CORE TESTS SPEED COALBED METHANE GAS DEVELOPMENT

Charles Willis
River Gas Corp.
Northport, Ala.

An extensive coring program permitted the rapid development of a coalbed methane gas project in the Black Warrior basin of Alabama.

By delineating the area of interest early in the life of the project, all aspects of the development phase could be coordinated.

Operated by River Gas Corp., an independent specializing in coalbed methane development and production for Emerald Gas Co., the objective of the Black Warrior project was to explore and produce commercial quantities of methane from medium-depth coal seams in central Alabama.

Because of the core tests and gas pricing considerations, the company's original plan to drill 800 wells was decreased to 535. The decrease in the number of wells resulted from increasing the planned 40 acre well spacing to 80 acres. Also, currently noncommercial reservoir areas were eliminated from the development.

The project commenced in October 1987 and the last well was tied into the sales line in May of 1991.

The 535 coalbed methane wells, as of May 1991, have an average on-line life of 1 year. Median production per well is 82 Mcfd. Gas rates are increasing as reservoir depressurization continues. Production, in May 1991, topped 42 MMcfd and 5,000 bw/d.

Project coordination and communication were achieved through the use of a central data base designed to track individual department progress. Status reports were provided to management and staff which accounted for each department's activities for the prior week and the planned activity of the coming week.

ACREAGE AND LOCATION

The Black Warrior project is a contiguous leasehold consisting of 32,480 developed acres situated along the west bank of the Black Warrior River approximately 10 miles northeast of Tuscaloosa, Ala.

Fig. 1 shows the project outline, major roadways, individual modules, well locations, large water bodies, and core holes. All wells were drilled in the Deerlick and Blue Creek degasification fields as designated by the Alabama Oil & Gas Board.

Several property owners controlled large blocks of surface acreage which facilitated development. Their cooperation and support was a key factor in the development of the property in a cost-effective manner. The speed with which large acreage blocks were leased was the impetus that allowed site construction, drilling, fracing, and completion crews to move quickly and work year-round.

Work was timed so that each phase of well development occurred at approximately 1 month intervals. This allowed the project to keep experienced contractors employed and allowed for several economies of scale in pricing goods and services.

CORING PROGRAM

Prior to initiating the coring program, the operator's geological consultants reviewed Krcra (known recoverable coal resource assessment) and coal company coring results for the area. The Krcra program was sponsored by the federal government throughout the Black Warrior coal field from 1979 to 1982.

The coal company's coring program was designed to evaluate coal mining potential for lands under lease. All Krcra core holes were drilled down to the Mary Lee coal group, and two coal company holes were taken down to the Black Creek group. All core holes near the project had gas desorption tests performed on the coals of interest. These holes also provided coal depths and thicknesses.

Fig. 2 is a generalized stratigraphic column of the major coal seams developed in the Black Warrior project. River Gas Corp. drilled six additional core holes. Core rigs were used and the wells were cored from surface to TD.

Total core drilled measured 31,844 ft.

Of this total, 319-05 ft of coal were recovered.

To determine gas content, direct-method gas desorption tests were performed on all coal seams of interest. Gas content and thickness data were compiled. Volumetric calculations determined recoverable gas per acre for the project. These data were mapped and contoured to delineate the final area of interest.

Contouring was validated by estimating gas content for the final three core holes prior to drilling.

Predicted and actual gas contents for these holes were within 5%.

The nature of the coal resource allowed accurate subsea mapping of both the coal thickness and the associated gas contents.

This provided the opportunity for area development plans to be made in advance of construction with only minor alterations necessary when drilling data were received and analyzed.

The ability to accurately determine the area of interest for development, combined with the leasing situation discussed previously, permitted rapid development to proceed with limited risk of drilling nonproductive wells.

DRILLING OPERATIONS

Of the 539 wells drilled, four were abandoned due to drilling problems. Three were redrilled successfully, and one was reformed into a larger unit. A total of 1,190,557 ft of hole was drilled and 1,131,194 ft of 51/2-in. casing run.

Considerable funds were expended to design and build roads that could provide access to locations by heavy equipment except in the worst weather. This approach to road building has allowed the operator to maintain and work over wells all year, even during the rainy season, without generating excessive lease-operating expenses.

