By Joe Lisenbee, Steve Fulwider and Jeff Knight, Centrilift

Transcription

1 Centrilift ESP Recirculation Systems Aid Gas Well De-watering By Joe Lisenbee, Steve Fulwider and Jeff Knight, Centrilift G as well de-watering is a significant application for electrical submersible pumping (ESP) systems in North America. One technology for de-watering consists of an ESP recirculation system, which locates the ESP string below the perforations to minimize gas interference in the pump and maximize fluid drawdown. In a cooperative project with Marathon Oil Co. at the Indian Basin Unit in New Mexico, Centrilift has proven the capabilities of recirculation systems. Today the technology is being applied all over the Permian Basin and in other producing regions of North America. In fact, new applications such as CO 2 floods are further expanding the use of recirculation ESP sytems. 56

2 Indian Basin Field Production Challenges The Indian Basin Field was discovered in 1962 and has produced over two trillion cubic feet of gas and 26 million barrels of oil from the Upper Pennsylvanian Cisco and Canyon formations at approximately 7,500 ft (2286 m). The original gas column was more than 1,000 ft (305 m), but by 1991 gas production had declined about 60% from the peak of 240 million cubic feet of gas daily. Nearly half the field was shut in due to water encroachment. By the mid-1990s, Marathon recognized that removing large volumes of water from the reservoir was the key to reversing the production decline. Removing the water reduces reservoir pressure, which mobilizes gas trapped at high pressure by the encroaching water. Marathon employed high-volume ESPs to produce the water, as well as a variety of ESP technologies to produce more water, increase pump efficiency, and reduce failures in the challenging artificial lift environment. Initial completions used high-volume ESPs in 7-in. casing, and these systems typically could lift up to 4,000 barrels per day of fluid up the tubing. In addition to the gas zones, an oil target deeper in the Upper Pennsylvanian was identified, and completions from that zone achieved rates up to 1,000 barrels per day. However, with the high fluid rates in each zone, wells usually targeted either oil or gas. To produce both simultaneously, a larger ESP system was designed that required in. casing, but could produce up to 10,000 barrels per day of fluid through the tubing including up to 10 MMCF gas and 1,000 barrels of oil daily. These wells proved the benefits of increased fluid withdrawal, but the cost associated with such large diameter holes limited the number that could be drilled. Production Zone Pump Size like a normal installation Standard options can be used Recirculation System Intake for both pumps Seal Conventional Motor Recirculation Tube Gas Well De-watering Solutions In an effort to achieve higher fluid rates in less costly 7-in. cased wells while eliminating high gas rate pump interference, Marathon turned to Centrilift for solutions. Marathon s previous attempts to de-water potentially high-rate gas wells to increase overall gas production employed shrouded 4.5-in. outside diameter motor ESP systems Figure 1 Recirculation pump application Tubing Clamp 57

3 Centrilift ESP Recirculation Systems Aid Gas Well De-watering Figure 2 Recirulation Tube set below the perforations to avoid gas interference with the pump. The shroud directs fluids around the motor for cooling. The motor was re-rated above the standard nameplate horsepower due to the hydraulic requirement necessary to lift about 4,500 barrels of water per day. However, even after increasing the horsepower rating, these shrouded systems limited the amount of water and gas production because of size and horsepower restrictions. Based on the needs at the Indian Basin Unit, Centrilift drew on its experience with smaller recirculation systems to develop a 500 series recirculation ESP system designed for 7-in. casing. This system increased available motor horsepower, resulting in increased fluid drawdown and higher oil and gas production, longer run times, and lower power and equipment lease costs all leading to increased well profitability. Prior to the Indian Basin installation, recirculation systems were designed for 5.5-in. or smaller cased wells. The equipment strings included 4-in. outside diameter pumps and 4.5-in. outside diameter motors in conjunction with a recirculation tube. Recirculation systems are designed to take a portion of the produced fluid, before much pressure has been added, and recirculate it past the motor set below the perforations. The fluid is directed through a custom-manufactured narrow cross section flat tube, down past the motor and released to flow up over the motor and back into the pump intake. The fluid is recirculated via a small, auxiliary ESP 58

4 Figure 3 Low-profile recirculation tube requiring only minimal additional horsepower. This approach allows higher horsepower ESP systems for higher volume wells and the most cost-effective ESP system for lower volume wells. Prior to Marathon s interest at Indian Basin, recirculation systems were confined to applications with relatively low flow volumes and small horsepower requirements. As flow and horsepower requirements increase the flat profile recirculation tube provides the most cost-effective way to use larger ESP equipment and achieve the necessary flow/horsepower output. New Recirculation System Recirculation systems include a standard ESP pump, an additional, smaller ESP pump with a recirculation port in the modified pump head, recirculation tubes, and assembly connections (Figure 1). The recirculation pump is the intake for the unit and ideally consists of as few pump stages as possible to minimize the horsepower added to the overall system. The recirculation pump is staged to generate sufficient total dynamic head to overcome frictional losses down the recirculation tube. The recirculation stage must be volumetrically larger than the main pump stage to produce enough excess fluid to be circulated past the motor for cooling. The recirculation tube, as used in previous installations with 5.5-in. cased wells with 400 series ESP equipment, was a flat profile, custom-made conduit as shown in Figure 2. Other low-profile tubing has been designed and utilized in low-clearance situations (Figure 3) For Marathon s total flow and horsepower requirements, Centrilift s 513 pumps and 562 motors were incorporated into a low clearance recirculation system for 7-in. casing. In addition, Centrilift s AutographPC sizing and simulation software was modified to incorporate recirculation system sizing. AutographPC software was used to size the application as in a standard ESP installation. The recirculation pump program was then used to select a suitable recirculation pump stage along with the number of stages, based on the required recirculation tube length and estimated recirculation tube flow area. Case Studies Well IHU-10 Marathon was producing the IHU-10 well with a shrouded ESP system comprised of a 145-stage 538 series pump, rated at 4,700 barrels of fluid per day; a 400 series seal section; and a name plated 288 hp 450 series motor re-rated to 350 hp with a 5.5- in. motor shroud to direct the well flow past the motor. The system was set below the perforations to take advantage of annular gas/liquid separation. The ESP unit was producing 6.6 million cubic feet of gas, 312 barrels of oil and 3,803 barrels of water per day with the ESP operating on a variable speed drive (VSD) at 64 Hz. However, the aggressive re-rating of the 59

