CO2 Sequestration Potential of Texas Low-Rank Coals

Abstract

Fossil fuel consumption is a primary source of CO2, a major greenhouse gas. Geologic methods, such as injection and sequestration of CO2 in coals may offer viable methods of reducing CO2 atmospheric emissions, while providing the added benefit of enhanced coalbed methane recovery. The potential for CO2 sequestration in low-rank coals is unknown, and it differs significantly from bituminous coals. To evaluate the feasibility and the environmental, technical, and economic impacts of CO2 sequestration in Texas low-rank coal beds, we are conducting a 2-year study to characterize the coals located near major electrical power plants. We have identified potential of CO2 sequestration sites in coals near 3 Texas power plants. These 3 power plants discharge 34,068,751 short tons of CO2 annually, accounting for 14.6 % of Texas' point-source emissions. On the basis of preliminary modeling of one site, using assumed permeability, CO2 storage and methane content values, we conclude that 360 wells on 80-acre spacing could sequester CO2 emissions for the Gibbons Creek plant for 11 years, while producing 90 Bcf of coalbed methane. However, reservoir properties are poorly known and, thus, the volumes of CO2 that may be sequestered and the amount of methane that may be produced are speculative at present. Future study will focus on better reservoir characterization and additional modeling. Introduction Texas, with 7% of the population of the United States, ranks first in the nation in consumption of petroleum, natural gas, coal, and electricity. Texas is also the biggest producer of electricity in the country.1 In 2000, 46% of electricity in Texas came from natural gas-fired plants, 41% from coal- and lignite-fired plants, and 13% from nuclear plants.2 We estimate that approximately 425,000,000 short tons of CO2 are emitted in Texas each year. This is based on an average CO2 emission rate of 20.57 metric tons per year per capita.3 In Texas, 274 power plants of various types produced 233,075,220 short tons of CO2 in 2001.4 This suggests that power plant emissions account for more than half of the total CO2 emissions in Texas. Sequestration of CO2 from power plants could have a direct and significant effect on Texas CO2 emissions. Approximately 54% of the total CO2 emitted by power plants is emitted by the 15 largest (major) power plants, and nearly 33% comes from the top 5 plants (Table 1 and Fig. 1). Martin Lake Plant, which is the largest lignite-fired generating plant in the world, produces 8% of total emissions from Texas power plants. Thus, sequestering CO2 from a few very large power plants could significantly impact CO2 emissions in Texas. Lignite and coal are the fuels used by the largest Texas CO2 emitters (Fig. 2). Major Texas power plants are concentrated in the most heavily populated eastern and southeastern parts of the state (Fig. 3). CO2 Sequestration in Coals A primary man-made source of greenhouse gas is CO2 from energy consumption. Viable methods of reducing CO2 emissions are greatly needed. Recently, researchers have suggested sequestering CO2 in coals. CO2 injected in coal beds may have the dual benefits of CO2 abatement and enhanced coalbed methane recovery by displacement.5 Primary recovery efficiencies of coalbed methane range from 20 to 60% of original gas in place. CO2 injection can improve methane recovery and help maintain reservoir pressure, thus offsetting operational costs by reducing the amount of gas compression required.

Evaluation of the Technical and Economic Feasibility of CO2 Sequestration and Enhanced Coalbed-Methane Recovery in Texas Low-Rank Coals

“…Based on USGS and Sandow mine analyses, Garduno et al had used a 10:1 ratio of CO 2 :CH 4 in preliminary reservoir modeling for this study. 6 The 2.5:1 ratio obtained for the APCL2 samples is similar to results from bituminous coals in other basins. 13 However, those results were for much higher rank coals than those of the Wilcox Group in the APCL2 well.…”

“…Vitrinite reflectance of the composite coal sample, which was measured over a 148-ft depth interval (6,262 ft), ranged between 0.47% and 0.61%; mean value was 0.54%. Coal rank is borderline between highvolatile C and high-volatile B bituminous.…”

“…We assumed a project area of 30,000 acres, based on preliminary studies. 6 Studies were run at different well spacings (40, 80, 160, and 240 ac). The number of 5-spot patterns required was computed by dividing the project area by the pattern area corresponding to each well spacing.…”

Effects of Water Salinity, CO2 Solubility, and Gas Composition on Coal Wettability

“…The combustion of fossil fuels (coal, natural gas, and oil) for energy and transportation is the main human activity that emits CO 2 (Houghton et al, 2001;State, 2010). CO 2 has be sequestrated in coal seams to enhance the coalbed methane production (ECBM) in addition to CO 2 storage (Garduno et al, 2003;Mavor et al, 2002;Schroeder et al, 2002;Seidle, 2000). Different authors have discussed the applicability of CO 2 sequestration in underground formations (depleted oil reservoirs, saline water aquifers, or salt caverns), in order to reduce the effect of global warming and also to enhance oil recovery (EOR) (Benson and Cole, 2008;Espie, 2005;Mohamed and Nasr-El-Din, 2013;Pilisi et al, 2010).…”

CO2 Sequestration in Unmineable Coal Seams

“…CO 2 has been sequestrated in coal seams to enhance coalbed methane production (ECBM), in addition to CO 2 storage (Seidle 2000;Mavor et al 2002;Garduno et al 2003;Ibrahim and Nasr-El-Din 2015c). In general, the coal structure consists of a cleat network system (Ͼ 50 nm) and a coal matrix system (Ͻ 50 nm).…”