Download a PDF Version of this case study: Bahrain Vertical Well 480.pdf or head to Downloads
SPE articles can be found at: The Society of Petroleum Engineers

MxL 50 KVA INDUCTION HEATER SYSTEM

1. Background
SPE paper 68220 THERMAL STIMULATION OF RUBBLE RESERVOIR USING DOWN HOLE INDUCTION, by V. C. Babu Sivakumar, Bahrain Petroleum Company was presented in March of 2001.

Initial production at 50 BOPD, for well 480, commenced on gas lift in 1992 and declined rapidly to 15 BOPD. Remedial workovers temporarily increased production to as much as 80 BOPD only to drop back to about 10 BOPD within a month with a water cut of 60%. Production of the 16 API, (350 cP at 40 °C) oil had dropped to less than 10 BOPD at installation of the Induction Heating System.
After four years of production on gas lift a beam pump was installed and production continued to decline. Well completion is 7 inch perforated casing, 3 ½ inch tubing with 2000 PSI wellhead and gas produced through 2 inch casing vent to gathering line with several hundred PSI back pressure.

Oil production is from the middle Cretaceous Mishrif formation with core data showing 19 to 24% porosity and permeability of less than 10 mD. It has been reported that production tests indicate the effective permeability to be much higher. The upper 26 feet of the forty foot production zone logged unbroken high resistivity whereas the lower section is variable.

2. Installation
The system had been shop tested and shipped before the field modifications were made to system number one and accordingly modified circuit boards were installed in the field and software altered to update the system. The downhole instrumentation was installed in systems one and two on the understanding that while it was working in our shop tests it was considered experimental. The updated downhole circuit board and the PCU modifications failed to pick-up the signals from downhole. The other
modifications, to ignore the fluctuations in flow line readings, to include two thermocouple readings and to include a communications link were implemented however the external communications (telephone line) was not installed.

During March of 1998 Induction Heating System PCU00002 was installed in the Rubble Limestone, Mishrif formation, of well 480 at a depth of 1660 feet. A thermal gradient of one degree F per 100 feet and a bottom hole temperature of 98 0F were determined from tool resistance readings taken during run-in and prior to start-up. The available log information reported a value of 114 0F indicating that production had introduced 16 degrees of cooling, in the wellbore, over six years of production.

Run-in of the tubing and tool was slower than we are accustomed to largely due to manual spooling and banding of the ESP cable. Resistance run-in readings were taken at 500 foot intervals which was adequate to determine a thermal gradient however as standard practice we now take resistance, temperature sensor and pressure readings every ten joints. Number 4 flat lead sheathed EPDM galvanised armor ESP cable was installed with a lead splice at the tool pig tail and a taped and armored splice at the feed through.

3. Operation
Start-up calibration was completed based on the inductor winding resistance using a Fluke bench top meter capable resistance readings to three decimal places and lead resistance cancellation settings. Forward and reverse tool resistance readings were taken and then averaged to ensure that any galvanic influences were cancelled as we noted the depressed temperature values and took all precautions to ensure confidence in the readings. The inductor winding resistance values when combined with ESP cable , surface wiring and their respective ambient temperature compensations give an average temperature over each section of the tool.

At start-up the maximum temperature was set to 110 °C and 23 KVA of power was needed to bring the tool temperature up to the set point. Within a day the flow had increased two fold and the amount of power used started to drop. By the end of the third day the power had reduced to 12 KVA and the production had increased seven fold.
Graph “Well480” shows the production before and after the Induction Heating system was installed.

During start-up kill fluid and then produced fluid, including tubing gas, was discharged to a sump formed by about a fifteen metre diameter berm. Once the kill fluid had been produced back the slugs of water and oil were quite distinct as they belched from the discharge pipe but as the reservoir temperature increased the “slugging” nature of the oil and water components diminished considerably but the bursts of gas continued with perhaps some loss of vigor. The change in flow characteristics was evident on the flow line meter log and we continue to see that pattern in most wells.

On site training was conducted over the first four days wherein approximately twenty operators and supervisors were trained which resulted in the system starting and stopping many times a day.

A workover in November 1999 was needed as the well had been shut-in due to tubing leaks The Heater and tubing string was pulled and we examined the heater and found it to be in excellent condition with no erosion or corrosion and with the same electrical values as on the original installation. Several joints of tubing were replaced and the system run back in the hole and upon hydro testing the tubing another leak was created so the tubing and heater was pulled and new tubing was installed. During the re-installation the ESP cable was squeezed in the slips whereupon it was examined by the Reda ESP cable representative and the electrical characteristics measured and since all indications were that it had not been permanently damaged the installation carried on to completion.

After a few days of operation a short developed in the downhole system and that section was turned off. Measurements taken were inconclusive but it was thought that a short had developed where the cable had been squeezed. Since replacement cable was not readily available it was decided to carry on with heating with the fuse for that conductor removed. Operation continued that way until February of 2000 when the second conductor failed and was isolated in the same fashion however production was maintained at 45 to 60 BOPD with the remaining section.

4. Observations

Babu Sivakumar conducted several experiments and found that the production declined quickly when the power was turned off and inreased rapidly as the power was restored.

Previous Page: METHANE HYDRATE RESEARCH