Assessing Stimulation of Complex Natural Fractures as Characterized Using Microseismicity: An Argument the Inclusion of Sub-Horizontal Fractures in Reservoir Models
- Theodore I. Urbancic (ESG) | Adam Baig (ESG Engineering Seismology Group) | Shoshana Beth Goldstein (ESG Solutions)
- Document ID
- Society of Petroleum Engineers
- SPE Hydraulic Fracturing Technology Conference, 6-8 February, The Woodlands, Texas, USA
- Publication Date
- Document Type
- Conference Paper
- 2012. Society of Petroleum Engineers
- 4.3.4 Scale, 2.5.2 Fracturing Materials (Fluids, Proppant), 5.8.2 Shale Gas, 3 Production and Well Operations, 3.2.3 Hydraulic Fracturing Design, Implementation and Optimisation, 4.1.2 Separation and Treating, 1.2.2 Geomechanics, 5.1.2 Faults and Fracture Characterisation
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The inclusion of fracture networks in reservoir models is generally based on the concept of failure associated with subvertical fractures. In general, it is surmized that fractures can grow irregularly in a stress field that is perturbed by a hydraulic fracture injection. It has also been considered that structural weaknesses in the rock such as pre-existing fractures and naturally occurring laminations commonly found in shale-gas reservoirs can be conduits for fracturing during stimulation and active pathways for fluid flow. We postulate that local stress perturbations through stress transfer allows for fractures to propagate and initiate failure along pre-existing fracture sets, which include sub-vertical and sub-horizontal fractures. Additionally, the degree of fracture interconnectivity and the type of fracturing will play a role in whether effective proppant transport is achieved. Through moment tensor inversion of microseismic events related to stimulation in the Horn River Basin utilizing well-conditioned geophone arrays, we have been able to define a three dimensional discrete fracture network consisting of sub-horizontal and sub-vertical fractures. Geologic data from the site provided corroborative evidence to the validity of the observed discrete fracture network, the presence of sub-horizontal fractures and fracture orientations in-line with current regional stress field. The fracture intensity and complexity appeared to be directly related to the degree of interaction between the sub-horizontal and sub-vertical fractures. Regions dominated by sub-horizontal fractures were also regions exhibiting poor fracture intensity and complexity. Based on these observations and moment tensor derived failure modes (opening component of failure), we were able to identify regions of enhanced fluid flow, further identifying regions of effective fluid transport. Regions with poor connectivity and dominance of sub-horizontal fractures also were identified as regions of poor fluid flow; these then become regions for potential re-stimulation. Based on these analyses, it can be suggested that sub-horizontal fractures can play an important role in the overall fracture development.
Early hydraulic fracture failure models were based on a simple symmetric singular bi-wing vertical fracture, propagating in the direction of maximum regional principal stress. Adaptations to this concept have been investigated over the years, including the addition of microfracturing, joints, and stress induced fracture networks (e.g., Perkins and Kern, 1961). These models, however, do not account for individual fractures. The injected volume is accommodated through absorption into the pore space of the rock. Others approaches have employed dendritic fractures based on jointing and assuming fracturing along pre-existing shear failures below fracture pressure (e.g., Pine and Batchelor, 1984). More recently, the concept of fluiddriven- fracture meshes has been introduced, wherein over-pressured fluids result in meshed or interlinked shear, extensional, and shear-extensional faults (e.g., Sibson, 1996).
All of these approaches are based on the concept of failure associated with sub-vertical fractures. Microseismic monitoring has shown that these models are, for the most part, oversimplifications (Urbancic et al., 2010). Fractures grow irregularly, in a stress field that is perturbed by the injection and along structural weaknesses in the rock such as pre-existing fractures and along naturally occurring laminations that are commonly found in shale-gas reservoirs (e.g., Reine and Dunphy, 2011). It can be postulated that local stress perturbations associated with these failures (stress transfer mechanism) allows for fractures to propagate and initiate failure along different sub-vertical and sub-horizontal fracture subsets as a result of temporal local rotations in the principal stress-strain axes and or their magnitudes. Changes in stress-strain magnitude and natural fracture spacing would then dictate the degree of fracture complexity and interconnectivity.
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