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STATEMENT OF WORK

Project Title:

Determination of CO₂ Storage Capacity and ECBM Potential of Lignite Coals

Contractor:

University of North Dakota Energy and Environmental Research Center
15 North 23rd Street
PO Box 9018
Grand Forks, ND 58202
(701) 777-5287

Program Area:

Emissions Reduction and Environmental Sciences

Partners:

North Dakota Industrial Commission Oil and Gas Division
North Dakota Department of Commerce
Division of Community Services State Energy Program
Montana Board of Oil and Gas

Project Description:

The objectives of the EERC's project are to develop estimates for the gas content and CO₂ storage capacity of lignite coals in the Fort Union Group of the North Dakota and Montana portions of the Williston Basin and to determine the potential for application of CO₂-based ECBM in those coals.

Management Plan

The unique nature of STAC requires that projects be supported by multiple State entities, and to the extent necessary any other entity. As indicated in the STAC Agreement, it is the Contractor’s responsibility to coordinate the execution of work under the Contract, incorporated by reference hereto. Contractor, in conjunction with the other State entities, and to the extent necessary any other entity, shall conduct the project in accordance with the Management Plan –  described below.

Administration

As principal contractor, the University of North Dakota Energy and Environmental Research Center (UNDEERC) will handle the majority of the management of this project.  The UNDEERC will execute the contract with STAC as well as any and all subcontracts necessary to complete the project.

For the duration of the project, James Sorensen will serve as the point of contact for technical and procedural questions.  Mr. Sorensen will be responsible for ensuring that all contractors perform their tasks as promised and at the proposed budget.  Mr. Sorensen will also be responsible for completing all project reports including the final report.

Administration of contracts and billing will be completed by UNDEERC staff members.  Sheryl E. Landis, EERC Manager of Contracts; Intellectual Property, will oversee contracts and legal issues related to the contract.  UND, will manage purchasing, billings, and accounting.  UND will also be responsible for verifying that cost share and matching funds are obtained appropriately and reported under the contract.

Task 1: Field Site Selection

The focus of this task will be to assemble and analyze existing stratigraphic information, including geologic cross sections, overburden, and coal thickness isopachs and structural contour maps that have been generated by the U.S. Geological Survey and North Dakota Geological Survey (NDGS) in order to identify areas of Harmon coal seam occurrence that may be technically suitable for CO₂ storage/ECBM.  The geologic analysis will focus on identifying areas where the coal seam thickness is 15 ft or greater and the overburden thickness is 500 ft or greater.  Of those areas, one will be chosen as the location for the field activities.  The study site will be chosen based on its geologic characteristics, land ownership, and accessibility.  As a teaming partner, the North Dakota Industrial Commission (NDIC) has agreed to waive any and all permitting fees.

Task 2: Sample Collection

The focus will be to obtain coal samples for gas content, gas storage capacity, and permeability analysis.  Samples will be collected from the Harmon coal seam at a location in North Dakota.  The Montana Board of Oil and Gas (MBOG) will also provide samples of subbituminous coal collected during the drilling of a coalbed methane well in the Montana portion of the Powder River Basin (PRB).  Since the sorptive properties of PRB coals are relatively well understood compared to the Williston Basin lignites, these samples will allow for comparison of the analytical methods with respect to differences in coal rank. Sample collection will involve drilling with a conventional water well drilling rig to collect cores and drill-cutting samples.  It is anticipated that the core and drill-cutting samples will be collected from a single borehole at both the North Dakota and Montana sites.  The gas content analysis will be initiated at the field site and completed at the EERC laboratories in Grand Forks, North Dakota.

Portions of the core and drill-cutting samples will be preserved for use in the gas storage capacity and permeability tests at the EERC laboratories.  The core samples will be stored in PVC pipe.  The drill cuttings will be stored in friction lid metal cans.  Air will be excluded from both containers.

Task 3: Gas Content Evaluation

The focus of this task will be to evaluate the effects of sample type and test conditions on the accuracy of the gas content analysis results for the lignite coal samples recovered from the Harmon coal seam.  The reliable analysis of the gas content of lignite coal presents several unique challenges.  It is the assertion of the researchers proposing this work that these challenges have not been adequately addressed in previous evaluations of the gas content of lignite coals (Warwick et al., Montgomery et al., 2003).

The gas content of coal is commonly determined by placing freshly cut core or drill-cutting samples in sealed canisters and measuring the volume of gas that desorbs. A critical shortcoming of this analysis method is that the void space in the canisters at the start of the gas desorption measurements is filled with air (Nelson, 2003b).  Lignite coals are very sensitive and undergo rapid oxidation (Nelson, 1989).  For lignite coal, depletion of oxygen from headspace air will occur during the canister gas desorption tests, which will mask negate gas pressure increase in the canister because of gas desorption from the coal.

Previous case study analyses have demonstrated that gas desorption test must be performed using an oxygen-free canister headspace atmosphere in order to accurately quantify the sorbed-phase gas from subbituminous coalbed reservoirs (Nelson, 2003b).  Such oxygen-free conditions are even more critical for evaluation of the lower-ranked lignites.  Aerial oxidation of lignite coal in the desorption canisters could result in significant underestimation of the actual desorbed gas volume from lignites.

