Controls of lithological heterogeneity on self-sealing behavior of propped fractures in Marcellus shale

Abstract

Developments in drilling technology and hydraulic fracturing methods have made shale formations an important geo-energy source and repository for energy-related wastes. Their very low permeability and outstanding potential for self-sealing are assets in sequestering wastes, but a limitation in sustaining shale gas production. We measure the permeability and self-sealing evolution of proppant-filled fractures in Marcellus shale, including the effect of time, normal stress, loading and unloading conditions, temperature and fluid composition. Permeabilities are measured over a first cycle of 24 h, with a hiatus of 56–91 days, and then remeasured to define impacts of physicochemical degradation (slaking) in a second cycle. Permeability reduces by up to 63 % where proppant crushing is isolated as a mechanism in embedment-eliminated steel split cores. In shales, lithological heterogeneity causes micro-slaking of clay-rich laminae appearing as stripes on the proppant oriented parallel to bedding planes with different proppant embedments resulting in differentiation in initial permeability values at the same proppant loading concentration. Intact rock compaction and mechanical closure-based self-sealing reduces permeability between 7.7 % and 21.6 % with an average value of only 14.5 %. Slaking, embedment, and swelling behavior in the Marcellus shale are responsible for all of the other remaining reductions in permeability. The reductions in permeability during all loading conditions correlate exponentially with time and can be defined by a single relation. The dimensionless constants of this equation depend on normal stress, physicomechanical properties and effective aperture. Long-term permeability measurements in a second cycle after 56–91 days show self-sealing through mineral precipitation in the Marcellus shale with a cohesive layer in the otherwise cohesionless proppant. The significant reductions between the initial and the second cycle permeability measurements reveal that time is a significant controlling factor in the self-sealing behavior of Marcellus shale in terms of reflecting creep deformation, slaking, and long-lasting geochemical processes.