Coalbed Methane Resources In The Southeast

Based on a workshop presented by PTTC's Central Gulf Region on June 8, 2004 at the University of Louisiana -Lafayette Energy Institute, Lafayette, Louisiana.

BOTTOM LINE

Louisiana coalbed methane (CBM) development has lagged activity in other U.S. basins. This is partly due to the relatively undeveloped coal industry in the region, lack of exploration of the potential coalbeds, and a lack of understanding of the criteria necessary for CBM development. The United States Geological Survey (USGS) and several states have recently investigated the lower Trinity Group, Cretaceous Hosston Formation and the Midway and Wilcox Groups (Paleocene-Eocene). Greatest potential is seen in the Wilcox groups. Maximum thickness of the Wilcox in Louisiana is 20 ft and the gas is primarily biogenic. Speakers outlined a six-step process for CBM development, a key element of which is gathering key data early on before extensive drilling occurs. It is important to employ wireline logs developed specifically for defining CBM reservoirs. Information on successful methods of evaluation, drilling and completion from other regions (including the Black Warrior Basin) provide guidance for future CBM development in Louisiana. Still, experience indicates that optimum approaches vary for each basin-the key is a structured effort to define those practices early on that will deliver maximum economic benefit in the given play.

PROBLEM ADDRESSED

Coalbed methane development has become a significant portion (over 8%) of domestic natural gas production in the United States. However, most of this development occurs in the Western Interior states, and the Gulf Coast has largely been bypassed. This workshop addresses the potential for coalbed methane (CBM) development in Louisiana and the Gulf Coast.

KEY WORDS:

Biogenic/Thermogenic, Coalbed Methane (CBM), CBM Life Cycle, Coal Rank, Desorption, Pinnate Drilling Systems

SPEAKERS:

An Introduction to Coal Seam Natural Gas (CSBG), AKA, Coalbed Methane (CBM),
F. Clayton Breland, Jr., Louisiana Geological Survey

Coal Gas Resource Potential of Cretaceous and Paleogene Coals of the Eastern Gulf of Mexico Coastal Plain,
Peter Warwick, U.S. Geological Survey

Laboratory, Drilling and Completion Techniques Used in Coal Bed Methane,
Steven A. Tedesco, Atoka Geochemical Services Corp.

Unconventional Drilling Methods for Unconventional Reservoirs in the U. S.,
Doug Wight, CDX Gas

Using Wireline Logs in Evaluating Unconventional CBM Plays,
Derek Crowson, Halliburton Energy Services

Coalbed Methane Industry in Alabama and its Influence from Regulatory Agencies,
Terry Burns, GeoMet Operating Company, Inc.

Panel Discussion:
Kirk Ross, Vintage Petroleum; Deane Foss, Harvest Gas;
Diana Chance, Conner Minerals
 

TECHNOLOGY OVERVIEW

The factors that control coalbed methane development include the type of coal, age of coal, thickness, coal maturation, fracture permeability, and the depth of burial of coal seams. Coal bed methane or natural gas development has become a domestic boom in the past decade. Strategies and methodologies that have proven successful in other regions are offered as examples of how to evaluate coalbed potential for CBM development and the best methods of drilling and completion. Regulatory concerns, particularly relating to produced water disposal are addressed, using the Black Warrior Basin of Alabama as an example.

Introduction
For Louisiana and Gulf Coast operators interested in getting into the coalbed natural gas market, the question of what makes CBM development possible is of paramount interest. With vast coalbeds underlying the region, why hasn't CBM been developed to the extent of areas in the Rocky Mountains or the much closer Black Warrior Basin of Alabama?

CBM is natural gas, generated during conversion of plant material to coal and released from coal seams. Coalbed natural gas was first vented to improve mine safety and reduce the hazard of gas explosions. Thus, CBM production started in the early 1970s as an unconventional energy byproduct of conventional coal mining.

Formation of coalbed methane relates to the type of coal, ranging from soft lignite through increasing harder coals: sub-bituminous, bituminous and anthracite. Moisture content in the coals is a key element in CBM development. Higher ranked coals with high moisture content produce the most CBM, and these deposits are found largely in the Midwest and western U. S.

Louisiana's history of coal production is quite recent and the coals are both poorly developed and relatively unexplored. Most coal zones in the Upper Cretaceous Navarro Group, Paleocene Wilcox, and Eocene Jackson and Claiborne Groups are insufficiently thick and low ranked to provide economic amounts of CBM for production. Limited CBM production has been established in central Louisiana from the Wilcox lignite beds.

