By Jake Kardash, Technical Specialist-Engineering & Stephen Lund, Technical Specialist, Mountain Coil Tubing
Introduction
Completing horizontal wells with the plug-and-perf and the ball-and-seat styles of completion has remained one of the most common methods for stimulating tight oil and gas or shale formations across North America. The use of coiled tubing, coupled with a milling bottom hole assembly, is often used to remove these plugs and restrictions after fracturing operations are completed.
The costs associated with the milling operations are often scrutinized by the operators, as they can increase dramatically if the strategy for each aspect of the milling process is poorly designed. Implementing a technical approach to the optimization of treating fluids throughout the entire milling process has resulted in significant improvements in plug milling efficiencies over traditional methods.
Current Industry Practice
Many of the industry rules-of-thumb and best practices have their roots in the jointed pipe drilling and workover world at a time where 99 per cent of all wells worked on were vertical. The physics of cuttings transport in a world where circulation had to stop every time another joint of pipe was added to the drill string dictated what was pumped. Fluid viscosities were raised using polymer gels to suspend the solids long enough to connect the next joint of tubing before circulation could resume.
This method continued to be successful with the advent of horizontal wells since pipe rotation and the turbulence around each tool joint enabled the cuttings which settle out of the flow stream along the long horizontal section to be re-entrained. Even though workover equipment evolved from jointed pipe rigs to coiled tubing units over the next decade, the rules-of-thumb stayed the same. The need for viscosity to support cuttings during pipe connections disappeared, but the gel did not.
The ability to re-entrain cuttings along the horizontal section of the well by pipe rotation was lost (coiled tubing cannot be rotated) so it was replaced by mechanical agitation in the form of wiper trips. In a wiper trip, the bit is regularly pulled back to the heel of the well to fluff up the solids lying on the low side of the well. These conventional methods of plug and ball seat removal often use relatively large amounts of chemical.
Despite the large number of chemicals used to aid in debris removal, the wiper trip is still invariably used to ensure sand and debris created from milling are transported to the vertical section of the well. A wiper is the act of pulling the coiled tubing from the working depth in the horizontal to near the vertical section of the well. This is often done after a prescribed number of plugs or ports have been removed, this can vary from two to five plugs or seats, depending on the experience of the service provider. This is time consuming and significantly reduces the remaining fatigue life of the coiled tubing string.
The performance and repeatability of this conventional method have been lacking since the amount of time and cycled meters needed to perform a mill-out can vary greatly depending on the amount of wiper trips made throughout the operation.
Fluid Rheology Control System (FRCS) Optimized Approach
In an effort to reduce the number of stuck pipe incidents, reduce total job time, and reduce chemical consumption, a number of companies during the last 15 years have focused on the optimization of debris removal as it was noted that improving debris transport was the area that could yield the largest increase in milling efficiencies.
Mountain Coil Tubing’s answer to the debris optimization question is the fluid rheology control system (FRCS). The FRCS unit combines a chemical injection system and a field laboratory that is tailored specifically for coiled tubing. The unit aims to improve chemical dosing for coiled tubing operations in three key areas, consistency, verification, and optimization. This allows the unit to create a consistent working fluid, even when re-circulated. By controlling the rheology of the fluid, a specific Reynolds Number to be targeted downhole which allows for the debris to be return to surface without the use of gel or wiper trips.
Figure 1: MCT Large Convectional Unit
Case Study
On a recent six well post-frac bridge plug milling project the FRCS unit was used to increase efficiencies and performance of the operation. Mountain Coil Tubing sent two large conventional 2 3/8” coiled tubing spreads. One FRCS unit with trained fluid technicians was utilized to successful supply both spreads with continuous precision chemical dosing and fluid testing.
The project, targeting the Duvernay formation, contained 245 plugs, with a total of 33,196 m drilled, completed with 5.5” casing from heel to toe, with an average WHP ranging from 25-35 MPa. One well, measuring 5,809 m to TD and containing 54 plugs was successfully milled to PBTD without the use of a wiper trip. This was while averaging 30 minutes per plug, including milling time, and wash time to the next plug as tagged.
A second well, measuring 5,465 m to TD and containing 34 plugs was successfully milled out to PBTD, with a total in-well operating time of only 38 hours. This was while only pumping 29 litres of gel and not performing one wiper trip.
When comparing previous campaigns the client has stated this campaign using Mountain Coil Tubing and FRCS technology was able to mill out twice the number of plugs in half the time, with half the number of cycled meters, and a substantial reduction in chemical costs, resulting in a substantial reduction in the overall CT completions costs.
Conclusion
The drive to reduce the costs related to coiled tubing milling operations has led to a re-evaluation of current industry practices and its understanding of debris removal in horizontal wellbores. It has been proven that by utilizing the FRCS unit, debris can be successfully removed from the wellbore without relying on gel and wiper trips. It is clear that the implementation of this optimized approach is successful in improving the efficiency, and reducing the costs of mill-out operations.
Focusing on fluid rheology, and the working fluids Reynolds number resulted in a reduction of operating time, chemical consumption, cycled meters and operating time, greatly reducing the overall cost of coiled tubing in well completions.
For more information contact Mountain Coil Sales and Engineering.
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