Speaker
Description
Unconsolidated sandstones form high-quality reservoirs and aquifers, playing a key role in subsurface energy activities. Hydraulic fracturing in these formations is known to be governed by plastic shear localization and particle transport; however, the exact mechanisms by which these processes operate remain poorly understood. As a result, accurately predicting the onset of fracturing, as well as the directions and lengths of fracture propagation, remains a significant technical challenge. We present the main outcomes of several years of investigation on the mechanisms of hydraulic fracturing in unconsolidated sandstones conducted in the Navier Laboratory, which includes experimental testing [1,2] and numerical modeling [3].
The experimental tests consist of the radial injection of water into compacted mixtures of Fontainebleau sand and silica particles. Several initial stress states were investigated, and size effects were assessed using two experimental setups: a small triaxial cell and a larger chamber. In these tests, injection was performed at a controlled flow rate that was increased in a stepwise manner. The occurrence of an initial pressure drop upon increasing the flow-rate step is interpreted as the onset of fracturing (Figure 1c). The measured fracturing pressures exhibited a consistent ratio with the confining stress within each experimental setup; however, size effects were observed in this ratio when comparing results from the triaxial cell and the larger chamber. Post-mortem micro-CT scans (Figures 1a and 1b) and microscope observations revealed that the fractures were vertical porous channels, propagated in the radial direction, from which the small silica particles had been washed out.
We developed finite-element numerical models to reproduce these experiments, with the aim of aiding their interpretation and allowing extrapolation to field conditions. We used three coupled models: fluid flow, particle transport, and mechanical equilibrium. The numerical model successfully reproduced both the geometry and the nature of the observed fractures (Figure 1d), as well as the measured fracturing pressures (Figure 1c) and their dependency on applied stresses. They also shed light on the observed size effects, which are attributed to the existence of a threshold flow velocity required to trigger particle mobilization. Moreover, the models elucidate the highly coupled mechanisms that lead to the hydraulic fracturing of unconsolidated sandstones. The onset of localized plastic shear dilation creates small zones of high permeability and flow rate, in which particle transport is initiated. This enhanced particle transport, in turn, induces local pressure increases, leading to the extension of plastic shear bands in the direction of flow.
| References | [1] Nguyen, T. T., Sulem, J., Muhammed, R. D., Dupla, J. C., Canou, J., Boero-Rollo, J. G., & Ochi, J. (2022). An experimental setup with radial injection cell for investigation of fracturing in unconsolidated sand reservoirs under fluid injection. Journal of Petroleum Science and Engineering, 213. https://doi.org/10.1016/j.petrol.2022.110362 [2] Nguyen, T. T., Sulem, J., Muhammed, R. D., Dupla, J. C., Canou, J., Boero-Rollo, J. G., & Ochi, J. (2023). Experimental investigation of plugging and fracturing mechanisms in unconsolidated sand reservoirs under injection of water containing suspended fine particles. Geoenergy Science and Engineering, 221. https://doi.org/10.1016/j.geoen.2022.211346 [3] Loyola, A. C., Sulem, J., Dupla, J. C., & Ochi, J. (2024). Numerical modeling of the fracturing mechanisms of unconsolidated sand reservoirs under water injection. Geomechanics for Energy and the Environment, 38. https://doi.org/10.1016/j.gete.2024.100550 |
|---|---|
| Country | France |
| Acceptance of the Terms & Conditions | Click here to agree |