Coal Resource of the Gulf of Mexico Coastal Plains

In an effort to determine the feasibility of CBM development in the Gulf Coastal states, the USGS and several states have recently investigated the lower Trinity Group, Cretaceous Hosston Formation and the Midway and Wilcox Groups (Paleocene-Eocene). Coalbeds in Arkansas and Louisiana at depths of 1,500 to 6,000 ft contain intervals greater than 10 ft thick, which should be suitable for coal gas production. Texas, Arkansas, Mississippi, Alabama and Louisiana may have gas in the Wilcox, and in Mississippi and Alabama the Midway Group is a good candidate for coal gas production. Maximum thickness of the Wilcox in Louisiana is 20 ft and the coal gas is primarily biogenic.

The surveys have studied the structure and stratigraphy of potential coalbeds across the coastal Plain states, and looked at the types of coal that have the potential for CBM development. The extensive CBM development in the Black Warrior Basin of Alabama (cumulative = 1.5 Tcf) gives an incentive to development in the Gulf Coastal Plain. The greatest potential for the Gulf Coast is seen in the Wilcox coalbeds where there is also the potential for using CO₂ for enhanced CBM production and opportunity to take advantage of CO₂ sequestration benefits.

Laboratory, Drilling and Completion Techniques in CBM

This section outlines the steps in a pilot CBM exploration process: 1) Evaluate the resource, 2) Determine the coal thickness, 3) Determine gas content, 4) Establish permeability, 5) Establish water production rates, and 6) Determine the optimal drilling and completion methods. For determining the thickness of coalbeds density-neutron logs are superior to gamma ray or sonic logs; however log data need to be combined with core data.

More than one test well to core and sample the project area is recommended for resource evaluation. Determining gas content from coalbeds is difficult due to loss of gas through different coring and sampling techniques. Specific coring devices have been developed to give accurate values for desorbed gas and adsorption characteristics of a given coal.

Accurate measures of permeability and water rates are necessary to determine the best completion methods. Evaluation of completion from open hole to cased hole, fracing options, and water disposal options significantly impact the economics of a CBM operation. Due to the high volume of produced water and environmental regulations, careful analysis of surface vs. reinjection disposal must be made prior to drilling. The final step in planning for a CBM pilot is to evaluate all information to determine optimal well spacing.

Unconventional Drilling Methods for Unconventional Reservoirs

A discussion of surface degasification methods and the advantages and disadvantages of vertical vs. horizontal drilling is presented. Vertical open-hole wells work best for high permeability coals, using closely spaced low-cost wellbores. Fracture stimulation of vertical wellbores is best applied for medium permeability coals, thin beds and where multiple coal seams are penetrated.

The problem is that most vertical wells are inefficient due to low gas recovery rates, long term dewatering, the large number of wells needed to de-pressurize and limitations of surface access. In addition, the mud systems traditionally used for vertical wells may damage the formation and reduce permeability.

Horizontal open-hole wells can be used for thick coal seams, low permeability coals, and in areas where good lateral continuity is present. A pinnate drainage pattern established by drilling multiple side laterals off a main horizontal lateral provides maximum CBM production under ideal reservoir conditions. An example of ultimate application is a Z-pinnate drilling and completion system using one well site with pinnate development from four main horizontal laterals. This system can drain 1,280 acres (or more) in a 3600 pattern. The Z-pinnate system replaces 16 standard 80-acre drilling locations and provides significant economic and environmental advantages. When using such a complex drilling pattern special attention must be given to pressure management. Overbalanced drilling may cause formation damage and fluid loss. Underbalanced drilling requires additional attention to safety issues in handling the produced gas.

Horizontal wells and pinnate drilling systems have been successfully applied in several basins, including the Appalachian, San Juan and Arkoma basins. As drilling and completion strategies rapidly increase, complex horizontal drilling systems can become the most economically efficient way to produce coal bed methane. The objective of selecting the optimum drilling method for each reservoir is to economically produce unconventional reservoirs.

Using Wireline Logs In Evaluating Unconventional CBM Plays

Coalbed resources have been difficult to evaluate in the past and required expensive and time-consuming coring to evaluate. The use of wireline logs to identify coal thickness, quality, permeability and estimate gas in place could significantly reduce evaluation costs. Previous problems with electric logs were that Spontaneous Potential and Induction logs responses for coal beds often mimicked sandstones and evaporates. SP and induction logs also provided poor vertical resolution resulting in difficulties in identifying boundaries.

