GEOLOGIC SCREENING CRITERIA FOR SEQUESTRATION OF CO2 IN COAL: QUANTIFYING POTENTIAL OF THE BLACK WARRIOR COALBED METHANE FAIRWAY, ALABAMA

Abstract

Sequestration of CO{sub 2} in coal has potential to reduce greenhouse gas emissions from coal-fired power plants while enhancing coalbed methane recovery. Data from more than 4,000 coalbed methane wells in the Black Warrior basin of Alabama provide an opportunity to quantify the carbon sequestration potential of coal and to develop a geologic screening model for the application of carbon sequestration technology. This report summarizes stratigraphy and sedimentation, structural geology, geothermics, hydrology, coal quality, gas capacity, and production characteristics of coal in the Black Warrior coalbed methane fairway and the implications of geology for carbon sequestration and enhanced coalbed methane recovery. Coal in the Black Warrior basin is distributed among several fluvial-deltaic coal zones in the Lower Pennsylvanian Pottsville Formation. Most coal zones contain one to three coal beds that are significant targets for coalbed methane production and carbon sequestration, and net coal thickness generally increases southeastward. Pottsville strata have effectively no matrix permeability to water, so virtually all flow is through natural fractures. Faults and folds influence the abundance and openness of fractures and, hence, the performance of coalbed methane wells. Water chemistry in the Pottsville Formation ranges from fresh to saline, and zones with TDS content lower than 10,000 mg/L can be classified as USDW. An aquifer exemption facilitating enhanced recovery in USDW can be obtained where TDS content is higher than 3,000 mg/L. Carbon dioxide becomes a supercritical fluid above a temperature of 88 F and a pressure of 1,074 psi. Reservoir temperature exceeds 88 F in much of the study area. Hydrostatic pressure gradients range from normal to extremely underpressured. A large area of underpressure is developed around closely spaced longwall coal mines, and areas of natural underpressure are distributed among the coalbed methane fields. The mobility and reactivity of supercritical CO{sub 2} in coal-bearing strata is unknown, and potential exists for supercritical conditions to develop below a depth of 2,480 feet following abandonment of the coalbed methane fields. High-pressure adsorption isotherms confirm that coal sorbs approximately twice as much CO{sub 2} as CH{sub 4} and approximately four times as much CO{sub 2} as N{sub 2}. Analysis of isotherm data reveals that the sorption performance of each gas can vary by a factor of two depending on rank and ash content. Gas content data exhibit extreme vertical and lateral variability that is the product of a complex burial history involving an early phase of thermogenic gas generation and an ongoing stage of late biogenic gas generation. Production characteristics of coalbed methane wells are helpful for identifying areas that are candidates for carbon sequestration and enhanced coalbed methane recovery. Many geologic and engineering factors, including well construction, well spacing, and regional structure influence well performance. Close fault spacing limits areas where five-spot patterns may be developed for enhanced gas recovery, but large structural panels lacking normal faults are in several gas fields and can be given priority as areas to demonstrate and commercialize carbon sequestration technology in coalbed methane reservoirs.