COILED TUBING DRILLING REQUIRES ECONOMIC AND TECHNICAL ANALYSES

S. C. Gary
Schlumberger Dowell
Sugar Land, Tex.

Over the past few years, coiled tubing drilling has become a viable method for drilling some reentry and new wells (Fig. 1) (27610 bytes). This increasing level of interest has brought advances in coiled tubing drilling equipment and application techniques.

Coiled tubing drilling has been effectively used for drilling new shallow wells, sidetracking or deepening conventional wells, reentering a well through tubing, and drilling underbalanced.

Field experience has proven that coiled tubing drilling is a technical and economic option on some wells; however, coiled tubing drilling is not the solution to every drilling prospect or production-enhancement job. To determine if coiled tubing drilling is viable, the geographic, technical, and economic aspects of each project must be considered in detail.

Generally, with some limitations, coiled tubing drilling is feasible primarily when jointed pipe cannot be used effectively. Also, coiled tubing drilling may be more appropriate because of some special well site requirements, such as environmental regulations requiring less surface disturbance.

TECHNICAL CONSIDERATIONS

The technical evaluation begins with the size of hole required. In through-tubing reentries, the hole size is limited by the inside diameter of the current completion. In new wells and conventional reentries, the restriction is more complicated. Because of the small sizes (1-3 1/2 in.) of coiled tubing currently available, the hole size will be restricted by the flow velocity needed for cuttings removal from the annulus.

The annular velocity must be calculated at 80% of the maximum motor flow rate for each motor/hole diameter combination. An acceptable annular velocity depends on the hole size, cuttings size, deviation, and mud system. As a guideline, this velocity should be 50 fpm in the largest vertical hole diameter expected. In a horizontal hole, a minimum velocity of 100 fpm is suggested.

If the annular velocity criterion is met, the injection pressure at the coiled tubing inlet must be then determined to ensure that the working limitations of the coiled tubing are not exceeded. Also, the coiled tubing tension during pulling out of the hole must be determined to ensure that the maximum tension of the coiled tubing and the maximum pull capacity of the coiled tubing injector head are not exceeded.

In highly deviated or horizontal well bores, the compressive loads on the coiled tubing must be evaluated to ensure that the coiled tubing can achieve the reach required. Included in this computation are the compression on the coiled tubing, the bending friction forces on the bottom hole assembly through the build-up section, and the resulting weights on bit.

If the calculated annular velocity, coiled tubing tension, or coiled tubing compression is restrictive, the project may have to be redesigned. Larger coiled tubing sizes, heavier walled pipe, or if permissible, modifications to the build-up rate, hole trajectory, or hole size may be necessary.

ECONOMICS

Generally, the economic feasibility of a coiled tubing drilling job depends on the location and scope of the project. Evaluating a coiled tubing job differs a bit from evaluating a conventional drilling job.

With a conventional rotary rig, the required equipment typically arrives on location with the drilling rig. These rigs are built to be mobilized and demobilized many times.

In contrast, with a coiled tubing drilling unit, ancillary equipment is not typically part of the basic rig package and must be rented separately. Less equipment is typically needed for a coiled tubing drilling operation than for conventional rotary drilling. The pipe, injector head, power pack (for the coiled tubing unit Only), coiled tubing blowout preventers (BOPs), and control cabin can be expected with the coiled tubing unit (Fig. 2) (85756 bytes). The mud equipment, fuel tanks and pump, generator, electrical equipment, and accommodations are some of the additional surface equipment that may be required.

If a job requires pulling or running a jointed completion, a jack-up frame and power tongs will be needed, in addition to the normal completion hardware. For example, a coiled tubing unit can have a jacking structure capable of handling 7-5/8-in. pipe at a maximum pull of 200,000 lb with an 8-ft stroke (See magazine cover).

Conventional drilling quipment will be required for the bottom hole assembly. For vertical holes, this equipment should minimally include bits, drill collars, mud motor, and drilling jars. For deviated holes, other equipment, such as steering and orientation tools, monel drill collars, whip-stocks, casing collar locator, cement bond tools, and mills for window milling, may be required.

Acquiring this additional equipment can be costly and time consuming. For this reason alone, some coiled tubing drilling jobs may not be feasible. For more conventional operations, multiwell projects may be required to justify the cost associated with mobilizing this equipment.

WELL TYPES

New shallow wells, conventional reentries, through-tubing reentries, and underbalanced wells have all been drilled successfully with coiled tubing, but these applications have some limitations that must be considered.

