Coiled Tubing Gas Lift Revives Dead Wells in South Pakistan
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This paper describes a coiled tubing gas lift (CTGL) technique successfully used to restart production from two pilot wells in a mature field in Pakistan that had been shut in since 2015. The project yielded production gains that resulted in a short payback period and verified that the technique represents an effective and economical solution to restart production in mature fields where conventional artificial-lift methods challenge well economics. The pilot results have encouraged the operator to further implement CTGL in Pakistan. According to the authors, suitable candidate selection, including data accuracy for nodal analysis and production forecast, and proper planning played fundamental roles in the project’s success.
Exploration and production companies in Pakistan struggle with multiple challenges including pressure from regulatory bodies to increase oil and gas production to meet requirements of an energy-starved country in a period of historically low oil prices. With little new significant discovery, operators are challenged to produce economically from mature and declining fields. Several fields in south Pakistan have been forced to shut down because of uneconomical production rates. Most of the wells in these mature fields do not flow naturally because reservoir pressure is insufficient to overcome the hydrostatic column.
Existing techniques have their own challenges, including very high associated costs. Realizing these challenges, the government of Pakistan recently prioritized developing mature fields and optimizing production in declining fields.
A detailed screening of candidate wells was carried out to identify innovative artificial-lift solutions. The deployment of gas-lift (GL) valves using coiled tubing (CT) soon emerged as the best option, offering the highest return in a cost-benefit analysis. CTGL is a rigless solution that enables setting GL valves at the required depth during any stage of a well’s life. CTGL strings can be installed without making permanent changes to already-existing wellbore tubulars, and make it easy to install in wells with either monobore or through-tubing completions. The technique allows gas to be injected through CT with production through the CT-to-tubing annulus. The system also is retrievable and replaceable when needed.
Gas-lift mandrels (GLMs) and valves are added to the CT in a similar fashion to any traditional GL completion. A workover rig is not required, and installation of the CT string also provides the opportunity to modify well cross-sectional flow areas, thus improving well hydraulics and production flow. Two flow configurations are possible: (a) tubing flow (i.e., production flow up the insert tubing string, gas injection down the newly created annulus) or (b) annular flow (i.e., gas injection down the insert string, production flow up the newly created annulus between parent tubing and the CT insert string).
The work flow to identify possible candidate wells for a pilot trial is presented in Fig. 1. The screening process consisted of the following steps:
- Collect well data (information on 10 wells was gathered initially)
- Classify whether the wells are oil or gas production wells
- Review parent-tubing size; a larger tubing provides more options for insert-string sizing
- Review parent-tubing condition; several wells were filtered out because of old well-tubing-integrity issues
- Model each well to estimate production potential for an oil production well using the inflow-performance-relationship (IPR) method, and a gas production well by analyzing critical velocity calculations.
From this initial review, prospective candidate wells were selected for detailed nodal analysis. Criteria included availability of lift gas on wellsite, parent-tubing size, static reservoir pressure, well productivity, formation gas, deepest injection point for GL, lift-gas response (production vs. gas-injection rate), and CT availability.
After the nodal analysis review was completed, two wells were selected for the pilot CTGL project. Details of the final GL installation design (including mandrel and valve depths) are provided in the case study section of the complete paper. Because this was the first CTGL operation in Pakistan, a detailed feasibility analysis was conducted for deployment through CT, followed by implementation of the system. Among factors that had to be taken into consideration were CT material, software modeling, downhole assembly, surface equipment, and installation procedure. These aspects are discussed in detail in the complete paper.
Conventional GL valves were selected for the CTGL installations but installed in a special internally mounted GLM. Unlike conventional installation of a GLM with a rig, where GL valves can be easily connected to the completion tubulars by making up between different joints of pipe, connecting GLMs on a continuous CT pipe requires cutting the string at the required depths and making connectors on both ends of the cut CT section. During GLM deployment, the downhole end of the CT pipe is secured by the CT blowout preventer, and the uphole end of the pipe is held by injector head chains. As illustrated in Fig. 2, a swivel connector is required to allow a connection because both CT ends are rotationally locked. The CT connector at the uphole end is pull-tested and pressure-tested per standard operating procedures. However, the inverted connector at the downhole end of the pipe cannot be pulled and pressure-tested using conventional methods. Therefore, custom-designed CT connectors were used to pressure-test the CT connector sealing element separately.
Two pilot wells were selected from four shortlisted candidates through nodal analysis. Well A had been drilled in 2015 and completed as a 4½-in., single-string monobore. Well B had been drilled and completed with 7-in. liner and 2⅞-in. production tubing in 2004. The CTGL system for each well consisted of one CT string with multiple stations of unloading and orifice valves spaced out at depths engineered to maximize the well’s productivity. In addition to a GL valve, each station included two CT external connectors and one self-aligning connector, which enabled efficient and safe connection between both CT ends during the deployment of the CTGL station.
Additionally, specific CT pressure-control equipment and wellhead adapters were used to secure and hang the CT string in the production tree and provide connection with the gas-injection facilities at surface.
The Well A installation deployed a 1½‑in. CT string with four CTGL stations. The Well B installation was performed with 1¼-in. CT and five CTGL stations. The wells were commissioned using existing surface infrastructure and were unloaded smoothly until the production stabilized at optimum rates near 420 and 325 B/D, respectively.
Well B CTGL system installation was completed in 5 days, including rigup and rigdown, compared with 8 days for Well A, as a result of implementation of lessons learned. Two GLMs per day were deployed compared with one for Well A, showing a marked increase in operational efficiency. The production rate of the well was higher than the designed values, and the well produced at a nearly constant rate of approximately 325 B/D.
Design and installation of the two new CTGL systems helped identify numerous best practices and lessons learned, which are described in the complete paper and will speed implementation of the methodology in other parts of the world.
Coiled Tubing Gas Lift Revives Dead Wells in South Pakistan
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