CBM production operations

Production operations in coalbed methane (CBM) wells are not significantly different from other gas wells except for one important distinction. Conventional wells typically begin production with high gas/water ratios (GWR) that decrease with time, whereas CBM wells start with low GWRs that increase with time. This distinction requires that equipment and facilities for water handling and disposal be built at the start of a project, which requires significant lead time and capital investment.

Critical aspects

The initial operational goal of nearly all CBM wells is to depressure the reservoir by continuously producing water at a low flowing bottomhole pressure. This requires an artificial-lift system that can be modified as the gas rate increases and water volumes decrease. Smaller tubulars and pumps are typically required with time as the reservoir pressure decreases and water rates drop. Initially, produced gas may be flared, especially in frontier areas without access to gas transmission systems. If the gas is to be sold, analyses will be required and treatment facilities may be needed to meet pipeline specifications.

The acquisition of high-quality reservoir surveillance data is a key element of production operations. Initial reservoir pressure values from each well and subsequent reservoir pressures are critical for determining whether depressuring is occurring. These data can be captured with downhole gauges or by measuring static wellbore fluid levels. Similar data should be obtained under producing conditions to ensure that wells are being pumped off. Both static and flowing bottomhole pressures should be measured every few months in a new pilot project or field development.

Production logging tools also should be run to determine which coal seams are contributing; however, these tools typically are limited to either flowing wells or those with a downhole assembly that can accommodate the tools under pumping conditions [ such as an electrical-submersible pump (ESP) with a Y-tool] . Accurate gas and oil rates are extremely important and should be measured frequently. In most projects, the production rates from new wells are measured daily to capture fluctuations in early production

Water production and artificial lift

Initial water rates in a CBM well are a function of the average coal permeability and aquifer strength. Because permeability often varies by more than three orders of magnitude within the same field, produced-water rates will vary by this magnitude as well. For example, in the Black Warrior basin of Alabama, initial production rates for 420 wells ranged from 17 to 1,175 barrels of water per day (BWPD), averaging 103 BWPD. Initial water rates may be unusually high if the coals are overpressured because of coal recharge along the basin margin. Initial water rates may be unusually low if the productive area has been depressured by nearby mining operations or previous well production. Water rates should peak within the first few years and decline thereafter, unless the aquifer is extremely strong or the number/spacing of producing wells is insufficient to depressure the reservoir.

Nearly all CBM wells require artificial lift at some point to accelerate dewatering and reduce reservoir pressure. The most common artificial-lift types include electric submersible pumps (ESPs), progressive-cavity pumps (PCPs), beam pumps, and gas lift. The method and criteria for selecting lift equipment is similar to other wells and is governed primarily by the expected production rate. Because many CBM wells are drilled in frontier areas where there is little coalbed-well experience and a limited maintenance infrastructure, it is often best to choose the lift system that is simplest to operate and least troublesome.

ESPs are ideal for pumping volumes in excess of l,000 BWPD from coal wells, but these pumps require reliable electricity and can be damaged by coal solids (fines), which are common in the early productive life of a well. PCPs are popular in many CBM projects because they can produce 100 to 1,000 BWPD), handle coal fines effectively, and require little maintenance. The versatile beam pump handles low-to-medium water volumes of 5 to 500 BWPD and requires little maintenance. Gas lift is the least expensive lift system to operate. It requires no electrical power and handles low water rates of 5 to 50 BWPD. Gas lift, however, requires specific well pressure tolerances to work effectively. The bottom line is that no matter which artificial-lift system is used, it is crucial to minimize downtime and keep the well pumped off.

Water disposal

Water disposal is one of the most important considerations in a CBM development. It can be very costly to build water-handling facilities, drill disposal wells, and comply with numerous environmental regulations. In marginally economic projects, water-disposal costs can be the deciding factor as to whether the project moves forward. It is important to remember that water production in CBM wells is viewed as an early, relatively short-term problem that must be overcome to produce gas economically.

To decide which disposal method is most applicable, a complete chemical analysis of a representative water sample is needed and anticipated water rates must be determined. There are three common techniques used for disposing of produced water in the CBM industry:

• Subsurface injection requires that a well be drilled or an existing well be worked over to accept produced fluids into an approved disposal zone.
• Surface evaporation, uses active evaporation ponds and a spray/mist system to evaporate the produced water.
• Stream discharge, requires an elaborate treating and monitoring system to ensure that chlorides, total dissolved solids, and other impurities are lowered to acceptable levels.

Because CBM reservoirs are shallow, most disposal wells must be drilled to deeper horizons, resulting in disposal wells costing more than development wells.

Facilities

Production facilities for CBM wells must be capable of handling:

• Produced water
• Coal fines
• Low-pressure gas

Accurate forecasts of early water production are necessary to size:

• Separators
• Flowlines
• Transfer pumps
• Storage facilities

Separators can remove most of the produced water from the flow stream, but heated separators or dehydration units are needed to extract the remaining water. Filters may be required to remove coal fines produced with the water to keep valves and equipment functioning properly. If scale-forming minerals are present in the water, chemical treatment may be needed to protect steel tubulars and surface equipment. If the water is to be disposed of off site, trucks or additional pipelines will be required for water transport. If water-disposal wells are used, injection wellhead assemblies and flow control equipment will be needed.

Produced coal gas rarely contains any H₂S but may contain other impurities. For example, produced gas from the Oak Grove field in the Black Warrior basin contains 3.4% N₂, while gas from the Piceance basin contains 6.4% CO₂. If these concentrations are more than pipeline specifications, the impurity levels will have to be reduced with amine scrubbing, molecular sieve dehydration/treatment, and/or cryogenic processing.

After the produced water is separated from the gas stream and the impurities in the gas have been removed, the coal gas is piped to a compressor. This compressor may be installed at the wellsite if the produced gas volume is sufficient, or centralized compression can be used to handle several wells and reduce costs. The volume of gas being compressed will dictate the ultimate size of the compression unit. The amount of compression required will vary depending on trunk- or transmission-line specifications. Some pipeline companies will accept low-pressure gas in the 50- to 150-psi range, while others require compression of up to 900 psi. After the gas is compressed to a sufficient line pressure, it typically requires a final dehydration before delivery.

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See also

Coalbed methane

CBM well drilling and completion

CBM case histories

CBM reservoir fundamentals

CBM reservoir evaluation

CBM economics

PEH:Coalbed_Methane

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George J. Koperna, Jr.