Downhole options developing around coiled tubing

Slimhole, fishing, casing remediation operations

Kevin McMillin
Engineering Editor

A number of factors are driving decisions to utilize coiled tubing applications. They are: portability, a small footprint, the presence of pre-existing tubing, and under-balanced drilling situations.

In contrast to the historical industry practice of permanent rig installation of large service equipment, the relative ease of mobilizing, setup, and operating portable coiled tubing equipment makes it a very viable alternative for use in slimhole environments.

The small footprint relates to the physical size of the equipment involved and rig/platform space available. A lot of offshore situations exist where free space to set up temporary equipment is at a minimum. Tight positioning of equipment may pose operational problems, not to mention worker safety concerns.

The recent operator practice of utilizing minimal fixed platforms and liftboats has also created a need for smaller service equipment. The positive economic benefits of these platforms guarantees them to be around for a while. Redesign of existing service equipment or remote control system technology will need to be developed to address the space limitation.

Utilizing pre-existing tubing has advantages. Wells do not have to be killed before remedial work is performed on them. Not taking the time to pull pre-existing tubing saves time and money.

A conveyance system capable of operating inside existing tubing is an added plus when it comes to well control issues. The under-balanced pressure situation is required in some producing fields where formations are highly susceptible to wellbore fluid damage. This poses unique engineering problems. The use of coiled tubing allows for continuous well pressure control in the treatment of these highly permeable formations. The conventional conveyance method for workover tools resulted in limited well control in tubing pulling and running operations. Coiled tubing has eliminated this problem.

Coiled tubing drilling

Coiled tubing drilling (CTD), once considered the "new drilling frontier", has gone through a very rapid aging process. Now that the functional and economical benefits have been evaluated, companies are looking back at past successes and failures to try and get more from this proven drilling technique.

New and improved additions to existing CTD systems are allowing operators to drill slimhole wells more efficiently than ever. Additions such as integrated weight-on-bit, at-the-bit inclination measurements, downhole motor innovations, and multi-lateral well design innovations, have given and will provide operators in the future an option for enhancing existing production and reservoir reserves.

Drilling problems associated with shallow formation gas in some areas of the world require an innovative application of coiled tubing drilling techniques. Operators in the Lake Maracaibo area of Venezuela have drilled 13-in. surface holes with 2 3/8-in. coiled tubing. Motor sizes to 6 3/4 in. are utilized for the operation. Downhole motors are primarily optimized for the low flow rates incurred, sacrificing torque and power. This primarily being a jetting operation results in very minimal bit torques.

The large annular volume resulting from this process demands the highest flow rate possible for more effective hole cleaning. To limit personnel risk, the coil tubing control unit resides on a nearby barge while drilling operations are in progress. This technique has proven very effective, and utilized widely throughout the area.

Drilling mechanics and formation evaluation questions have stimulated the development of measurement and logging while drilling tools for the coiled tubing market. One service company has marketed a system boasting the capability of continuous downhole motor directional control and measurements while drilling information. Powered and controlled via wireline electrical conductor inside of the tubing, it provides electrical control of motor orientation, directional, and formation evaluation measurements.

Continuous motor rotation allows for a better "gun barrel" wellbore. No hydraulic orienter or separate MWD system is necessary. Internal and annular pressure measurements give more control with motor stall prevention. Integrated downhole weight on bit and a resistivity measurement are in development and will yield more real-time drilling mechanics and geological information, improving geological steering and drilling control.

CT fishing jar design

The coiled tubing completion/workover market to date has been increasing with advances in existing technology, equipment, and tools. Slim hole environments are requiring smaller OD tools with large ID's for ball-dropping operations, and shorter tool lengths for use in smaller lubricator pressure assemblies.

Several service companies have responded to this demand with new tools and services. Traditional tool design and engineering specifications have simply been adapted for use in this smaller working environment. With the emphasis on "small", IRI International - Downhole Products Group has developed an industry first Bowen fishing jar for use in slimhole environments. The unique design separates itself from other industry jars with a shorter length, yet similar performance characteristics. High load ratings and stroke capability in both the up and down directions allow the jar to be run as a drilling or fishing jar. Tool sizes range from 1 11/16 in. to 3 1/8 in. OD, with weights ranging from 36 lb to 126 lb and lengths ranging from 67 in. to 84 in. Unlimited stroke life has been observed in field tests and job applications to date. The shorter length makes this jar a very practical choice for conveyance on wireline.

New CT applications

The increased application of coiled tubing drilling has generated the need for more downstream products and services.

• Slimhole operations: Industry professionals have indicated that the slimhole completion and production market will be a hot area of development in the coming year. Service companies are already responding with new products and services for application in this slimhole environment.

Several companies have recently introduced new technology for well fracturing and stimulation utilizing coiled tubing. The obvious benefits are:

1. No drilling rig is required
2. No tubing must be pulled
3. It is not necessary to kill the well.

The cost savings of not requiring drilling or workover rig mobilization and working through existing tubing, is evident. Past techniques required hydraulically killing the well before operations began. This is not necessary with a coiled tubing system, and as in the drilling application, constant well pressure control is an added benefit. This technique has allowed small and large operators to financially substantiate going into older fields and to stimulate production.

• Tubing remediation tool conveyance: Operators have recently been utilizing coiled tubing to convey tubing remediation tools. The practice of using downhole motors and very abrasive mills has been a popular method of mechanically eliminating tubing scale. Experience has shown that this practice, if not monitored properly, can easily damage the internal surface of the tubing. Constant movement of the tool string was required to prevent excessive mechanical action on the tubing ID. One company has taken the initiative and developed a hydraulic alternative. The technique utilizes a recently developed abrasive fluid additive pumped through a new-design, rotating, high pressure head. This has proven to be a very viable financial and operational alternative. Common sand had been utilized in the past as an additive to the fluid system to give it more abrasive qualities. Experience has shown that the use of sand as an abrasive additive is very detrimental to tubing ID integrity and also a problem to remove from the well. Inattention and lack of tool movement during this process in the past has errantly perforated the production tubing. An alternative is a man-made bead which has proven to be as abrasive as sand, yet reduce mechanical erosion effects on the tubing. Other useful attributes such as zero toxicity and acid solubility make it a safe and easily eliminated by-product after work is completed.
• Fracture multiple zones: Another innovative application becoming popular with operators is using the coiled tubing technique to fracture multiple formation production zones simultaneously, with the convenience of constant well control. Operations requiring three days in the past can now be completed in one day. New slimhole packers to enhance the effectiveness of this application are in development. Several service companies have experience in this area with each company pushing the technology with new and exciting achievements.

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