First-Time Insights into Hydraulic Fracturing of Unconsolidated Sands from Novel Laboratory Experiments with in-situ CT-Scanning

Abstract

Waterflooding in unconsolidated sands has been observed to frequently result in injectivity decline of injectors when operated under ‘fractured’ conditions, resulting in reduction of waterflooding value creation and potential premature injector failure. Optimization of injector design and operation is currently limited by an insufficient understanding of the mechanics of ‘fracture’ and its associated mechanisms in unconsolidated sands, and the lack of adequate quantitative tools to predict injection performance. Utilizing in-situ CT scanning during large-scale laboratory injection experiments delivered novel insights into generation, closure and re-opening of ‘fractures’ in sand packs built from synthetic sands and highly unconsolidated downhole core material. Cavity formation was identified as main ‘fracturing’ mechanism. The cavities opened at injection pressures exceeding the confining stress and subsequently enlarged against a very low cohesive strength for the downhole core sands. This suggests material fluidization rather than shear failure as the immediate cavity initiation mechanism. Injectivity was found to be controlled by the interplay of fines transportation away from the injection region, resulting in injectivity increase, and the permanent compaction of the sand around the cavity, resulting in injectivity decrease. These first-time insights challenge the current understanding of matrix vs ‘fractured’ injection in unconsolidated sand reservoirs and highlight the role of sand fluidization and cavity formation. Furthermore, the injectivity behaviour is dependent on the combined effects of the sand material, the presence of fines, and the injection flow regime. The knowledge of the sand destabilization and mobilization processes enable design and operation optimizationof water injectors with implications on sand control strategies and remediation measures.

Propagation of Cryogenic Thermal Fractures from Unconfined PMMA Boreholes

“…While reporting some cryogenic fracture patterns, notably tortuosity, the studies inferred them from boundary observations [24][25][26] or post-mortem observa-tions after breakdown tests [15]. That is, the studies involved no real-time observations of internal cryogenic fracturing processes around the boreholes of 3D specimens, an approach similar to the CT-scan monitoring of hydraulic fracturing by de Borst et al [27]. Rather than flowing cryogen through boreholes, which maximizes the cooling rate and sustains a cryogenic temperature at the boreholes, most studies involved injecting cryogen one way, pressurizing the borehole and thus elevating the temperature, but the borehole temperature was not monitored [11,28,29].…”