With extremely tight margins, drillers of coalbed methane (CBM) wells are looking to save money any way they can. Here's an innovative solution.

CBM production is a growing component of the North American natural gas supply. The San Juan, Black Warrior and Raton Basins are recognized as successful areas of CBM development and many other basins hold larger reserves of coalbed natural gas. In recent years, the Powder River Basin in Wyoming and Montana has emerged as one of the most active natural gas plays in the United States. It is estimated that more than 11,000 wells have been drilled to date in the basin. More importantly, another 66,000 wells may be required to develop fully the basin's CBM resources.

CBM development has occurred primarily in the east central portions of the basin in shallow, thick seams.

The completion method for these wells is typically the openhole, single zone completion. The accepted practice is to drill a production well into each coal seam. Thin seams of less than 20 ft (6.1 m) are usually bypassed for more favorable thick seams. While using single-zoned completion technology is fairly simple, it has numerous limitations:

• The practice makes most zones less than 20-ft (6.1-m) thick non-commercial;
• A larger infrastructure of wells, pipelines and roads is required;
• There is greater environmental exposure and damage; and
• Large areas of the basin are made uneconomic due to basis differential and high gas gathering costs.

Multi-seam completion solution

The potential benefits of developing a successful multi-seam completion technology appropriate to Powder River Basin coalbeds are numerous:

• Access to vast CBM resources residing in thin coal seams;
• Increase the economically recoverable CBM resources in the basin by 21 Tcf;
• Reduce environmental impact resulting from less wells and infrastructure; and
• Improved water management.

Multi-seam completion technology provides economic and physical access to previously bypassed thin seams. Successful application of the technology would increase the accessible coalbed methane gas-in-place to 75 Tcf and raise the technically-recoverable CBM resource of the basin to 50 Tcf. Multi-seam completion technology is particularly relevant to the northern region of the basin, which is dominated by multiple thin coal seams.

Multi-seam completion technology would significantly reduce the required number of CBM wells by enabling a single well to produce jointly several coal seams. With the proposed completion system, which uses underbalanced multilateral, multi-seam technology, as many as 25,000 wells and associated infrastructure could be eliminated leading to a greatly reduced environmental footprint.

Past efforts unsuccessful

The Powder River basin possesses unique geological and reservoir characteristics that preclude the use of traditional multi-seam drilling schemes that have been successful elsewhere. Power River coals are shallow, under-pressured, low-rank (low strength) coals surrounded by aquifers. These reservoir conditions cause standard cased hole well drilling and completion practices to create extensive damage to the coal face which calls for a customized solution to overcome this problem.

To better understand the performance and problems associated with using traditional completion practices in thin Powder River coalbeds, it is necessary to review their applicability in other regions to determine why they are inappropriate. Multi-seam completions are used extensively in the San Juan, Black Warrior and Raton Basins. These basins use hydraulic fracturing stimulations with sand proppant to enhance gas and water rates. In addition to hydraulic fracturing, the dynamic openhole cavity completion method is also used in the San Juan Basin. The type and design of hydraulic fracture stimulations may vary among the basins, reflecting different geological and reservoir characteristics. Hydraulic fracturing is utilized mainly to overcome formation damage caused during drilling and cementing and to connect the well bore with the coal seam's natural cleating system.

Powder River coals have high inherent permeability and associated high natural flow potential; therefore, the permeable, under-pressured coals suffer deep invasion. Consequently, they can become severely damaged during well construction operations. In addition, the coals are aquifers or are in close proximity to aquifers.

Hydraulic fracturing ruled out

Although hydraulic fracturing has enjoyed success in other CBM producing areas, the technology is not applicable in the Powder River Basin. The stratigraphic section of the basin has aquifers inter-bedded with the coal seams. Both the aquifers and the coal seams could be adversely affected by large interval stimulations. The use of certain additives normally used in fracture slurries could be restricted or prohibited in coal seams that are aquifers or those with aquifers nearby. There is a high potential for lawsuits, resulting from actual or suspected contamination of the aquifers. In fact, several suits are still being vigorously pursued regarding compliance with Section 1422 of the Safe Drinking Water Act. In addition, the inclusion of stimulated aquifers would only add to the amount of water production, thus hindering the dewatering that is necessary before gas production can commence.

Openhole dynamic cavitation

The San Juan Basin stimulations are designed for maximum vertical growth, to connect as many coal seams to the well bore as possible. Although openhole cavitation has been attempted in several US coal basins, it has only been successful in the San Juan Fairway. A unique set of favorable reservoir properties is required to achieve a successful cavitation. In the Powder River Basin, the clays and shales that are inter-bedded with the coal seams are very unstable and cannot be included in an openhole multi-seam completion. This precludes use of the openhole cavitation technique.

