Abstract

Hydraulic fracturing is the primary completion method in the unconventional Sohagpur Block coal seam gas (CSG) project targeted at low to high-volatile bituminous Barakar coal seams. Each well contains up to four to six seams with an aggregate thickness of 6 to 32 m and depths ranging from 400 to 800 m.

The coal seams are mostly comprised of naturally fractured material (butt and face cleats) with very unique geomechanical properties, which poses many challenges in terms of coal stimulation. The coal is inherently prone to creating multiple fractures during hydraulic fracturing stimulation treatments, including T-shaped, parallel, or branched fractures. Commonly, during multiple seam stimulation, the probability of complex fracturing increases dramatically. The higher value of pressure-dependent leakoff tendency and high process zone stress (PZS) leads to higher treating pressures (relative to depth) during fracturing treatment, which, in turn, normally causes breakout into and possibly through the lower stress boundary zones. Poor proppant placement and inconsistent areal proppant distribution leads to minimized proppant pack conductivity and ultimately productivity loss. As a result, the hydraulic fracturing treatment options dwindle significantly, thus reservoir stimulation results are not optimum in each seam.

During the CSG unconventional stimulation case discussed in this paper, the determination of fracture height and proppant pack quality was of the utmost importance. To evaluate fracture height and proppant pack quality and minimize complex fracturing growth and breakouts, a unique and environmentally acceptable high thermal neutron capture compound (HTNCC)-coated proppant technology was adopted for the first time in this CSG basin in India. The new proppant system was pumped during multiple seam stimulations to verify the post-fracture analysis of the coal seam observed using a three dimensional (3D) planar numerical model. The results were then used to sensitize fluid systems and volumes and tune proppant volumes with optimum treatment pumping rates against the standard "cookie cutter" method of 4,000 lbf/ft of coal. The normally observed extensive height growth and proppant distribution was then re-evaluated during the next interval treatment.

The application of this nonradioactive (RA) tracer technology has resulted in achieving increased effective fracture length of each individual seam, increased proppant conductivity, and significant fracturing pressure drop during the treatment execution. Overall, this technology has greatly helped the development of effective coal seam fracturing design and validation. Selection of the correct diagnostics has helped improve the understanding of some of the significant questions remaining in regard to unconventional CSG basins. This paper discusses the lessons learned to achieve optimized seam completion design and verified results, which can be applied in other CSG basins.

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