Recently, hydraulic fracturing has been widely applied in unconsolidated sandstone reservoirs, leading to the development of an integrated fracturing and sand control technique called fracture packing. Unconsolidated sandstone typically has high porosity and permeability, poor cementation effects, and low strength. Furthermore, the mechanisms of fracture initiation and propagation are complex, involving multiple rock deformation and failure modes, such as tensile, shear, and plastic compaction. These mechanisms are not yet fully understood, making it difficult to optimize fracturing fluid and process parameters and to control fracture morphology. A major reason for this knowledge gap lies in the experimental methods used. Conventional laboratory methods involve separate sample preparation and fracturing processes, which introduce stress disturbances and pressure changes during cementing. These issues are particularly severe in unconsolidated sandstone, potentially compromising the accuracy of the results. To address these challenges, this study developed a new experimental methodology to more accurately investigate the hydraulic fracturing behavior of unconsolidated sandstone.
We developed a large-scale experimental setup for the physical simulation of in situ sample preparation and the true triaxial hydraulic fracturing of unconsolidated sandstone. It allowed continuous operation from sample preparation to fracture propagation under stable stress conditions, eliminating stress disturbances caused by sample transfer. Our methodology involved the designing of an artificial unconsolidated sandstone formula based on natural core analysis. Chemical cementing agents were deliberately avoided to replicate realistic formation strength properties. Each experiment began by establishing a simulated wellbore model, followed by filling a sand mixture inside the load frame. After the target stresses stabilized, the fracturing simulation began immediately without sample movement. Using this setup, we conducted a series of comprehensive tests to investigate the effects of varying permeability on hydraulic fractures in unconsolidated sandstone reservoirs. Furthermore, by leveraging the setup’s sample preparation efficiency, we explored the fracturing behavior of heterogeneous unconsolidated sandstone, including scenarios with near-wellbore damage zones and vertically stratified formations containing shale barriers. The fracturing fluid used in the experiments was an organoboron crosslinked gel, with variations in polymer concentration and injection rate as key parameters. Throughout fracturing, pressure data were recorded in real time, and the post-test fracture geometries were characterized through detailed layer-by-layer dissection.
Experimental results revealed the substantial leaking of the fracturing fluid at the initiation site and along the fracture path in unconsolidated sandstone, representing a typical characteristic and the primary control of fracture propagation in these reservoirs. Furthermore, the results showed that active leak management is important for transforming the fracture mechanism from a complex shear failure to one involving the development of more effective tensile fractures. Direct visual evidence from our tests showed that hydraulic fractures successfully traversed the near-wellbore damage zone in the unconsolidated sandstone. Moreover, in vertically heterogeneous formations with negligible stress contrast, fractures traversed low-permeability shale layers, leading to significant vertical growth.
In summary, this study developed an innovative experimental framework for modeling true triaxial hydraulic fracturing in unconsolidated sandstone. Its pivotal achievement was the integrated apparatus that enabled sample preparation and subsequent fracturing without stress-induced disturbances, providing reliable, representative results. Our findings advance understanding of fracturing mechanics in unconsolidated sandstone formations, highlighting the important role of fluid leak control and revealing previously overlooked fracture height containment issues in heterogeneous reservoirs. These results provide a validated physical basis for optimizing key design parameters, such as the fracturing fluid composition and pump rate, in unconsolidated sandstone reservoirs.
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