Dick Ghiselin • Special Correspondent
Editor's note: For those of you who were expecting to hear from Eldon Ball on this page, we should tell you that our friend and colleague has left PennWell to pursue another opportunity. We will miss him and wish him well.
According to regulatory authorities, there are more than 2,000 single jacket producing wells in the shallow Gulf waters offshore Texas and Louisiana. Lying in 100 ft (30 m) of water or less, many of them are operated using the hub-and-spoke technique, where a large central production platform acts as the hub and each remote satellite well is connected to the hub by a flowline, like the spokes on a wagon wheel.
In spite of the good maintenance and repair capability offered by the hub, it is very difficult and sometimes uneconomical to take care of sick or broken satellite wells in a timely fashion. Many satellite wells are on platforms so small that it is virtually impossible to deploy workover or remedial equipment on them. Mobilizing a cantilever workover rig, while effective, is usually too expensive and time consuming. Even if completely successful in rejuvenating a well, the out of pocket cost exceeds the net present value of the potential production improvement. The same story applies to many coiled tubing interventions using lift boats. Even the most economical slick line services often face overwhelming challenges on some of the smaller platforms. In every case, it all boils down to cost versus incremental added value.
Despite these challenges, the fact is that in many cases residual reserves remaining in the reservoirs tapped by these wells exceed 5% to 15% of the original oil in place. There's still a lot of unproduced oil down there. For the oil wells, the problem is usually associated with declining reservoir pressure. For the gas wells, most are drowning in water influx. Either way, practical solutions exist through the application of artificial lift techniques or dewatering.
This is exactly what Tammany Oil & Gas found in its shallow water offshore assets. The challenge for the company was providing the means to implement those solutions on a small, single-well jacket. The company's engineers could sit and watch the wells go down one at a time, or they could figure out a way to bring them back to life within the cost constraints imposed by tight commodity prices, especially for gas, and the cost of the remedial work necessary to rejuvenate the wells. Having already performed cost analyses on workover and coiled tubing solutions, the company knew that unless a well had exceptional potential, there was no way it could be rejuvenated profitably using those techniques.
However, most of the satellite wells are connected to the hub production facility by a 4-in. steel flowline. Production separation and field processing are taken care of at the hub. One of the solutions the engineers had considered involved supplying the satellite wells with high-pressure gas to operate a gas lift system for the oil wells, or a jet dewatering scheme for the gas wells. The challenge was how to get the high-pressure gas to the satellite platforms safely and economically. A major snag in this solution existed in two areas: firstly, the cost of running a high-pressure gas line over a mile to the platform was prohibitive; and secondly, getting the regulatory authorities to permit such a scheme in a timely fashion represented a significant challenge.
An innovative and elegant solution was developed by Tammany. Why not pig a flexible high-pressure gas line through the middle of the existing flowline? Gas could be routed to the satellite well from the hub, while production flowed back to the hub. No additional lines needed to be permitted. After the engineering work was completed, Tammany applied for authorization to proceed. The authorities recognized that the solution was unique and did not add any risk, so after considerable review it was approved.
A single run of 6,023 ft (1,836 m) high-pressure thermoplastic 1.25-in. diameter line was chosen. Flexible line was used because it could negotiate bends as sharp as 5-D without kinking. The line was pulled from the hub platform to the satellite in a single operation with line pull less than 400-lbf. Installation was closely scrutinized by the Texas Railroad Commission (RRC) pipeline inspectors. Among their requirements was to pull an extra 50 ft (15.2 m) of line through so it could be inspected for any damage from the pull-through operation. No defects were found.
A lift boat was positioned at the satellite to facilitate the installation, but the time to do the project was dramatically shorter than laying a new steel line or installing coiled tubing. This saves a huge expense where risers would have to be cut and a new radius riser installed. During subsequent make-up of the high-pressure gas line on both the satellite well and the hub platform, the line was pressure tested to 1,500 psi for eight hours. No leaking was observed.
This concentric pipe solution is thought to be the first of its kind ever attempted in the Gulf of Mexico. Like many first attempts, there were valuable lessons learned. Resolution of these lessons is expected to further reduce costs when the solution is implemented on subsequent wells.
By the simple fact that an elegant solution was made available in a timely way, a well was salvaged that ordinarily would have been slated for abandonment because remediation costs exceeded the value of estimated incremental production.
Similarly, the satellite well was restored to life with an initial rate of 244 b/d of oil. Maximum gas lift volume required to achieve this rate was 303 Mcf/d of gas at 900 psi. The well has been subsequently choked back and stabilized at 50 b/d. Despite the delays associated with implementing a first-time solution, the project was deemed to be a success. At Tammany, the feeling is that a new technique to rejuvenate under-performing wells offshore has been pioneered. The company is actively looking for well candidates where the conditions are suitable for high-pressure gas injection via a concentric high-pressure flexible line. Even though the first attempt was performed on an offshore well, there is no reason why the technique would not be suitable for a land satellite well. Being able to thread a high-pressure gas conduit through the protective environment of the existing flowline eliminates risk, and saves both time and money.