Investigating the Mechanisms and Time-Scale of Imbibition During
Hydraulic Fracturing Flow-Back Operation in Tight Reservoirs
Mingyuan Wang, Juliana Leung, University of Alberta
Economic production of tight oil reservoirs requires multi-stage hydraulic fracturing. This
stimulation technique would further accelerate production if hydraulic fractures can
connect to sweet spots with enhanced permeability due to the presence of natural
fractures. However, it is often reported that only 10-50% of the water-based fracturing
fluids can be recovered during flow-back operation. This water may potentially be trapped
in closed discontinuous fractures, but some may also be imbibing into the rock matrix.
This study investigates the impacts of water imbibition in reservoirs with multi-scale
fractures.
In this research, we build a series of discrete fracture networks consisting of both natural
fractures and hydraulic fractures to simulate imbibition and flow-back of hydraulic
fracturing fluid. In our models, the positions and sizes of natural fractures are distributed
stochastically. The DFN models are subjected to numerical multiphase flow simulation to
model fluid movement and distribution during shut-in (soaking period) and flow-back.
Counter-current imbibition may either lead to simple fluid redistribution within the fractured
rock or, alternatively, it can cause blockage of oil from being able to flow back into the
hydraulic fractures. Although high capillary pressure in the matrix enhances spontaneous
imbibition, but the process is quite slow, and the time scales for this imbibition to occur are
studied. The presence of oil-filled natural fractures could also provide alternate pathways
for redistribution of oil and water in relation to the hydraulic fracture. Impacts of various
natural fracture parameters including fracture connectivity, configuration, and intensity are
illustrated in a systematic sensitivity analysis.
This research demonstrates the applicability of DFN models in describing heterogeneities
observed in tight reservoirs. These models allow detailed analysis of fluid flow within a
complex fracture system. Estimating the time scales required for imbibition to occur would
help to assess the impact of shutting in a hydraulic fracture system prior to flow-back. This
work aims to demonstrate the positive and negative impacts of shut-in duration preceding
flow-back. This is a significant consideration for hydraulic fracturing design. Results
obtained from this research would help considerably in optimization of
production/development strategies and assessment of stimulated reservoir volume.