The majority of the wells were drilled with 7-7/8 in. bits and 6 in. air hammers. Average drilling time was 36 hr (excluding waiting on cement time) with penetration rates often reaching 30 ft/hr.

Hole conditions were very stable. After reaching total depth, drilling rigs were moved off location and holes were logged.

Workover rigs were used to run 5-1/2 in. casing. "Lite" Class A cement was used to limit hydrostatic head on the coal formations.

COMPLETION PRACTICES

Except for a few special cases, all formations open to production have been hydraulically stimulated. Fracturing the coal is necessary to achieve economic production rates and efficient formation drainage.

The scale of the project and well accessibility were important factors when pricing drilling, fracturing, and other services.

Table 1 summarizes average sand and fluid pumped for each formation developed. The majority of the sand pumped was 12/20 Brady sand. By coordinating all efforts involved in the fracing procedure, the operator was able to frac as many as six zones in 1 day using one crew.

Water for most frac jobs was drawn from the Black Warrior River or its many tributaries adjacent to the project acreage. Frac water was pumped through temporary pipeline systems into trucks or into frac tanks.

Water was filtered prior to being gelled. After fracturing, solid bridge plugs were drilled out and the wells cleaned up by operator-owned service rigs.

Tubing and rods were run in preparation for setting the pumping unit.

PRODUCTION OPERATIONS

Gas and water-gathering pipelines are laid to every well. The road rights-of-way were purchased wide enough to accommodate the water and gas pipelines.

All water-gathering lines are polyethylene and tested to 200 psi.

Gas-gathering lines lead to field trunk lines which terminate at field compressor sites. Field compressors take the gas from 5 to 100 psi.

Gas is then moved to one of two sales sites where it is compressed to approximately 450-500 psi and delivered into the buyer's transmission line.

The water pipeline system takes the produced water to one of five settling and treatment ponds. Here water is checked against Npdes (National pollution discharge eliminations system) permitted criteria and treated if necessary. Water is then discharged into the Black Warrior River. Fig. 3 shows a road leading to a well site. Gas and water pipelines are buried along the roadside.

Fig. 4 is a typical completed well schematic and shows a barefoot completion for the Black Creek formation. Some wells were cased through the Black Creek for logistical reasons.

Most pumping units are Lufkin 114-119-72 conventional type. Separators were tested to 250 psi and were designed to handle 300 bw/d.

Wellhead meter runs are flange-type orifice meters running with 7-day charts.

As shown in Fig. 4, most gas is produced up the tubing-casing annulus, with water and entrained gas being produced through the tubing.

Each pumper is responsible for approximately 20 wells and the operation of one field compressor. Compressor/pipeline technicians also assist with the operation of field and sales compressors and pipeline maintenance such as pigging lines and line repair.

At present, River Gas Corp. has seven sales compressors and 24 field compressors totalling 4,318 hp. Eleven additional compressors are expected to be in place by the end of 1992. These will be needed to handle peak production of more than 70,000 MMcfd.

The most common remedial action taken to maintain production rates is a pump change. The operator owns and mans two workover rigs solely for this purpose. In addition to reducing overall cost, rigs are available when needed and workovers can be prioritized,

Another step taken to reduce operating costs and maintain quality control was the installation of a pump shop. This has proven to be economic as pump repair costs have been reduced over 40% by performing pump maintenance and repairs in-house.

Environmental concerns are of prime importance to the coalbed methane operator. River Gas Corp. keeps a crew working full time on erosion control for the 32,480 acre project.

The amount of rain received in the area can exceed 70 in./year. Erosion control is achieved by hydroseeding all rights-of-way, access roads, and locations. Hillsides and stripped areas not disturbed by the company have also been seeded to prevent erosional runoff.

Hay bales to slow water velocity are placed in culverts and ditches while siltation fences are used to control erosion runoff.

Current gas production rates are exceeding early projections used for initial economic feasibility studies. Fig. 5 is a plot showing the average daily rate for gas and water for all wells on line. All production information on this plot has been shifted to a common starting point for all wells.

The objectives of the Black Warrior project to develop and produce economic quantities of coalbed methane have been met.

Table 2 is an average cost breakdown for a well completed by River Gas Corp. The total amount of $190,000 compares favorably with other per well coalbed methane development costs in the Black Warrior basin. These have varied between $250,000 and $500,000/well.