The learning curve has evolved in the last few years for operators in shale plays. Early wells started with relatively large cluster spacing and small proppant volumes resulting in suboptimal initial completions. Over the years, perforation cluster spacing has declined. Consequently, the number of hydraulic fracturing stages has increased. The total proppant pumped per lateral foot has also increased. The majority of the existing wells were completed with geometrically spaced multiple perforation clusters per stage. Sometimes more than six clusters per stage have been employed. Studies have shown that one-third of these perforation clusters are not productive (Miller et al., 2011). Noncontributing perforation clusters could be due to not initiating hydraulic fractures, insufficient proppant placement, or loss of near-wellbore connection due to over-flushing or severe drawdown. Furthermore, during the development phase, the depletion from parent wells leads to asymmetric hydraulic fracture growth on closely spaced infill wells. Parent wells may also be negatively impacted due to hydraulic fracture interference from new completions. These factors have led to poor hydrocarbon recovery factors, sometimes less than 10% in horizontal shale wells.
Recovery factors from existing wells can be improved through restimulation. Candidate selection is a key in achieving economically successful restimulation. Restimulation of appropriate horizontal shale wells resulted in significant production uplifts based on early field results. Designing a fit-for-purpose restimulation treatment is dependent on initial completion, offset well distance, infill plan, and, above all, economics. On top of the design aspect, operationally achieving effective restimulation on long horizontal wells with tens of perforation clusters is a challenging task. Thus real-time monitoring and control is a key for field execution.
This work uses an integrated petrophysical, geomechanical, hydraulic fracture, and reservoir modeling workflow and field observations to develop restimulation strategies for improving hydrocarbon recovery. This integrated workflow includes a multistep calibration process to reduce uncertainty. One of the key calibration steps is to model hydraulic fracture growth accounting for local geological heterogeneity and match with observed treatment parameters and microseismic interpretations. Another critical calibration step includes automatic gridding of hydraulic fracture geometry to run numerical reservoir simulation to match realized production results. Reservoir pressure distribution at the end of the production history is used to recalculate stresses for modeling the refracturing scenarios.
Multiple practical refracturing scenarios were constructed for addressing near-wellbore connectivity issues and ineffective drainage along the lateral. Creating new surface area in undrained rock and restoring productivity of existing hydraulic fractures resulted in higher recovery. Higher proppant amounts in undrained rock on one well pad or laterals with wider well spacing improved recovery. However, larger jobs can lead to significant interference for closely spaced wells. In conclusion, this paper demonstrates that properly designed restimulation treatments lead to improved recovery.