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Hydraulic fracturing and wellbore completion of coalbed methane wells in the Powder River Basin, Wyoming: Implications for water and gas production

¹ Department of Geophysics, Stanford University, Stanford, California 94305; present address: Kalchbuehlstrasse 20, Zurich 8038, Switzerland; lbcf{at}pangea.stanford.edu
² Department of Geophysics, Stanford University, Stanford, California 94305

Lourdes Colmenares got her undergraduate degree as a geophysics engineer at the Universidad Simón Bolívar in Caracas, Venezuela. She worked as an instructor in the Department of Earth Sciences of this university for one year before joining the Stress and Crustal Mechanics Group at Stanford University, from which she received her Ph.D. in geophysics in January 2005.

Mark Zoback is the Benjamin M. Page Professor of Earth Sciences and professor of geophysics at Stanford University. His principal research interests are in the fields of crustal stress and reservoir geomechanics. He is currently one of the principal investigators of the SAFOD Project (San Andreas Fault Observatory at Depth) and chair of the Science Advisory Group of the International Continental Drilling Program.

Excessive water production (more than 7000 bbl/month per well) from many coalbed methane (CBM) wells in the Powder River Basin of Wyoming is also associated with significant delays in the time it takes for gas production to begin. Analysis of about 550 water-enhancement activities carried out during well completion demonstrates that such activities result in hydraulic fracturing of the coal. Water-enhancement activities, as the operators in the basin call this procedure, consists of pumping 60 bbl of water/min into the coal seam during approximately 15 min. This is done to clean the wellbore and to enhance CBM production. Hydraulic fracturing is of concern because vertical hydraulic fracture growth could extend into adjacent formations and potentially result in excess CBM water production and inefficient depressurization of coals. Analysis of the pressure-time records of the water-enhancement tests enabled us to determine the magnitude of the least principal stress (S₃) in the coal seams of 372 wells. These data reveal that because S₃ switches between the minimum horizontal stress and the overburden at different locations, both vertical and horizontal hydraulic fracture growth is inferred to occur in the basin, depending on the exact location and coal layer. Relatively low water production is observed for wells with inferred horizontal fractures, whereas all of the wells associated with excessive water production are characterized by inferred vertical hydraulic fractures. The reason wells with exceptionally high water production show delays in gas production appears to be inefficient depressurization of the coal caused by water production from the formations outside the coal. To minimize CBM water production, we recommend that in areas of known vertical fracture propagation, the injection rate during the water-enhancement tests should be reduced to prevent the propagation of induced fractures into adjacent water-bearing formations. In areas where S₃ is unknown, a minifrac should be done to determine the magnitude of S₃ (to know whether fracture propagation will be vertical or horizontal), so the water-enhancement activities at the time of well completion are done to minimize water production and optimize gas production.

This article has been cited by other articles:

				H. E. Ross and M. D. Zoback Sub-hydrostatic pore pressure in coalbed and sand aquifers of the Powder River Basin, Wyoming and Montana, and implications for disposal of coalbed-methane-produced water through injection Rocky Mountain Geology, November 1, 2008; 43(2): 155 - 169. [Abstract] [Full Text] [PDF]

				C. E. Campbell, B. N. Pearson, and C. D. Frost Strontium isotopes as indicators of aquifer communication in an area of coal-bed natural gas production, Powder River Basin, Wyoming and Montana Rocky Mountain Geology, November 1, 2008; 43(2): 171 - 197. [Abstract] [Full Text] [PDF]