Traditionally, hydraulic fracturing simulations have been based on elastic theories. Although this is adequate for hard rocks the fracture geometry predictions fall short when applied to fracturing soft rocks that are at incipient plasticity, or that are prone to compaction. These phenomena are usually encountered during fracturing for sand control, FracPacks and disposal of slurries and drilling cuttings in soft layers. The capacity of the created fracture to store solids, the conditions of the rock strength near the fracture faces and the near well/fracture rock permeability are all highly impacted by the rock compaction during fracture propagation.

The objective of the presented work has been to develop a comprehensive understanding of the impact of rock compaction and plasticity on the fracture geometry (and volume) and formation properties around the fracture. In particular, it is important to quantify the details of the geometry of factures generated during FracPac and waste disposal operations as well as the permeability changes in the vicinity of the fracture associated with these processes.

The rock behavior is modeled using a variation of the Cam-Clay model. The model is an inelastic work hardening model that, depending on the loading path, could be used to predict both compaction and dilatancy, in a given formation. The current work has been directed at quantifying the increased fracture volume as a result of rock compaction during injection of viscous fluids.

The results of fracturing modeling in plastic/compacting rocks that incorporate the compaction behavior of soft formations is presented in terms of fracture volume capacities and permeability alteration associated with the fracture propagation. This is especially necessary and useful in modeling soft or elastic-plastic compacting formations since the fracture propagation is heavily driven by the leak off into the formation. Formations with low permeability lead to lower leak off rates, especially if the injected slurry has a low leak-off property. This scenario leads to compaction of the rock along the fracture sides. Formations with high permeability cause a large amount of fluid to leak off into the formation causing the formation to dilate during slurry injection and fracture propagation.

Results from fracturing models and injection simulation have been benchmarked against finite element models and other fracture simulators to insure that the calculated widths are accurate. Extent of formation disturbance and permeability alteration are also shown. Finally, the strong implications of rock compatibility on tip-screen-out treatment design are also addressed.


1.1. Background

Hydraulic fracturing has been the work horse of the oil and gas industry for enhancing well productivity and in performing remedial workover tasks. Recently, its use has been expanded to encompass newer applications such as FracPack (sand control) treatments, storage (injection) of drilling cuttings, disposal of solid waste slurries and other returns from the wells. At present it has also been used in conjunction with remedial/initial operations in improving the response of gas storage repositories and controlling water flooding and/or injector performance.

A significant part of the optimization of fracturing operations requires accurate predictions of the created fracture extent and its geometry.

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