Objective
The aim of this project is to improve the efficiency of hydraulic fracture treatments, by determining the geometry of hydraulic fractures in the vicinity of the borehole, when the final propagation plane differs from the initial propagation direction. This will be accomplished by physical and numerical model studies of the fracture process.
The research will aid in innovation of the design of hydraulic fracture treatments, with regard to fracture reorientation.
A) ACOUSTICS
The final design of the complete data acquisition system has been finished. We can now perform fast measurements with 32 transducers. It is possible to register clear signals from a hydraulic fracture whose serial extent is large compared with the wavelength, but whose width is very small compared with the wavelength. Waves reflected from the fracture can be used to produce an image of the fracture. The variation of the amplitude of normally transmitted waves which propagate across the fracture can be mainly attributed to the interference between the direct arrival and the arrival scattered from the fracture tip.
A software package has been written to be used as a forward elastic modelling tool for prediction of the wavefield observed during the hydraulic fracturing experiments.
B) MODEL TESTS
We derived a complete set of 5 dimensionless groups for planar fracture propagation, which form the basis of universal scaling laws for hydraulic fracture propagation. For testsat in-situ stress, the scale laws showed that rock-like material should be used with a low fracture toughness and a low permeability. Furthermore very high fluid viscosity must be used to reduce the influence of toughness and to compensate for the low injection rate in laboratory models. For tests on cement the pressure, width and radius of radial fractures agreed, within experimental uncertainty, with a fully coupled simulation based on linear elastic rock behaviour. Tests on sandstone with a large variation in flow rate and viscosity scaled at low confining stress. At high confining stress a higher pressure was observed, compared with simulations that assumed elastic material behaviour.
Also, the influence of fracture pressure was studied on the fracture geometry. In some respects they counteracted each other. This was due to the fact that the perforations became more important when the liner was cemented in a loaded block and the same held true for high fluid viscosity since the fluid was unable to penetrate the cement annulus. Representative tests should however be conducted with correct cementing chronology. An extensive study was performed of the influence of fracture pressure. The geometry of the fracture at the wellbore changed with increasing pressure. Modelling and experiments agreed on the fact that fracture reorientation was more gradual for higher fracture pressures. After removal of the tortuosity, with further opening of the fracture, this effect could be beneficial since it could yield a larger width in the region where the fracture re-oriented towards the preferred plane.
C) MATERIAL PROPERTY TESTS
Fundamental fracture mechanisms in concrete and sandstone were investigated in uniaxual tensile tests; re-orientation of fractures was studied in single-edge-notched and double-edge-notched four-point-shear-experiments.
A) Acoustics
The first steps will be to prepare the acoustic technique for application to a model test with a simple fracture. We will develop sensors, assess resolution, model the acoustic diffraction and process the data obtained in the tests.
After the first phase the acoustic technology will be applied to both transmission and reflection measurements in the model tests.
B) Model tests
Model tests will be performed on blocks of rock and cement with a cased borehole from which fractures are propagated by applying fluid pressure in the borehole. The experimental set-up will be made and the design of the model tests will be developed :
- construction and testing of injection system, casings
- preparation of blocks
- dimension analysis and design of models
. relation model to field application
. scale effect on fracture propagation
The following model tests will be conducted :
- tests with two different borehole diameters (scale effect)
- reorientation tests with variation of : inclination, stress condition, perforation position and rock type.
C) Material property tests
Advanced material property tests will be used for the interpretation of the model tests. The following experiments/activities will be performed :
- uniaxial tension tests
- mixed mode testing
- study of fracture reorientation
- construction of load frame for Iosipescu-beam shear tests (specialized four point bending tests)
D) Numerical modelling
We will perform computations to :
- study initiation of cracks
. 3D finite element analysis of model tests (DIANA finite element program)
- modelling of crack reorientation with a Boundary Element code
- simulations using a simple hydraulic fracture model.
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Coordinator
2600 Delft
Netherlands
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