Fracability Index - More Than Rock Properties
- Michael J. Mullen (Stimulation Petrophysics Consulting LLC) | Milton Bock Enderlin (Texas Christian University TCU)
- Document ID
- Society of Petroleum Engineers
- SPE Annual Technical Conference and Exhibition, 8-10 October, San Antonio, Texas, USA
- Publication Date
- Document Type
- Conference Paper
- 2012. Society of Petroleum Engineers
- 2.5.2 Fracturing Materials (Fluids, Proppant), 1.6.9 Coring, Fishing, 1.10 Drilling Equipment, 5.1.10 Reservoir Geomechanics, 2.5.1 Fracture design and containment, 1.2.2 Geomechanics, 3.2.3 Hydraulic Fracturing Design, Implementation and Optimisation, 1.2.3 Rock properties, 2.4.3 Sand/Solids Control, 4.3.4 Scale, 4.1.2 Separation and Treating, 1.6 Drilling Operations, 5.8.2 Shale Gas, 5.6.1 Open hole/cased hole log analysis, 3 Production and Well Operations
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The geometry and complexity of hydraulic fracture stimulation treatments is largely controlled by the heterogeneous and anisotropic nature of rocks. Conventional practice in designing fracture stimulation treatments revolve around the parameters that can be changed and considers the rock being stimulated as homogenous and isotropic. Engineers can influence the frac geometry to some degree by changing the pumping rate, fluid viscosity and proppant loading. If the goal of a frac job is to get the fluid and proppant below the ground, most any formation can be frac'd. The degree to which a reservoir will generate complex fracture networks, break into fresh rock or create a simple bi-wing fracture can be influenced by the stimulation treatment job design. The creation of a complex fracture network is a function of the stimulation fluid rheology, rock properties and presence and orientation of preexisting planes of weakness acting in the stress state of the reservoir.
The index of brittleness or fracability is a term frequently used to describe formations that are likely to create complex fracture networks when fracture stimulated. In a broader view, fracability is much more than just calculating mechanical rock properties. A new definition of fracability, the Complex Fracability Index (CFI), proposed here integrates the sedimentary fabric, stratigraphic properties, mineral distribution and the presence and orientation of preexisting planes of weakness operating in the present day stress state into a single metric. This approach establishes a means to qualitatively or quantitatively determine the degree to which the rock will have the ability to create a complex fracture network or just a simple planer hydraulic fracture using the mechanical rock properties and the dip and orientation of the preexisting planes of weakness in the rock in the modern day stress state. The CFI methodology is not limited to shale reservoirs. Examples from the Barnett Shale and the tight sand in the Piceance Basin will be discussed.
Fracability is a term that has been used and abused when describing the effect that a hydraulic fracture stimulation treatment has on reservoir rock. High fracability has been associated with very complex fracture geometry. In the engineering world, hydraulically created fractures are generally characterized as being planar, dipless and orthogonal. (SPE 1157697, SPE 774418). A typical description of natural planes of weakness (joints, faults, fractures, and bedding planes) includes spatial attributes such as dip and strike (Jaeger et. al. 20071). When planes of weakness are activated through shear motion a seismic event results. If these surfaces are small and the slip is small, the seismic events have been characterized as a microseismic event (Halliburton 2011) which can be recorded during a fracture stimulation treatment. The occurrence and distribution of these microseismic events are frequently used as an indication of the degree of complexity created by the hydraulic fracture stimulation treatment and the volume of reservoir rock that could be connected to the wellbore. Reactivation of preexisting planes of weakness allows for fluids to flow along the reactivated surface (Barton et al, 19952). If the reactivation occurs during hydraulic fracturing stimulation, the fluids flowing along these reactivated surfaces must contain stimulation treatment fluids.
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