Abstract

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 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 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 greater than 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 (less than 1800 ft), fracture growth appears to be limited by the PC sand, fracture pressure rises, and ISIP greater than 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 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 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.

Introduction

Fracturing of coal seams in the San Juan Basin is used to stimulate production of natural gas. The Fruitland coal seams overlie the Pictured Cliffs (PC) sandstone, and there have been indications of fracture break-out from coal seams into the PC sandstone, although the PC sand is apparently nonproductive in our areas of interest in the northern part of the basin. Whether a fracture breaks out from the PC sand, or is confined by shale zones, is important in regard to fracture modeling predictions. This question has also been considered by other authors. We have tentatively identified another class of fractures in the San Juan Basin: fracture confinement by the PC sand at shallow depths (less than 1800 ft).

The fracturing treatments that are discussed in this paper use 30 lb/1000 gal borate cross-linked fracturing paper use 30 lb/1000 gal borate cross-linked fracturing fluid, with about 5000 lb of proppant per foot of coal height (35–45% of fluid is pad). They are mostly graduated proppant treatments, with 40/70 mesh sand to about 3 ppg preceding 12/20 sand to about 10 ppg. There are often two separate stages of fracturing: a lower one for seams close to the PC sand, and an upper one for the remaining seams well-removed from the PC sand. Amoco's method of completion is casing plus perforations in the individual coal seams. Analysis of some of these fracture treatments has been published elsewhere. A data set of many fracturing treatments is now available. The set contains (a) pressure-time profiles, and (b) wellbore gamma ray (GR) surveys of radioactive tracer added to proppant.

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