The efficiency of a thermal enhanced oil recovery project with surface steam generation can be significantly increased by using insulated tubing in the injection wells. In order to evaluate the performance of various insulated tubulars it is necessary to obtain detailed temperature measurements and accurate heat loss data under actual in-field conditions. A system to provide this information has been developed and is in operation at the Aberfeldy steam pilot near Lloydminster, Saskatchewan, Canada.
Temperature measurements are made using thermocouples inside and on the outer wall of the injection string; on the outside of the casing, and in a set of three 25 mm (1 in) ID thermowells attached to the casing. In addition, thin film heat flux sensors are bonded directly to the wall of the injection string. A probe system was designed to measure circumferential temperature variations in the thermowells at depths down to 100 m. This makes it possible to obtain detailed axial temperature profiles. Anticipated hot spots on an insulated joint will be detected in this manner. All of the data is recorded on a datalogger and detailed analysis is performed on a computer system.
To date a short test has been carried out using bare 60 mm (2–3/8 in) injection string tubing. This bare string provides data for comparison with insulated strings. High resolution radial temperature profiles were obtained during this test. Variations in heat loss from the string as functions of time and operating conditions have also been successfully monitored. Heat losses from the string during initial start up on the order of 1.0 Kw/m (1050 Btu/hr-ft) were observed with the heat flux sensors. This is in good agreement with the expected heat loss. The heat flux sensors make it possible to both simplify and improve the determination of insulated tubular thermal performance.
Heavy oils are an increasingly important resource in meeting American energy requirements. Much of this oil is at depths where conventional steam drive technology is inefficient. Bare tubing is normally used in shallow steam injection wells. In deeper wells, however the heat loss from bare tubing to the surroundings, primarily by radiation, becomes excessive. Not only is the injection string longer, but steam pressure and thus temperature is much higher. Under these conditions use of insulated tubing can result in substantial fuel savings as well as the delivery of higher quality steam to the reservoir.
In 1978 Project DEEP STEAM was begun by the Department of Energy to develop the technology needed for the recovery of heavy oils (< 18 ° API) at depths below 750 m (2500 ft). One of the approaches being pursued is the reduction of wellbore heat losses in systems with surface generators. This entails testing of commercially available insulated tubulars and high temperature packers. Most of this work has been done in an above-ground test facility constructed by the Thermal Systems Group of General Electric in Tacoma, Washington. In these tests, tubulars and packers were subjected to pressures of up to 14.5 MPa (2100 psi) at temperatures of 378 °C (640 °F).