Measuring Hydraulic Fracture Propagation and Closure in Sandstone

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ABSTRACT: This study investigates experimentally hydraulic fracture propagation, arrest and closure in permeable rocks. Through a controlled laboratory experiment on a Molasse sandstone block under a true triaxial state of stress, hydraulic fracture propagation and closure was monitored via the measurements of fluid pressure, fracture opening at the wellbore and continuous recording of passive acoustic at sixteen sensors. The combination of acoustic data, fluid pressure, and fracture opening measurements offer an interesting dataset to better understand fracture closure. Both the width and pressure behavior upon closure are consistent with recent theoretical estimate. This notably justify the use of pressure derivatives with respect to the square-root of shut-in time to pick fracture closure. The scaled opening versus reverse time and scaled pressure over the square root of reverse time exhibits a linear behavior, in line with the expected theoretical behavior. As a result, the width evolution near closure can be used to estimate the leakoff coefficient. Interestingly, our measurements also reveal the existence of a irreversible/residual opening at the wellbore even after complete fracture closure, whose value is of the order of the Molasse sandstone grain size. This may be attributed to a small amount of shear slip/fracture mixed modality, potentially causing non-alignement of asperities of the fracture surfaces at closure. 1 INTRODUCTION Hydraulic fracturing is a technique widely employed to enhance well productivity. It is also a reliable method to measure in-situ stresses at great depths from wellbores. The hydraulic fracturing process involves injecting a fluid at pressure sufficiently large to propagate a tensile fracture in the rock. Upon cessation of the injection, the fracture eventually closes due to the combined effects of the confining in-situ stress and the depressurization of the fluid associated with its permeation into the surrounding permeable rock matrix. The behavior of hydraulic fracture once the fluid injection has stopped depends on the dominant propagation regime. Indeed, in cases where toughness dominates, the fracture immediately arrests, and then close due to fluid leak-off. Conversely, if fluid viscous dissipation is dominating when the injection stops, the fracture continues to grow for a while driven by the energy elastically stored in the solid during the propagation phase (Möri and Lecampion (2021)). Following fracture arrest, fracture recession and closure starts. Its dynamic is mostly governed by fluid leak-off. Although a large number of numerical and theoretical models have been investigated (Nolte (1979a); Desroches and Thiercelin (1993); Papanastasiou (2000); Mohammadnejad and Andrade (2016); Peirce (2022); Peirce and Detournay (2022b,a)), experimental investigation with precise measurements of fracture arrest and closure are limited (De Pater et al. (1996)).