Numerical simulations of vertical growth of hydraulic fractures and brine migration in geological formations above the Marcellus shale

• Hydraulic fractures propagate above the stimulated zone by hundreds of feet.
• Natural fractures are a minor conduit for hydraulic fluid flow to overburden.
• Distance between hydraulic fractures affects pressure perturbation in overburden.
• Brine migration into overburden remains local and is not a threat of contamination.

One of the critical environmental questions about hydraulic fracturing in shales is the potential for contamination of ground and surface water. There are two specific concerns arising from hydraulic treatments: 1) whether hydraulic fractures extend upward through overlying strata to reach overlying aquifers containing drinking water, and 2) whether injected fluids push native fluids upward into these overlying aquifers. In this work, the extent of likely fracture growth through overlying layers during hydraulic treatment of the Marcellus shale was estimated using a hydraulic fracture model. A wide range of material and fluid flow properties in a multi-layered geologic model was considered. The model was based on conditions and characteristics applicable to the Marcellus shale in that part of the Appalachian basin within southwestern Pennsylvania. Predictions of vertical termination frequencies for hydraulic fractures were used in a multi-layer model of the strata and natural fractures for studying brine migration through the natural and induced fracture network. NFFLOW, the software for explicitly modeling flow within networks of fractures, was utilized to compute transient flow rates according to the schedule of injected fluid during hydraulic fracturing. To aid our analysis, the modeled sequence of geologic strata was capped with a fictitious unfractured, but moderately-permeable layer, which serves as a monitoring zone. The analysis assumes one well lateral was placed in the middle of the Marcellus shale with hydraulic fractures penetrating layers in the model. The newly-developed geomechanical module within NFFLOW was used to represent stress-sensitivity of the fractures. This allows the opening and closing of fracture apertures with changes in fluid pressures within fracture segments. Pressure increases in the formations overlying the Tully limestone, indicating fluid flow, was observed due to the hydraulic stimulation; and the impact of these increased pressures on brine migration towards the surface was considered.