Hydraulic Fracture Propagation in Pre-Fractured Natural Rocks
- Chunfang Meng | Hans J. De Pater (StrataGen Delft Bv)
- Document ID
- Society of Petroleum Engineers
- SPE Hydraulic Fracturing Technology Conference, 24-26 January, The Woodlands, Texas, USA
- Publication Date
- Document Type
- Conference Paper
- 2011. Society of Petroleum Engineers
- 2.5.4 Multistage Fracturing, 3.2.3 Hydraulic Fracturing Design, Implementation and Optimisation, 1.14 Casing and Cementing, 2.4.3 Sand/Solids Control, 2.2.2 Perforating, 2.5.2 Fracturing Materials (Fluids, Proppant), 5.8.6 Naturally Fractured Reservoir, 1.2.2 Geomechanics, 1.2.3 Rock properties, 4.3.4 Scale, 1.6 Drilling Operations, 2.5.1 Fracture design and containment, 5.8.2 Shale Gas, 3 Production and Well Operations, 1.7 Pressure Management, 5.3.4 Integration of geomechanics in models, 5.9.2 Geothermal Resources, 5.1.2 Faults and Fracture Characterisation
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We conducted a series of hydraulic fracture initiation tests with fractured natural rock samples. The objective is to characterize the interaction between hydraulic fracture initiation and natural fracture infiltration and opening. The natural fracture was simulated by cleaving a rock cube into two or three layers and putting the parts back together. The hydraulic fracture is initiated by pressurizing the borehole drilled perpendicular to the layers. The test parameters include rock type (sandstones and a tight limestone), confining stress and fluid type (silicon oil and cross-linked gel). A detailed model was built to simulate the hydraulic fracture interaction with a natural fracture. Qualitative agreement was found with the test results.
Natural fractures come in many different forms. As one extreme they may be just a weakness governed by the rock fabric, another extreme is a very conductive and wide open shear zone. Since shear dilatancy is not only occurring on a microscopic scale but also on macro-scale by fault block rotation, shear can generate very high conductivity along fault zones. If a hydraulic fracture grows into such a fault with a large aperture, it is likely that the fluid will flow into the fault. This is often
the objective in geothermal reservoirs where hydraulic fractures are needed to connect the well to shear zones that produce large amounts of water1. Very conductive faults may be rare in sedimentary reservoirs, but interaction with natural fractures is common, especially in tectonically stressed reservoirs. In those reservoirs, active faults will be conductive and acceptance of fluid will enhance the interaction.
Another kind of interaction with natural weakness planes is possible on layer interfaces. Under normal stress conditions, the largest stress will act on the horizontal plane, but even in that case interaction is possible, since all stress components fall to zero at the fracture tip. Moreover, near the well the fluid pressure often exceeds vertical stress, so that the fluid may flow into a horizontal natural fracture. Interaction is not only governed by virgin fracture aperture, conductivity, stress on the fracture plane and fracture surface friction. More important are factors that control the evolution of natural fracture conductivity near a hydraulic fracture. That means that the effect of shear and decreased effective normal stress must be related to fracture stiffness and conductivity. Geologists classify fractures according to orientation and origin, but in this context it is more relevant to classify based on current orientation with respect to the preferred fracture plane and on mechanical properties: in particular conductivity and its relation to stress and fluid pressure.
Hydraulic fractures propagate by two mechanisms: as a tensile fracture that is opened by a wedge of pressurized fluid and during fluid injection into a reservoir due to lowering of the effective stress. The latter situation can also occur naturally, when the pore pressure cannot escape during geologic compaction and the least effective stress falls to zero. Although the mechanism is quite different we regard both as hydraulic fractures. However, the current discussion focuses on the wedgedriven fractures.
Lab tests showed that strong interaction may occur between hydraulic fractures and natural fractures, depending on the angle of approach, the ratio of net pressure to stress difference and the frictional properties of the natural fracture2,3,4,5,6. This is of course no surprise: when a hydraulic fracture meets a joint the fluid has a choice of continuing the hydraulic fracture or flow into the joint. In addition there is another effect in tectonically stressed formations: the hydraulic fracture may induce shear failure and thereby extend existing shear fractures to grow into the hydraulic fracture39.
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