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.
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