Laboratory studies of the compressibility and permeability of low-rank coal samples from the Powder River Basin, Wyoming, USA

We characterize the mechanical properties of coal samples from the Powder River Basin (Wyoming, USA) by conducting laboratory experiments. We present results from laboratory measurements of adsorption, static and dynamic elastic moduli, and permeability as a function of effective stress, pore pressure, and gas species. Notably, we observe that CO2 adsorption causes the static bulk modulus to decrease by a factor of two, while the dynamic bulk modulus remains essentially unchanged. Permeability of both intact and powdered samples decreases by approximately an order of magnitude in the presence of CO2, which is consistent with observations of adsorption-related swelling of the coal matrix. Interestingly, CO2 appears to change the constitutive behavior of coal; Helium saturated samples exhibit elastic behavior, while CO2 saturated samples exhibit viscous, anelastic behavior, as evidenced by creep strain observations. The Power River Basin, which extends from eastern Wyoming into southeastern Montana, is the largest and fastest growing CBM producer in the world, with approximately 17,000 wells currently producing 30 billion cubic feet of gas per month [19]. The primary production coal seams fall within the Wyodak-Anderson zone of the Fort Union Formation, at depths ranging from 500 to 1500 feet. For this study, we obtained fourinch diameter core samples from the Roland and Smith coal zones, from a depth range of 1300-1400 feet. The samples have an initial porosity of approximately 10%, an initial permeability of 1 millidarcy, and and ash content that varies from 10-20%. Our research focus for this study has been to better understand the effects of adsorption on the mechanical and flow properties of sub-bituminous coals. We conducted laboratory experiments on one-inch diameter core samples of coal, under hydrostatic and triaxial loading conditions, as a function of effective stress, using both helium and carbon dioxide as saturating gases. We present measurements of elastic stiffness, permeability, swelling strain, and creep strain, for both intact and powdered coal samples. 2. SAMPLE DESCRIPTION AND EXPERIMENTAL PROCEDURE We obtained 4-inch diameter core samples of subbituminous coal from the Roland and Smith coal zones of the Fort Union Formation in the Powder River Basin, Wyoming. The depth to these zones is approximately 1300-1400 feet. Note that these samples are from the same well and depth range as those reported on by Tang et al [20]. Figure 1 shows the field area, and Figure 2 shows the stratigraphic column. These samples were stored at room conditions, and so it was assumed that the initial methane gas was completely desorbed prior to testing. Due to the numerous cleats and micofractures, obtaining one-inch diameter core plugs for testing was difficult, and some samples were molded from powdered coal. For both the intact and powdered coal samples, sample preparation procedures followed those described by [20] in an effort to make comparison of the two data sets possible. For both the intact and powdered samples, cylindrical core plugs were prepared, with a nominal size of oneinch diameter and two-inch length. For reference, the initial porosity of the intact samples was approximately 10%, while that of the molded samples was approximately 30%. The samples contain 10-20% ash content, which serves to reduce the initial bulk density (~1.5 g/cc for the intact samples). Prior to testing, all samples were vacuum-dried until constant mass was achieved, to remove residual moisture and gas. Figure 1: Topographic map of the Powder River Basin, showing the area of active CBM development (adapted after [21]). Figure 2: Stratigraphic column of the major formations and coal zones of the Powder River Basin (adapted after [19]). Our laboratory apparatus is a conventional triaxial machine, commonly used in rock mechanics testing. For the purpose of this study, we modified it to enable simultaneous measurement of stress and strain, ultrasonic P and S wave velocities, and gas permeability. For schematics and specific details concerning the apparatus, please consult the 2006 GCEP Annual report (http://gcep.stanford.edu/). In an effort to establish a baseline set of measurements, we tried to isolate the effects of stress, pore pressure, temperature, moisture, and gas mixture by varying only one component of the system at a time. For all of the data shown below, measurements were carried out at room temperature, only single phase Helium or CO2 was used as a pore fluid, and moisture and humidity were minimized. However, both pore pressure and effective stress were varied during experiments. Measuring permeability as a function of both pore pressure and effective stress is particularly important, as permeability can vary due to swelling/shrinkage (a function of pore pressure) and due to mechanical opening/closure of pathways (a function of effective stress). 3. ADSORPTION ISOTHERMS We measured adsorption isotherms for both intact and powdered coal samples. Total adsorption isotherms (at 22°C) were determined for pure CO2 using the volumetric method. Our results are shown in Figure 3. While our isotherms fall slightly below those reported by Tang et al [20], given the difference in apparatus setups and innate heterogeneity of coal, we believe that a maximum of 15% error between the data sets provides a satisfactory match. Figure 3: Total adsorption isotherm for powdered and intact coal samples from the Powder River Basin, Wyoming. Experiments performed at a temperature of 22 C. Blue curves show data from this study. Black curves are shown for comparison, and are adapted after [20]. 4. STATIC AND DYNAMIC ELASTIC PARAMTERS We measured the elastic stiffness of intact samples of PRB coal using two different methods. Static elastic parameters can be determined from a stress-strain curve. In general, the slope of the tangent of the stress-strain curve at a particular value of strain gives the elastic stiffness at that point. For example, bulk modulus can be calculated by plotting hydrostatic stress against volumetric strain. On the other hand, assuming that the sample is an isotropic, homogeneous medium, dynamic elastic parameters can be derived from measurements of ultrasonic velocities. At a particular value of stress or strain, the dynamic bulk modulus is given by: K = ρ Vp 2 − 4 3 Vs 2 