Oil operators have faced technical challenges while drilling and producing wells in the Vaca Muerta formation (Argentina). Wells must be hydraulically fractured for stimulation and in situ stress strongly influence this process. During stimulation, several geomechanics-related issues can occur, which have the potential to negatively impact the operation of the field. Different studies suggest a strike-slip regime in the Vaca Muerta formation. Furthermore, several sections of the formation exhibit intercalations of organic shale with limestone, ash beds, and carbonate veins. The combined effect of the reduced stress difference and the material fabric may promote horizontal fractures. The present paper discusses numerical modeling of the stresses during hydraulic fracture propagation considering actual properties for Vaca Muerta formation. A case study is presented, where the effects of different features are being quantified.
Shale is characterized by thin laminate or parallel layering and exhibits transversely isotropic properties with symmetric axes perpendicular to the bedding. The rock behavior is modeled using Biot’s poroelastic theory (Detournay et al. 1993). A fracture initiation criterion and a non-linear fracture propagation model (interface elements) (de Borst 2018) are used in order to handle fracture propagation (induced and natural). The finite element method is implemented for this coupled problem (fluid flow and mechanical). A special scheme is developed in order to take the pumping rate as boundary condition. An in-house computer MatLab program is developed to perform the relevant calculations.
This paper presents numerical modeling and results of hydraulic fracture growth in a shale reservoir. This fracture growth is limited by a limestone layer, and there is a weak interface between the limestone and the organic shale. The analysis is focused on the interaction mechanism between this weak interface and the hydraulic fracture. Three possible ways of propagation may occur, which are: arrest, crossing and T-shapes fractures. In order to represent this phenomenon, a cohesive model with mixed damage (mode I and mode II)is developed.
Fracture-interface interaction mechanisms have been first studied by Blanton (Blanton et al. 1982) in an experimental way. Other approaches have been made using a numerical poroelastic fluid model (Celleri et al. (2017) and Nguyen et al. (2017)).
Currently, most geomechanical models used by the industry are based on simple assumptions. Vaca Muerta cannot be evaluated with traditional models or on the basis of planar vertical fractures (Geertsma et al. 1979) and simple relationships between isotropic elastic properties, far-field stresses, and reservoir and mechanical properties.
This paper discusses one of the required steps to move forward to a more realistic numerical representation. It has developed a consistent geomechanical characterization and applications for completion strategy in unconventional reservoirs. This information is critical for completion design and must be used as a decision maker for fracture stages and landing points.