Fracturing through perforations in the coal is often used in the San Juan Basin to stimulate production of natural gas from Fruitland coalbeds, which overlie the Pictured Cliffs (PC) sandstone. A data set of many fracturing treatments has been analyzed, including (a) pressure versus time profiles, and (b) wellbore gamma ray (GR) surveys of radioactive tracer added to proppant. First, factors that tend to confine a fracture's height growth are reviewed, and vertical stress profiles are discussed. Secondly, three classes of fractures are identified:

  1. When the PC sand is close (less than 20 ft) and the coal is deeper than 1800 ft, a fracture is likely to grow into the PC sand, accompanied by falling pressure.

  2. When the PC sand is more than 20 ft away, a fracture is largely confined by the shales and accompanied by rising or flat pressure, a proppant-induced pressure increase (PIPI), and ISIP > 1 psi/ft. There is likely to be a T-fracture. The limited (or zero) fracture height growth in bounding shales is explained.

  3. When the PC sand abuts one of the coal seams at shallower depths (<1800 ft), fracture growth appears to be limited by the PC sand, fracture pressure rises, and ISIP > 1 psi/ft. This may be due to factors which encourage interfacial slippage at shallower depths.

The rapid fall in tracer signal from a coal seam into bounding zones is interpreted by a discrepancy in fracture width, or due to zero fracture width in shale. Proppant tracer signals in shale-bounded coals are asymmetric: more peaks occur at or above the coal roof than at or below the coal floor. This may reflect a horizontal fracture component near the roof of the coal seam (i.e., a T-fracture), but more analysis is needed to confirm this. When several seams are stimulated simultaneously, if two coal seams are separated by more than 15 ft, they are unlikely to be linked by tracer, and fracture communication may not exist between the two seams. Although proppant appears to have access to every seam usually, this may not mean equal fracture lengths in these seams. Finally, an anti-correlation between 40/70 and 12/20 proppant tracer supports 40/70 proppant plugging smaller flow channels, thus diverting fluid to other channels which become wider, and can more easily transmit 12/20 proppant. This may explain success of "graduated proppant" fracture treatments.

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