Abstract

Rock properties are assumed constant or to vary only slightly along a horizontal well placed within a correlative target. However, in the presence of folds, faults and/or fractures, the geomechanical properties can change along the well leading to skewed hydraulic fracture development. During field and well planning, the challenge is to identify these features using a combination of pilot hole logs, drilling experiences and seismic maps to optimize well placement and maximize field economics. This can be difficult as smaller geologic features that may impact well performance are also sub-seismic and difficult to identify before drilling the well.

In this paper, we present a case study from a horizontal well drilled in the Permian Basin. In this well, we ran a suite of open hole quad-combo logs and a resistivity-based borehole image log. These enabled us to characterize the petrophysical, lithological and mechanical properties along the lateral wellbore and to map the near wellbore fractures and formation structure. Rock properties were validated by comparing with acquired pilot hole logs which showed that the rock properties were consistent throughout the entire lateral. However, the image log indicated that the well was drilled through numerous, sub-seismic folds. Near the toe of the lateral, three distinct intervals separated by ∼30-40 feet were identified: (a) normal bedding (less-folded), (b) the presence of a fold structure, and (c) presence of a fold structure with fractures. Three single cluster Diagnostic Fracture Injection Tests (DFIT’s) were done sequentially to independently evaluate the stress levels in each interval.

Significant differences were found between DFIT’s conducted at normal bedding and at folds with DFIT’s obtained within the folds showing higher breakdown, treating pressures and closure gradients. This paper gives a practical field example of how stresses change within the folds, even across short intervals and we show that the stimulated fracture pattern can vary significantly in the presence of folds.

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