Producing hot, high rate, high pressure wells can subject tubing threads to Producing hot, high rate, high pressure wells can subject tubing threads to compression and tensile forces that exceed the scope of current published data and specifications.

Mobil Oil Indonesia, contractor to Pertamina, Indonesia's National Oil Company initiated the development of test apparatus and test procedures to simulate actual downhole stresses on tubulars. Mobil's intention was to fill the gap in the published data and to determine the cause of thread leaks in Arun wells. It was postulated that the leaks were the result of extreme compression and tensile forces acting on the tubing threads. Wells completed in the Arun Limestone are capable of producing 250 MMSCFD at well-head pressures and temperatures of 4000 psi and 347 degrees F. Forces on the tubing string varying from 350,000 lbs compression to nearly 500,000 lbs tension during pressure and temperature reversals between conditions of shut-in, producing and stimulating.

The test apparatus and procedure can detect thread leaks while subjecting test specimens to:

  1. 7500 psi internal pressure

  2. Compression and tensile loads up to 500,000 lbs.

  3. Temperatures up to 350 degrees F

  4. Quenching of internal pin surface such that the temperature is reduced from 350 degrees F to 150 degrees in

Application of the equipment and test procedures confirmed the failure mechanism and helped evaluate other thread designs for use in the harsh Arun wellbore environment.

Introduction

Tubulars, worldwide, are being subjected to a more severe environment as the economics of oil and gas production permit the industry to produce from formations that are deep, hot, sour and high pressure. Producing from these horizons introduce tubing connections to compression and tensile forces that exceed the range of currently published data.

Mobil found it necessary to fill the gap between the conditions of published tubular performance data and actual producing conditions. Mobil published tubular performance data and actual producing conditions. Mobil Oil Indonesia operates the Arun Field in Northern Sumatra. High annular pressures in Arun wells suggested that tubing leaks were compromising an pressures in Arun wells suggested that tubing leaks were compromising an otherwise competent completion system. It was postulated that abnormally high wellbore temperatures and pressures were exerting forces on the tubing that exceeded the strength of the connections. Test apparatus and procedures were developed to stimulate actual downhole conditions. Use of procedures were developed to stimulate actual downhole conditions. Use of the equipment and techniques permits detection of thread leaks while subjecting test specimens to:

  1. 7500 psig internal pressure

  2. Axial loads from 500.00 lb tension to 500,000 lb compression.

  3. Temperatures up to 350 degrees F.

  4. Quenching of the internal pin surface such that the temperature is reduced from 350 degrees F to 150 degrees F in 30 seconds.

Test Apparatus

Figure 1 is a schematic illustrating the test equipment. A 10-¾ inch I>D. environmental test chamber (1) is mounted via an adapter flange (2) to a load cell (3). The chamber is rated at 10,000 psig burst pressure. The load cell is in the form of a hydraulic press capable of exerting 500,000 lb compression or tension. Two side outlets are fitted with blind hubs (40 and tapped for ½ inch NPT for recording chamber pressure and providing a relief valve.

The test specimen is made up of two 7-inch, 35 pounds per foot nipples or pup joints (5) joined by a coupling (6). The specimen has had the threads pup joints (5) joined by a coupling (6). The specimen has had the threads cut that are to be evaluated. A load cell crossover (7) and a 10-¾ inch × 7 inch casing hanger (8) has been attached to the test specimen. The specially fabricated hanger has been tapped to provide:

* Water injection to two spray nozzles (9) that direct a cold water sprayon to the seal area of the connection at each end of the coupling.

* Nitrogen injection (10) to pressures up to 7500 psig. The pressure inside the test specimen is also recorded at this point.

* A vent pipe to extract water and nitrogen and provide a means of stabilizing the internal pressure while injecting water.

* Temperature recording (11) from a thermocouple attached to the inside of the top nipple near the lower seal area.

* Two high pressure monitoring lines to record pressure from holes (12)tapped in each nipple opposite the sealing area of the connection. This monitors leakage within the connections.

The specimen is attached to the load cell piston (13) inside the chamber. The annular volume between the specimen and the chamber is filled with oil. The purpose of the oil is to effect homogeneous heat transfer from the chamber exterior to the test specimen. The specimen assembly is held in position by a seal clamp (14) and hold-down ring (15). Heat is provided by position by a seal clamp (14) and hold-down ring (15). Heat is provided by twelve electrical heater strips that are wrapped around the exterior length of the environmental chamber.

Figure 2 illustrates the peripheral attachments to the environmental chamber. Mobil used nitrogen to test the connections. A high pressure gas pump (A) and nitrogen gas supply (B) was used to pressure the test fixture. pump (A) and nitrogen gas supply (B) was used to pressure the test fixture. The spray nozzles are fed by a high pressure pump (C) capable of delivering 12 to 14 gpm against the internal test pressure. The water pump was manifolded (C) to circulate water through a 1/3 inch choke or to pump water to the fixture. A vent line (D) was provided to control the bleed-off of water and excess nitrogen pressure during the water injection cycle. The temperature control panel (E) regulates the heater strips. A strip recorder (F) monitors temperature and pressure exerted on the specimen. A relief valve (G) on the chamber upper port regulates the chamber annular pressure. pressure.

Arun Test Apparatus

The test facility is designed to simulate wellbore conditions. Table 1 shows the Arun wellbore parameters. Flow rates of up to 250 MMSCFOD through 7-inch tubing cause flowing wellhead temperatures to approach the 350 degrees F bottom hole temperature. Wellhead temperatures have been recorded at 342 degrees F at flowing pressure of 3500 to 4500 psig.

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