Presented at SPE Annual Technical Conference and Exhibition, Houston, Texas, September 2004.
Fracture Propagation and Formation Disturbance during Injection and Frac-Pack Operations in Soft Compacting Rocks
The widespread use of FracPack technology in deepwater reservoir has been a growing practice. Its purpose is sand control and well stimulation. To-date, field applications and fracture treatments have been designed using traditional hydraulic fracturing simulators that apply LEFM theories. While this is adequate for hard rocks (e.g., tight gas formations), the fracture geometry predictions fall short when applied to fracturing soft rocks. Soft rocks are normally at incipient plasticity and, hence, are prone to compaction. Compaction, or plastic rock deformations during sand control FracPacks operations and disposal of drilling cuttings slurries in soft layers. The capacity of the created fracture to store or accept solids, the conditions of the rock strength near the fracture faces and the near well/fracture rock porosity or permeability are all highly impacted by the rock compaction during the fracture propagation process. The objective of the presented research is to assess the impact of compaction and plasticity on fracture geometry and formation properties around the fracture. In particular, it is important to quantify the details of the geometry of factures generated during FracPack and waste disposal operations as well as the porosity/permeability changes in the vicinity of the fracture faces. In the current paper, rock behavior is described by a variation of the Cam-Clay model. This model represents an inelastic, work hardening model that, depending on the loading path, could predict both compaction and dilatency, in a given formation. This is particularly useful in modeling soft or elasto-plastic compacting formations since the fracture propagation is heavily driven by the leak off into the formation and the in situ stress profile. Formation low permeability leads to lower leak off rates, especially if the injected slurry has a leak-off control additive. This scenario leads to compaction of the rock along the fracture sides. High permeability at the tip results in a large amount of fluid leak off into the formation causing the near-tip zone to dilate during slurry injection and fracture propagation. The current paper presents results of fracture simulation in compacting rocks including fracture geometry, fracturing pressure and porosity/permeability alteration around the fracture. In the previous paper, results of finite element model provided a benchmark to simulation results. The present work, on the other hand, shows the extent of formation disturbance and porosity/permeability alteration, as well as propped fracture characterization in FracPacks. Finally, the model results addressed the disparity between conventional FracPacks designs and actual treatment data. The observations confirm the need for careful consideration of rock plasticity in fracture simulation to avoid FracPack failures and minimize the absence of TSO response in some field implementations.