has made identifying gas entry and quantifying three-phase contributions along the horizontal wellbore critical. In addition, it has been necessary to develop GSO techniques for the toe, heel, and midpoint of these horizontal producers.
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