NEW SHALLOW WELLS

Coiled tubing drilling units can handle small and shallow new wells, typically 5,000-6,000 ft deep, with hole diameters up to 8 1/2 in. In some softer formations, the hole size may reach 12 1/4 in.; however, the casing size will be limited to a final production casing of 3 1/2-in. Recent technical advances continue to push these limits.

In areas with environmental requirements, coiled tubing drilling is often advantageous because fewer rig loads are required and the resulting location size is smaller than that of a conventional drilling rig. In jungles, mountainous regions, national parks, and urban areas, a small well site footprint is desirable to reduce environmental impact. Offshore, the smaller location becomes especially attractive because many platforms have limited deck space (Fig. 3) (78798 bytes).

Coiled tubing drilling is effective for special applications, such as the new shallow gas relief wells being drilled in Indonesia and Venezuela. The small amount of equipment placed at risk, as well as the advantage of personnel safety, supports the use of coiled tubing drilling.

Many of today's new well projects, however, are not especially suited to coiled tubing drilling, especially because of current market economics. In many regions of the world, the relatively new coiled tubing drilling unit and its bulk of required ancillary equipment must compete against a depreciated drilling rig. In these areas, the coiled tubing drilling unit simply is not an economical alternative to the conventional rig.

CONVENTIONAL REENTRY

Deepening and sidetracking wells covers the bulk of the conventional reentry market, and coiled tubing drilling is suited to a portion of these operations. These techniques involve pulling the production string and drilling the well overbalanced.

For sidetracks, a whipstock is set at the kick-off depth, and a window is milled in the casing. Coiled tubing drilling can handle wells with up to a 6-in. hole size and requiring build-up rates not exceeding 30/100 ft. For horizontal sidetracks, the drain hole length, limited by the required weight on bit, may reach 4,000-5,000 ft.

The coiled tubing drilling unit must compete for most of these jobs with depreciated drilling rigs with low day rates. Coiled tubing drilling, however, does provide the same environmental advantages for conventional reentries as for new wells. Because of low mobilization costs, coiled tubing drilling also is economically viable for reentry work from large offshore platforms. Coiled tubing drilling is also useful offshore where depleted formations have reservoir pressures less than the hydrostatic column of water and drilling with foam is required.

THROUGH-TUBING REENTRY

The most technically and economically successful applications of coiled tubing are those in which a service, such as through-tubing reentries or underbalanced drilling, is performed that cannot be conducted efficiently or safely with a conventional rotary rig.

Through-tubing reentries are typically drilled to deepen or sidetrack a well and are performed without removing the well's production tubing. These projects are suited to coiled tubing because no additional equipment is needed to pull the tubing. A coiled tubing unit can move in, rig up, and begin drilling within hours. This quick rig up is especially attractive in offshore and arctic locations, where drilling and workover rigs have higher day rates.

With current downhole technology, vertical through-tubing reentries are limited to 3 1/2-in. production tubing and to hole sizes down to 2-3/8 in. For directional drilling, the bottom hole tools limit the applications to 4 1/2-in. tubing or hole sizes of 3 1/2-3 3/4 in.

Combined with underbalanced drilling, through-tubing reentry drilling projects provide for the highest potential cost savings for the operator.

UNDERBALANCED DRILLING

In true underbalanced drilling, the well flows while the hole is drilled. The well is not killed while pipe is tripped. This underbalanced condition prevents drilling fluid from entering and damaging the producing formation.

This drilling technique is possible with jointed pipe, such as with snubbing units, but it has proven safer and more efficient with coiled tubing.

Underbalanced drilling can be applied in combination with any other application (shallow wells, conventional reentries, or through-tubing reentries). The only limitations are those associated with one of these three applications.

OUTLOOK

Experience gained by the coiled tubing drilling industry over the first 4 years of its existence has led to an understanding of when and how to apply coiled tubing drilling techniques. Coiled tubing drilling can be a safer, more economical, and technically more efficient approach to some drilling projects, but it is not applicable for all wells.

Field applications have particularly proven the economic and technical feasibility of coiled tubing drilling for through tubing and underbalanced applications. The industry has learned that using coiled tubing drilling for new shallow wells and conventional reentries is technically possible, but not always economically attractive.

The outlook for further growth in the coiled tubing drilling industry is excellent. New applications and geographic areas for coiled tubing drilling trials should surface this year.

THE AUTHOR