An innovative solution

A proven process, developed and tested by Gardes Energy Services, Inc., meets the requirements for multi-seam completions in thin Powder River coal beds. The technique involves drilling multiple, radial wells with numerous lateral branches that penetrate the coal seams, using a unique, two-string drilling technique. Key to the new technique is a retrievable, multi-use Upstock (whip stock) that enables underbalanced drilling of multiple coal seams with multiple lateral branches within each seam. To understand how the technique works, refer to the numbered steps in the accompanying illustrations. First, a pilot hole is drilled that pierces all the coal seams that have been targeted. A small rat hole sump is drilled about +/- 200 ft (61 m) below the bottom coal seam, and the well is cased using 7 5/8-in. steel and fiberglass casing. The fiberglass is spaced across all the coal seams of interest while the steel covers the rest to surface. The Upstock is lowered into the pilot hole to the deepest coal seam to be developed on the end of 5 1/2-in. casing, referred to as the "carrier string," and is oriented in the proper direction (1).

Because the casing is hung off in the wellhead, a dual injection annulus is created. The multilateral drilling assembly is lowered through the 51/2-in. carrier string to the top of the Upstock, where it is oriented in the same direction. Drilling commences as clear drilling fluid is pumped down the drill pipe to activate the mud motor and guidance systems (2). The dual annulus process prevents damaging drilling fluid from invading the coal seams. During the drilling process the drilling fluid is pumped down the drill pipe and air is simultaneously pumped down the annulus outside the carrier string in order to lower the equivalent circulating density below that of the formation and create an underbalanced environment. Returns consisting of commingled drilling fluid, cuttings and air meet at the Upstock and return via the annulus between drill string and the carrier string (3). Branch laterals can be drilled in the coal seam simply by reorienting the bent housing on the drilling motor. When the last branch lateral has been drilled, the drilling assembly is then pulled out of the hole and the carrier string and Upstock are then moved up the hole and reoriented in the next coal seam to be drilled. There is no need to retrieve the Upstock. It can be repositioned by removing joints of casing at the surface. Once the Upstock is repositioned, the underbalanced drilling procedure and process, as mentioned above, can resume on the next seam. The process has been repeated for as many as 10 seams without retrieval.

Once all coal seams have been completely drilled the drill string is removed from the well bore and then the carrier string with Upstock is subsequently removed. A downhole electric submersible pump (ESP) or progressive-cavity pump (PCP) is lowered down into the sump area on tubing. As soon as sufficient dewatering has taken place, gas desorbs from the cleating system and makes its way through the branch lateral to the sump annulus, finally entering the 75/8-in. cased hole, where it is produced to surface.

Another possible production system would be to leave the carrier string in and lower it to the sump area. An annular casing packer which was previously positioned in the carrier string is set to isolate the annulus between the carrier string and the casing string just above the 7 5/8-in. casing shoe. Perforating guns are fired in the carrier string opposite each coal seam to be produced. An electrical submersible pump ESP is set on tubing in the sump, and dewatering commences up the tubing (5). As soon as sufficient dewatering has taken place, gas desorbs from the cleating system and makes its way to the branch laterals, finally entering the 7-in. carrier string through the perforations, where it is produced to surface.

Disposal

Where the practice is permitted, this system facilitates downhole disposal of produced water. This scenario requires location of a suitable disposal aquifer in advance. If the aquifer is above the 75/8-in. shoe, then it will be necessary to perforate the 75/8-in. casing before the carrier string is set in the hole to provide communication to the aquifer. Produced water can then be pumped down the casing/carrier string annulus into the open perforations. Often, the disposal zone can be located in the sump area, which simplifies the completion. Because produced water is first pumped to surface by the ESP before being re-injected, unwanted water from adjacent wells can be added to the water that is pumped back downhole for disposal if desired, depending on the capacity of the aquifer to accept water.

Advantages

The multiple string drilling and completion technology offers the following advantages:
Underbalanced drilling using clear fluid helps reduce or eliminate formation damage facilitating both dewatering and subsequent gas production.

The dual-string Upstock technique allows as many as 10 seams to be drilled on a single trip. As much as 11,000 ft (3,355 m) of lateral branches have been drilled into a 4-ft (1.2-m) coal seam from a single well bore.

Multidirectional laterals have been drilled from a central well bore to effectively drain 640 acres.
Minimal environmental impact can be realized. A single branch lateral well can produce as much gas as 16 vertical wells from two seams on an 80-acre spacing.
Net present value is increased because of higher recovery rates, faster dewatering, minimal infrastructure and broader areal sweep.

Deep, thin, coal seams can be successfully drilled and completed in over pressured coal deposits below 5,000 ft (1,525 m).

There is no risk of contaminating freshwater aquifers.

Comments