Geomechanics has become an important tool for engineers and geologists, with classical applications in wellbore stability, sand management, and hydraulic fracturing. However, unconventional shale reservoirs cannot be evaluated with traditional models or on the basis of simple relationships between isotropic elastic properties, far-field stresses, and reservoir and mechanical properties. These plays are geologically complex and the reservoirs are heterogeneous in nature. Wells need to be hydraulically fractured for stimulation and, in a complex tectonic environment, the rock fabric and in situ stress strongly influence this process. We discuss a new methodology developed for stress modeling, taking into account the tectonic environment. We present a case study for the Vaca Muerta formation in Argentina. The obtained results contribute to the understanding of stress distribution in unconventional reservoirs.

The Jurassic- and Cretaceous-age Vaca Muerta formation of the Neuquén Basin, Argentina, is a heterogeneous self-sourcing reservoir that is complicated by tectonic and volcanic influences. As an unconventional reservoir, its characterization requires the evaluation of key properties related to the production drivers: reservoir quality (RQ), drilling quality (DQ), and completion quality (CQ). A methodology is presented to maximize horizontal well success by identifying optimum horizontal well landing locations through integration of multidimensional petrophysical and geomechanical properties. This ongoing case study identifies of the key properties for RQ, DQ, and CQ through correlation analysis and production validation. The methodology uses a systematic and quantitative probability calculation to determine the lateral landing score (LLS) for all depths along a vertical pilot wellbore. Core-calibrated petrophysical evaluations of the interbedded siliciclastics, organic shales, ash beds, and tight limestones of the Vaca Muerta quantify uncertainty in estimates of reservoir properties (e.g., mineralogy, maturity, porosity, fluid saturations, and permeability) and RQ. Stress profiles, calibrated using core evaluation and stress tests, provide a predictive model of geomechanical properties (e.g., anisotropic elastic properties, in-situ stresses, wellbore stability, and rock fluid sensitivity). These parameters are critical for DQ and CQ predictions. Production results define appropriate normalization and weighting of properties for the LLS probability. Tectonic stresses in the Vaca Muerta may promote horizontal fractures that create restrictions to fracture growth and/or induce pinch points. Previous approaches to determine the target location in an unconventional resource play using geomechanical inputs alone may not apply to the Vaca Muerta because they overlook the effects of the fracture complexity induced by the stress regime of the prograding depositional environment. A high LLS occurs where positive RQ, DQ, and CQ values exist in sections thick enough to drill. The LLS brings together measurements from multiple domains to provide a qualitative, comparative, and repeatable ranking of ideal landing locations in tectonically active unconventional plays. Implementing the LLS as a decision-making tool for horizontal well placement generates both an optimized landing point and completion design. The workflow is iterated with available horizontal well production data to validate the relevant production drivers.

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A 1D Mechanical Earth Model (MEM) was built for the Albian section of a planned horizontal well in the south Campos Basin area in order to reduce risks for well placement, completion and stimulation in a fast track development program with reduced number of appraisal wells. Mechanical properties and stress profile along the wellbore for the design were determined from multiple data sources including core testing, advanced acoustic, wellbore image, processed petrophysics, micro-fracturing and in-situ formation pressure. These measurements enabled the definition of local correlations for the studied carbonate reservoir, estimation of geomechanical properties, and the creation of input for the completion design. The calibrated geomechanics model was used to evaluate the completion options for a horizontal well in carbonates to optimize reserves and maximize well productivity. Based on field characteristics and mechanical properties determined from this study, the horizontal well placement and drilling design was adjusted and an open hole completion with mechanical isolation was proposed to enable multiple fracturing in one continuous and efficient operation.