The Character of Coal Fracture Treatments in Oak Grove Field, Black Warrior Basin

Abstract

Forty-three gel fracture treatments are analyzed in this report -in both Mary Lee/Blue Creek seams and in Black Creek seams. Although 12/20 sand concentrations were added to 10 ppg, there were virtually no screenouts, presumably because pad volumes were so high (almost 50%). The Black Creek fractures are vertical, with substantial height growth, and are characterized by high treating pressures but relatively low fracture propagation pressures. There is conspicuous erosion by 12/20 sand of the fracture "entry region": perforations, perforation/fracture junction, or near-wellbore fracture constriction. It is postulated that erosion is conspicuous because (a) not all perforated zones are taking fluid, (b) the overall perforation/fracture junction may be more complex when subfractures from Black Creek seams join up 'to form the main fracture, or (c) the fluid, sand, and products of erosion are not confined to the coal seams.There are relatively few proppant-induced pressure increases, again possibly due to the fact that the 12/20 sand and products of erosion are not confined to the coal seams. The constant behavior of shut-in pressure with time in the majority of cases is consistent with an absence of any poroelastic effect (although about 25% of cases are consistent with a poroelastic effect). Approximately half of the Mary Lee/Blue Creek fractures are just like the Black Creek fractures and are interpreted similarly. The other half are different and exhibit high fracture propagation pressures. They are probably T-shaped fractures. A T-fracture is confined to a coal seam (there might be a T-fracture in more than one seam). Shut-in pressures measured throughout such fracture treatments are greater than 1 psi/ft, but generally decrease with time. In general the high pressure T·fractures are shallower than the low pressure vertical fractures. They do not show any correlation with a prominent fault block, which contradicts a previous finding.There are more proppant.-induced pressure increases. In the high pressure Mary Lee/Blue Creek cases, the pressure drops at final shut.-in range from References and illustrations at end of paper. 277small (~100 psi) to large (~750 psi). The former appear to be consistent with an elevated fracture tip resistance (or apparent fracture toughness). The latter are consistent with a near-wellbore flow constriction (discrete offsets/obstructions, or multistrands, or tortuous fluid flow path due to T-fracture geometry). The pressure decline after shut-in is not any faster in the high pressure Mary Lee/Blue Creek cases than in the low pressure cases. The most likely explanation is a reduction in coal permeability due t.o (a) high' ambient in-situ stress, (b) damage by the high fracturing pressures. Gas production appears better, by more than 50%, in low pressure Mary Lee/Blue Creek cases t.han in high pressure cases. This is consistent with the notion that a vertical fracture is a more effective stimulation than a T·fracture.

Modifying subterranean fluid flow patterns using barriers to improve pre-drainage in coal mining

“…Firstly, multiple, small seams may be stimulated together resulting in small fracture heights whereby high fluid velocities can sweep earlier placed tracers away from the wellbore when multiple tracers are used (Hanson et al, 1987). Next complex geometry may lead to "T"-shaped or "H"-shaped fractures which can only be inferred by 'hot spots' at boundary planes, unless some other diagnostic can confirm the complex geometry (Johnson et al, 2003;Palmer, 1990;Palmer et al, 1993;Palmer and Kutas, 1991;Scott, M. et al, 2010).…”