Wayne Baud
Shell Western E&P
Houston
Bernard Eastlund
Production Technologies International Inc.
Houston
Installation of a downhole heater in a well eliminated production interruptions caused by downhole paraffin and hydrate buildup, exacerbated by cooling effects of C02.
The well is one of 20 operated by Shell Western E&P Inc. in the C02-flooded portion of the South Crossett Devonian Unit (SCDU) in the Permian basin of West Texas.
Prior to heater installation, the well's production routinely choked off in 2 days unless the downhole impairment was mechanically removed. Mechanical cutting established that the paraffin and hydrate buildup was in the upper 2,000 ft of the 5,300-ft well.
The fluid exit temperature, depending on the choke setting, fluctuated between 14 and 30' F.
CONVENTIONAL TREATMENTS
Several conventional techniques were unsuccessful in treating the problem in this well. Results of these techniques were as follows:
- Cutting was required 6 days/week when the well flowed 24 hr/day. The addition of three daily 1-hr shut-ins to melt hydrates reduced cutting to 3 days/week. But cutting this frequently on this flowing well was undesirable from both a safety and manpower perspective. The shut-ins also reduced oil and gas production.
- Hot oiling was an alternative to cutting but was undesirable because of a packer in the production string. For proper treatment hot oiling would have required a pulling unit. Otherwise, failure to circulate could cause formation skin damage caused by pumping paraffin into the perforations.
- Back pressure on the tubing can reduce the buildup by increasing the exit temperature, reducing velocity, and maintaining the C02 in a saturated liquid state closer to the surface. But this method was not effective and reduced production.
- A bladed plunger was developed to keep the tubing clean. Mechanically, this operation succeeded, although it reduced production because of the required 3 hr/day shut-in to allow the plunger to fall and reset.
- An intermitter was used to automate shut-in in conjunction with mechanical cutting. The intermitter reduced the frequency of cutting by allowing hydrates to thaw, as mentioned previously.
HEATING
Based on simulation models, the cooling from gas breakout and expansion is equivalent to 30-60 kw. In principal, the uniform application of the same power level to the upper 2,000 ft of tubing should compensate for the cooling and prevent paraffin buildup.
The Paratrol electric tubing heater (Fig. 1) makes the production tubing and casing into a heating circuit. Uniform power is applied over the heated length.
Electric current is safely introduced into the tubing by a patented E-sub that electrically isolates the wellhead and landing joint from the rest of the production tubing. The electric current flows along the tubing to the bottom of the zone of interest. At this point, a contactor in the tubing string makes a solid electrical contact with the casing, completing the circuit. In the SCDU well, the contactor was installed at 2,000 ft. Three insulators, installed on each tubing joint between the E-sub and contactor, prevented inadvertent contact between the tubing and casing.
TUBING/CASING ANNULUS
Because the tubing/casing annulus must be free of materials that can conduct electricity, including fluids such as salt water, inhibited lease crude was circulated to occupy the well's annular space.
Upon start-up, an electrical short occurred at about 270 ft from the surface. It appeared that a tubing leak may have caused the short. After replacing the failed tubing, a second short occurred in a new location. Both shorts were above and next to an electrical insulator. Examination showed that paraffin with iron sulfide (FeS) inclusions had built up on top of the electrical insulators.
The production string was pulled and the casing cleaned of paraffin and FeS. After reinstallation of the production string, diesel was circulated in the annulus. Since then, the system has operated for 8 months without mechanical problems or impairment. A similar unit operated in a North Crossett (Devonian) Unit well for 3 1/2 years without a mechanical problem.
POWER CONDITIONING
The electrical connection from the control panel to the E-sub is through a pressure-isolating, feed-through assembly in the wellhead side outlet that provides access to the tubing-casing annulus. The E-sub cable runs about 30 ft from the feed-through assembly to the top of the E-sub.
Power is from a 90-kva, single-phase, 60-hz transformer that steps the available 480 v field voltage down to 22O v. The unit's operating point is 220 v and 225 amps with a power factor of about 0.8.
A total of 39.6 kw was uniformly applied over the upper 2,000 ft of the well. Exit temperatures have been maintained at roughly 45 F.
PRODUCTION DATA
The well's production (Fig. 2) showed an initial increase of 55 b/d. But after 2-4 months the production decreased, possibly because of:
- Reduced offset C02 injection
- Attempts to reduce electricity consumption
- Formation of paraffin or hydrates from the contactor to 400 ft below the contactor.
We feel that reduced tubing flowing pressure has pushed the problem deeper into the well bore.
BIBLIOGRAPHY
Bosch, G., Schmitts, K.J., and Eastlund, B.J., "Evaluation of Downhole Electric Impedance Heating Systems for Paraffin Control in Oil wells," IEEE Transactions on Industry Applications, Vol. 28, No. 1, January/February 1992.
Eastlund, J., Grisham G.R. HI, Meek, D.L., Schmitt, K J., and Anderson, D.C., "New System Stops Paraffin Buildup," Petroleum Engineer International, January 1989.
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