Sample type is another factor that affects gas content analysis results.  The gas content of lignite coals is often evaluated using drill-cutting samples (Warwick et al., 2000; Montgomery et al., 2003).  Recent case study analyses have demonstrated that gas desorption tests performed using drill-cutting samples significantly underestimate the actual gas content of bituminous and subbituminous coals (Nelson, 1999; 2003b).

To address the technical issues and challenges described above, an evaluation of the gas content of the Harmon lignite coal seam will be conducted using freshly cut whole core samples and the best practice methodology developed by Gas Research Institute (GRI) in the 1990s (Nelson, 2003b).  Following recovery to the surface, the coal core will be divided into 1-ft sections.  Each 1-ft core section will then be placed into a sealed canister.  The GRI best practice methodology involves displacing the headspace air within the canisters with an inert, nonoxidizing fluid such as water, thus minimizing or eliminating aerial oxidation of the coal during the gas desorption test.  The sealed canisters will then be immersed in a water bath maintained at the in situ temperature of the coal seam, and periodic measurements will be made of the volume of gas that desorbs.  It is expected to require several weeks to complete the gas desorption measurements.  Gas samples will be collected and analyzed to determine the sorbed-phase gas composition.

The gas desorption experiments will include direct comparisons of air versus water and whole core versus drill cuttings. It is anticipated that the data sets that will result from these experiments will enable identification of a best practice gas content analysis protocol for lignites.  At the conclusion of the gas desorption tests, the samples will be removed from the canisters and analyzed to determine the residual gas volume, sample mass, bulk sample volume, density, moisture content, ash content, moisture holding capacity, and Btu content (Nelson 1999, 2003b).

The analytical approach described above will be applied to the Montana samples to allow for comparison of method effectives and results between coals of different rank.

Task 4: Porosity and Permeability Evaluation

The focus will be to determine the effective cleat porosity and absolute permeability of the naturally occurring microfracture network in the North Dakota and Montana core samples.  These analyses will be performed by saturating a core sample from each site with brine and then measuring the volume of brine that is displaced when humidified helium is injected into one end of the sample (Pyrak-Nolte et al., 1993).

Task 5: CO₂ Storage Capacity Evaluation

The focus of this task will be to determine whether long-term contact with CO₂ affects the gas storage capacity properties of lignite coal.  Previous gas sorption studies of coal have only been conducted for very short time intervals lasting just a few days.  To evaluate the potential effects that long-term contact with CO₂ can have on the gas storage capacity properties of lignites, high-pressure CO₂ sorption tests will be conducted for time intervals lasting as long as one year.  The bulk compositional and gas storage capacity characteristics of the lignite coal will be evaluated prior to the start of the long-term high-pressure CO₂ sorption tests.  Periodically during the course of a year, lignite coal samples will be degassed and their bulk compositional and gas storage capacity characteristics will be evaluated.

The gas storage capacity characteristics of both CH₄ and CO₂ will be evaluated.  CH₄ is a nonpolar hydrocarbon that only interacts with coal by physical adsorption.  It is commonly assumed, but arguably far less well established experimentally, that CO₂ also interacts with coal by physical adsorption.  A recent laboratory study found that there was a relationship between the oxygen content of coal and the heat of CO₂ adsorption (Nishino, 2001).  This suggests the possibility of a chemical interaction between the CO₂ and surface oxygen functional groups.  Tests will also be performed to determine if the compositional properties of the lignite underwent change.  These tests will include proximate, ultimate, slurry pH, and Btu analysis.  It is anticipated that the data sets developed as a result of the gas storage capacity tests and compositional property tests will shed light on whether lignite coal is a stable matrix for either CO₂ sequestration or application of ECBM technology.

Gas storage capacity test results are affected by test conditions such as the analysis temperature and by coal sample properties, particularly moisture and mineral matter content (Nelson, 2003b; White et al., 2003).  The sample preparation and isotherm-testing methods commonly used by commercial service laboratories have inherent shortcomings that can significantly affect the accuracy of sorption isotherm analysis results (Lancaster and Hill, 1993).  As part of the project work scope, the EERC will construct an advanced isotherm-testing apparatus.  This will ensure the highest possible quality control for these critical experiments as well as the ability of the EERC to control the experimental test conditions to a far greater degree than would be possible if the tests were performed by a commercial service laboratory.

The analytical approach described above will also be applied to the Montana subbituminous coal samples to allow for comparison of method effectiveness and results between the two ranks of coal.

Task 6: Data Evaluation, Report Preparation, and Technology Transfer Activities

The focus will include data reduction, evaluation, and interpretation, as well as technology transfer activities, the preparation of task reports, and a comprehensive final report.  Technology transfer activities will include presentations of project results at technical conferences and the publication of papers in technical journals.

Project Tasks, Status, and Deliverables

Last Updated: 10/24/06