5 Centrilift ESP Recirculation Systems Aid Gas Well De-watering motor horsepower resulted in system failure after 212 days due to a grounded motor. A newly designed recirculation system for 7-in. cased wells was installed to offer higher horsepower and longer run times. Since this was the first installation of the new design, Marathon decided not to be overly aggressive and sized the unit to achieve production levels comparable to the shrouded system. The primary driver for the installation was the potential for lower monthly operating costs. The new system included a 191-stage 513 series main pump rated at 3,500 barrels of fluid per day; a five stage 513 series recirculation pump rated at 6,100 barrels of fluid per day; a 513 series seal section; and a 380 horsepower 562 series motor. The unit also was equipped with pressure/temperature sensing instrumentation with pump intake pressure and motor internal operating temperature parameters. The recirculation tube was 48 ft (14.6 m) long. Following installation of the new system, the well was producing 5.7 million cubic feet of gas, 306 barrels of oil and 2,714 barrels of water per day at 56.7 Hz. Marathon successfully achieved its overall objectives with system run life reaching 375 days and a reduction of about $2,500 per month in operating expenses. Well IHU-20 At Marathon s IHU-20 well, the shrouded system consisted of a 163-stage 513 series pump rated at 4,100 barrels of fluid per day; a 400 series seal section; and a name plated 288 hp 450 series motor re-rated to 350 hp with a 5.5-in. motor shroud. This unit produced about two million cubic feet of gas, 293 barrels of oil and 4,366 barrels of water per day at 62 Hz on a VSD. The system failed after 196 days due to a grounded motor. Two previous identical units also had to be pulled from the well due to grounded motors, which was a result of aggressively re-rating the 450 series motors. The recirculation system was installed with a 207-stage 513 series main pump rated at 6,100 barrels of fluid daily; a nine- 60

6 stage 513 series recirculation pump rated at 10,000 barrels of fluid per day; a 513 series seal section; and a 570 hp 562 series motor. The unit was equipped with pressure/temperature sensors and the recirculation tube was 64 ft (19.5 m) long. Following installation of the recirculation system, production jumped to 3.5 million cubic feet of gas, 639 barrels of oil and 6,680 barrels of water per day at 60 Hz operation. This was the third recirculation system installation in the well. The two previous units yielded similar production totals, but equipment problems unrelated to motor operating temperature limited operation. This recirculation system ran for 210 days and the longest running recirculation unit in the well achieved 484 days run time. Marathon s drivers for this well were different than for IHU-10. This well had additional potential pay behind pipe, which could be brought on stream. However, with the previous shrouded units already working over capacity, there was no opportunity to open the additional productive interval. Opening the new pay zone not only meant greater oil production but also a greater quantity of water production and a significant increase in hydraulic requirement. The high horsepower range of the 500 series recirculation system provided the necessary capabilities to open the additional pay. Although operating expenses increased at the well, this was more than offset by daily oil production that was almost doubled and daily gas production that increased by a factor of 1.5. Conclusions The recirculation ESP system technology has been so successful at Indian Basin that other operators in the Permian Basin are leveraging the technology to de-water natural gas fields. The Permian Basin is increasingly shifting to natural gas production and a significant number of older oil fields are currently on CO 2 flood both applications for recirculation ESP systems. CO 2 floods introduce too much gas into a standard ESP, but the casing is not large enough to employ a shrouded system. Therefore, the recirculation system is the only option to locate the ESP below the perforations to eliminate gas interference in the pump. This solution offers operators the ability to optimize production, increase ESP run life and lower operating costs. The new Centrilift Centurion line of pumps with larger shaft sizes will further push applications of the recirculation systems to the 8,000 barrels of fluid per day range. One technical challenge that must be addressed is the recirculation tube. Additional length is necessary to increase the fluid capacity and the current tubes do not provide enough fluid over that distance to adequately cool the motor. However, this modification to the recirculation tube can be addressed as need arises for larger recirculation systems. About the Authors: Joe Lisenbee has 21 years of experience in the ESP industry and has been with Centrilift for 15 years. He has worked as a field service supervisor on both domestic and international assignments and has been in the marketing group in the Permian Basin for 10 years. He served for five years as the Marathon Indian Basin Alliance Coordinator and is currently an Operations/Marketing Account Manager in the Centrilift Midland office. Steve Fulwider has been with Centrilift since He was a field service technician until September 2003 when he was named Marathon Indian Basin Alliance Coordinator. Jeff Knight is a Senior Applications Engineer with Centrilift. He earned B.S. and M.S. degrees in petroleum engineering from the University of Missouri- Rolla and has over 17 years of experience in the oil industry. Acknowledgment The authors would like to thank Marathon for permission to publish this work. This article is partially based on paper SPE Custom Recirculation ESP System in 7-in. Casing Increases Indian Basin Gas Well Production. 61