EFFECT OF FRACTURE NETWORK COMPLEXITY AND EVOLUTION ON FRACTURING FLUID RECOVERY AND ITS IMPLICATIONS ON AQUIFERS’ SAFETY

Abstract

It has been reported that the recovery of the fluid injected during hydraulic fracturing (HF) is considerably low. Hence, the fate of the fracturing fluid in the subsurface and its migration become a concern for water aquifers contamination. In this work, we study the migration of fracturing fluid and track its distribution in fractured reservoirs using a two-dimensional finite element model with multi-phase and multi-component flow. We determine the effect of fracture network complexity (represented by interconnecting hydraulic conduits or faults), HF shut-in period, formation and fault permeability and its temporal evolution - driven by pressure changes - on fracturing fluid recovery. Our results indicate that extending the shut-in period curbs the recovery and increasing network complexity results in a larger fluid loss into the formation. Moreover, we found that the correlation between the subsurface permeability structure and the recovery is not straightforward; the recovery increases when the permeability of faults increases and decreases when the permeability of the layers surrounding the HF zone increases. Our findings also show that the pressure-dependency of the permeability, i.e., fractures and cracks open during high pressure injection and close or partially close when the pressure is released, significantly affects the flow-back recovery. This can further justify the observed low fluid recovery rates in most cases. Finally, we show that the time needed for trapped fracking fluid to migrate to nearby aquifers ranges from a few months to hundreds of years. Consequently, it is crucial to assess the long-term effects of the hydraulic fracturing practices since the associated environmental hazards might require several years to be noticed or detected.