After a frac job has been performed, the well must be cleaned out to remove any proppant remaining in the wellbore. Using coiled tubing (CT), or continuous tubing as it is sometimes called, is an excellent way to get the well cleaned up in an underbalanced condition and ready to go on production.
Coiled tubing, although generally more expensive in Russia than other cleanout methods, provides a number of advantages for achieving improved well productivity. Using nitrogen mixed with brine, the well can be cleaned out in an underbalanced state, which improves solids removal and reduces formation damage. Afterwards, the well can also be lifted using only nitrogen for a period of several hours. This cleans the well up even further and provides valuable production data for pump sizing and well performance. Finally, coiled tubing cleanouts typically are completed in two to three days, several days faster than with non-CT methods.
CT in Nyagan
TNK-Nyagan began using coiled tubing for post-frac cleanouts for the first time in 2006. The PU recently completed a successful 28-well program with Schlumberger and the contract has been extended through year-end for further work. The majority of wells fractured by TNK-Nyagan are put on production using an electrical submersible pump (ESP).
One of the major problems experienced by the PU has been the necessity to run one or more "sacrificial" ESPs following the non-CT cleanout of a frac job. This is usually attributed to the fact that a non-CT cleanout does not sufficiently remove all proppant from the wellbore and it is then produced through the ESP, severely limiting its run life. Utilizing coiled tubing and nitrogen for underbalanced post-frac cleanouts significantly improves the solids removal process and reduces the chance of an ESP failure due to proppant in the production. Preliminary results from the TNK-Nyagan frac program this year, demonstrated in Fig. 1, have shown the ESP failure rate to be significantly less for wells cleaned out in this manner.
CT Benefits
Preliminary analysis of the TNK-Nyagan frac program indicates that wells cleaned out with CT show about 15 percent higher average volume rates than non-CT wells, but with approximately 30 percent higher total job cost. However, considering all factors including lower probability of ESP failure in CT wells, average cost per barrel is about 5 percent less for wells cleaned out with CT than non-CT wells. Utilizing coiled tubing for underbalanced cleanouts is an excellent way to insure the best possible conditions for returning a well to production after fracturing operations. This Hull and Machinery Insurance, however, comes at a price which must be offset by improved productivity, increased ESP run life and a faster turn-around time to production than non-CT methods. The challenge is to manage the process so that only the highest-potential wells are selected as candidates for CT cleanouts, in order to provide the greatest return on investment.
CT Technology
Coiled tubing has been in use for some time in the oil and gas industry but it is a relatively new technology in Russia. CT ranges in size from as small as 19 mm (0.75") diameter to as much as 114 mm (4.5") for use in special applications. For TNK-Nyagan, Schlumberger is using a 3,900 m reel of 38 mm (1.5") diameter CT, which is a more typical size.
The above photo shows some of the basic components of the coiled tubing fleet. The coiled tubing is contained on a large hydraulically-operated, truck-mounted reel, with all controls and gauges required to operate and monitor the equipment located inside the cab. The coiled tubing is spooled into the well over a tubing guide, or "gooseneck", and through an injector head which provides the thrust to push the CT into the well against pressure or wellbore friction. The CT then passes through the blowout preventer stack (BOP), past the wellhead and into the wellbore tubing or casing. The bottom hole assembly (BHA) at the end of the CT typically consists of a conventional nozzle and check valve, through which fluid and/or nitrogen are pumped. At the surface, returns are routed from the well through a choke line and manifold, and into a return tank.
In a typical Nyagan post-frac cleanout, CT is run into the well with periodic circulation of brine fluid until the proppant remaining in the wellbore is tagged. Subsequently, nitrogen is injected into the brine fluid at surface and this nitrified brine is pumped through the coiled tubing while cleaning out the well to the desired depth. The combined fluid and nitrogen rates are calculated to provide an underbalanced condition of 50 atmospheres or more at the producing formation. Samples are taken at periodic intervals and the return fluid checked for proppant and/or formation sand. Gel sweeps are added to the nitrified brine in order to remove all solids and circulate the well completely clean.
After the well is clean, it is "kicked off" for a six-hour flow test using only nitrogen as the lifting mechanism. Sampling is conducted at 30-minute intervals and data recorded as to pressures, well influx (cu. m per day), oil/water cuts and sand content. The nitrogen pumping rate is based on modeling for the first two two-hour flow periods, and then adjusted to optimum for the final two-hour flow period depending on actual results.
After the kickoff is complete, the nitrogen pumping is stopped and the well is allowed to equalize. The coiled tubing is then run in the hole to tag bottom and confirm the desired depth is still in effect. If some fill is found, it may be necessary to repeat the cleanout procedure using nitrified brine and gel sweeps. Finally, the coiled tubing is pulled out of the well while spotting kill fluid in the tubing string. The CT equipment rigs down and moves to another well while a workover rig moves in behind to run the final completion and put the well on production.
