Frac Hits: Good or Bad? A Comprehensive Study in the Bakken
Frac hits are a persistent phenomenon that operators face periodically during unconventional field development. With basin maturity and infill drilling, frac hits play a major role in dictating overall production from multiwell pads. This paper focuses on the causes of frac hits and their subsequent impact on well EURs with solution methods to minimize negative impacts resulting from frac hits.
A fully numerical model-based was built around a four-well pad in Mountrail County, N.D., by integrating high-tier data including 3D sonic logs, nuclear magnetic resonance imaging and downhole spectroscopy to build a mechanical earth model of the reservoir. The parent well(s) are history-matched and geomechanical properties recalculated to changes in in situ stresses from parent well production. Infill wells are evaluated for asymmetric frac propagation toward depleted wells, and EURs are estimated to compare with those of the parent wells.
The initial well stimulation program and the volume of production from the parent well has a huge impact on the degree of fracture asymmetry in infill wells. This preferential propagation creates additional stimulated surface area between wells. If the parent well was understimulated in the first place, the infill wells in general result in a positive frac hit, and additional barrels of oil are produced from the parent well with little or no impact on the infill well. However, if the parent well has been on production for a long period the hydraulic fracturing treatment deposits a huge volume of fluid and proppant in already depleted areas, and the reservoir pressure is not sufficient to flush out the excess water. This causes the parent well to experience a surge in water cut and reduction in oil rate for an extended period. In addition, the infill well's initial production will not match the parent well IPs, and EUR can reduce drastically. This paper will categorically illustrate the timing, spacing and stimulation recommendations to minimize or mitigate these impacts.
Quantifying frac hits requires a comprehensive multiwell approach incorporating geomechanics, fracturing and production. This paper showcases case studies from the Bakken, identifying fracture asymmetry and production forecast from multiple wells, carefully considering all the physics, rock and fluid interaction in the subsurface strata. This will be a valuable tool for the engineers and geologists in the oil and gas community to effectively plan future infill development programs in unconventional reservoirs.