THE CHARACTER OF COAL FRACTURE TREATMENTS IN OAK GROVE FIELD, BLACK WARRIOR BASIN

Forty-three gel fracture treatments are analyzed in this paper in both Mary Lee/Blue Creek seams and in Black Creek seams. Although 12/20 me sh sand concentrations were added to 10 ppg, there were virtually no s creen-outs, 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 mesh sand of the fracture ''entry region'': perforations, perforation/ fracture junction, or near-wellbore fracture constriction. It is postu lated that erosion is conspicuous because (a) not all perforated zones are taking fluid, (b) the overall perforation/fracture junction may b e more complex when subfractures from Black Creek seams join up to for m the main fracture, or (c) the fluid, sand, and products of erosion a re not confined to the coal seams. There are relatively few proppant-i nduced pressure increases, again possibly because the 12/20 mesh sand and products of erosion are not confined to the coal seams. The consta nt behavior of shut-in pressure with time in the majority of cases is consistent with an absence of any poroelastic effect (although about 2 5% of cases are consistent with a poroelastic effect). Approximately h alf 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 th roughout such fracture treatments are greater than 1 psi/ft, but gener ally decrease with time. In general the high-pressure T-fractures are shallower than the low-pressure vertical fractures. They do not show a ny correlation with a prominent fault block, which contradicts a previ ous finding. There are more proppant-induced pressure increases. In th e high-pressure Mary Lee/Blue Creek cases, the pressure drops at final shut-in range from small approximately 100 psi to large approximately 750 psi. The former appear to be consistent with an elevated fracture tip resistance (or apparent fracture toughness). The latter are consi stent with a near-wellbore flow constriction (discrete offsets/obstruc tions, 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 case s. The most likely explanation is a reduction in coal permeability due to high ambient in-situ stress or damage by the high fracturing press ures. Gas production appears better, by more than 50%, in low-pressure Mary Lee/Blue Creek cases than in high-pressure cases. This is consis tent with the notion that a vertical fracture is a more effective stim ulation than a T-fracture.