Modern log suites using Magnetic Resonance Imaging log (MRIL), electro-micro imaging (EMI), high resolution induction logs (HRI), spectral density logs (SDL), micro-log (MI) dual spaced neutron II log (DSN) and modern gamma ray log (NGRT) now provide complete analysis of coal identification and potential for CBM recovery. These log suites have been used extensively in well-developed CBM provinces in the Rocky Mountains, such as the San Juan Basin.

The different logs are used in combinations to evaluate coalbeds in various stages of exploration through natural development to depletion of natural gas. The newer logging tools give superior resolution for identification of bed thickness and boundaries and can assist in determining coal rank and permeability analysis. Several of the logging tools, SDL, EMI and ML, can be used for analysis of fracture networks, which ultimately determine CBM recovery. The MRIL log allows evaluation of porosity and permeability without correlation to core material, providing more economic means of analysis of large acreages.

The key to successful evaluation of CBM resources is to select and apply the correct log suite to the life cycle of the coalbed development to achieve optimal management and economic long-term production.

Brief Summary of the Coalbed Methane Industry in Alabama and it’s Influence from Regulatory Agencies

A summary of cumulative coalbed natural gas production from the major U. S. basins through 2003 shows that the Black Warrior Basins ranks between the San Juan and Powder River Basins, and if the mature San Juan Basin is deleted, the production from the Black Warrior Basin is significantly higher than from any other U. S. Basin. The Black Warrior Basin has produced far more CBM than all other basins in the eastern U. S. combined.

The major factor leading to development of the CBM industry was the growing concern in the 1960s over coal mine safety and the hazard of explosive coalbed gas. Coalbed gas venting and degasification projects led to recovery of coalbed gas as a byproduct. In 1980 the first CBM wells in Alabama were permitted for resource development rather than for mining.

The thick, extensive coals of the Black Warrior Basin were a natural target for CBM development. CBM production rose rapidly in the late 1980s and 1990s. By mid 2004 over 5,500 CBM wells had been drilled in the Black Warrior Basin in 20 fields. Currently 3,900 CBM wells are operated producing 330 Mmcf/day or over 120 Bcf/year. Cumulative production from the Black Warrior Basin is 1.6 Tcf. CBM production accounts for 30% of all natural gas production from Alabama, contrasting with 8% nationwide.

Regulatory agencies established comprehensive rules for CBM development and operations starting in 1983. Containment and disposal of produced water is a major environmental issue addressed by the regulations. The nine current CBM operators in Alabama have established strategies to operate within the regulatory structure and produce significant volumes of coalbed methane, offering an example for potential development in the Gulf Coastal Plain.
 

CONNECTIONS:

F. Clayton Breland, Jr.
Louisiana Geological Survey
LSU 208 Howe Russell
Energy Coastal & Environmental Bldg
Baton Rouge, LA 70803
Ph: 225-578-8300
E-mail: clayton@lgs.bri.lsu.edu

Terry Burns
GeoMet Operating Company, Inc.
5336 Stadium Trace Pkwy, @206
Birmingham, AL 35244
Ph: 205-425-3855 x109
E-mail: tburns@geometcbm.com

Derek Crowson
Halliburton Energy Services
719 Hangar Drive
New Iberia, LA 70560
Ph: 337-367-9261
E-mail: Derek.Crowson@Halliburton.com
 

Steven A. Tedesco
Atoka Geochemical Services Corp.
14550 East Easter Ave.
Englewood, CO 80112
Ph: 303-617-8919
E-mail: stedesco@atoka.com

Peter Warwick
U.S. Geological Survey
956 National Center
12211 Sunrise Valley Drive
Reston, VA 21092
Ph: 703-648-6469
E-mail: pwarwick@usgs.gov

Doug Wight
CDX Gas
5485 Belt Line Rd, Suite 190
Dallas, TX 75254-7672
Ph: 972-392-1880
E-mail: doug.wight@cdx.gas

For information on PTTC’s Central Gulf Region and its activities contact:

Mr. Robert H. Baumann, Managing Director
Center for Energy Studies, Louisiana State University
Energy, Coast and Environment Building
Nicholson Drive Extension
Baton Rouge, Louisiana 70803
Phone 225-578-4400 Fax 225-388-4541
E-Mail rbaumann@lsu.edu

Disclaimer: No specific application of products or services is endorsed by PTTC. Reasonable steps are taken to ensure the reliability of sources for information that PTTC disseminates; individuals and institutions are solely responsible for the consequences of its use.

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