Abstract

The deformation of unconventional reservoir rocks is often characterized by a linear elastic constitutive model. However, recent laboratory experiments show substantial plastic volumetric deformation under shear stress. This observation highlights the need of adopting advanced constitutive models to accurately capture this mechanical response and comprehensively analyze its implications on well completions and production. Observations of micromechanical deformation are informative to the role of rock fabric in accommodating plastic deformation sources, and we present laboratory test results to evaluate the hydrostatic and shear enhanced compaction (Curran & Carroll, 1979) response of Vaca Muerta mudstone and Indiana Limestone samples, where the latter is being used as a reference. Additionally, we evaluate the micromechanical deformation characteristics of the two reservoir rocks.

Stress-strain data from hydrostatic and triaxial compression tests are used to properly quantify two different compaction responses. Hydrostatic compression testing characterizes the isotropic compaction response, whereas triaxial compression testing is utilized to comprehensively analyze compaction enhanced by shear, using the hydrostatic trend as a baseline. Under triaxial test conditions, we define the critical values for the onset of compaction and subsequent dilation and evaluate the microstructure via scanning electron microscopy (SEM) imaging to identify the possible mechanisms which could explain different sources of volumetric plastic deformation. We compare our Indiana limestone results with those from published literature. Moreover, we evaluate the deformation profiles of the samples and discuss their micromechanical behavior for both reservoir rock types. Finally, for Vaca Muerta samples, we adopted the calibrated Modified Cam-Clay model presented in Hasbani & Kias 2023, to simulate a triaxial test as a step towards use in future field applications (e.g., wellbore stability analysis and hydraulic fracturing).

Triaxial test results show stress-dependent elastic behavior and compaction at in-situ stress conditions. The bulk behavior is observed to be similar in both rocks, exhibiting elasto-plastic deformation driven by shear enhanced compaction. In addition, the anticipated linear-elastic and brittle failure behavior of Vaca Muerta samples is not observed. These experimental results cause concerns regarding the traditional assessment methods of wellbore stability, fracture propagation during hydraulic fracturing, and in the interpretation of in-situ stresses. The consequences of non-linear elasticity and plasticity to field applications have previously been reported by other authors for conventional and poorly consolidated rocks but were not anticipated to be significant in low porosity, unconventional reservoirs.

The experimental evidence presented in this work, highlights the inherent complexity of mudstone and limestone reservoir systems and their different mechanical behavior compared conventional elastic-brittle rocks. More importantly, our results suggest that ignoring plastic deformation and stress-dependent mechanical properties in unconventional reservoirs may result in inaccurate predictions of field behavior, particularly as the in-situ conditions change as a function of production and pressure depletion.

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