C.J. de Pater

Experimental Verification of Dimensional Analysis for Hydraulic Fracturing

The authors have derived model laws that relate experimental parameters of a physical model of hydraulic fracture propagation to the prototype parameters. Correct representation of elastic deformation, fluid friction, crack propagation, and fluid leakoff forms the basis of the scaling laws. For tests at in-situ stress, high fluid viscosity and low fracture toughness are required. Tests on cement blocks agreed with the scale laws based on elastic behavior.

Numerical implementation of displacement discontinuity method and its application in hydraulic fracturing

Abstract A formulation of the quadratic displacement discontinuity method is presented to model two-dimensional elastic fracture problems. The new formulation is found by analytical integration of the fundamental solution along a straight-line crack within which three collocation points are taken. Each element has a quadratic distribution of displacement discontinuity values. For a curved crack, a general formulation of the displacement discontinuity method is proposed, based on the boundary integral equation. This formulation contains strongly singular and hypersingular integrals which can be evaluated in the sense of Cauchy principal value and Hadamard principal value, respectively. Special crack tip elements are adopted to show the r variation of displacements near the crack tips. Some examples are used to illustrate the accuracy of two formulations. General formulation is also used to simulate hydraulic fracturing propagation and interaction between a crack and a fault. The fracture propagation path is obtained by using the maximum tensile stress criterion.

The influence of steam pressure on thermal spalling of sedimentary rock : Theory and experiments

Thermal spalling is the process by which surface material cracks and breaks off due to variation of temperature. The process is complex and the physical mechanisms that cause thermal spalling are not well understood. The theory and the results of experiments carried out to determine the effects of water and steam pressure on the phenomenon of thermal spalling are presented. It is shown that in theory the rise in pore pressure due to the presence of water as gaseous steam or as compressed liquid is insufficient to cause thermal spalling in sedimentary rock. A new criterion for spalling due to steam saturation pressure which is based on the concept of critical stress is presented. The results from uniform heating experiments and from linear heating rate experiments have led to the conclusion that explosive spalling is caused by saturated steam alone, at local pressures in access of the unconfined tensile strength. The results of thermal shock experiments are more difficult to interpret because of the high compressive stresses at the heated surface. Spalling due to thermal shock is probably caused by a combination of steam pressure and fracturing associated with high compressive thermal stress.

Geometry of Hydraulic Fractures Induced From Horizontal Wellbores

In the Dan field, very high breakdown pressures were observed for wellbores drilled with a high azimuth with respect to the preferred fracture plane. The increased breakdown pressure was caused by significant near-wellbore friction. In scaled laboratory tests, variation in breakdown pressure was accompanies by a change in fracture geometry. Therefore, the variation in breakdown pressure in the field treatments could not be related simply to in-situ stresses.

Experimental Study of Nonlinear Effects in Hydraulic Fracture Propagation(includes associated papers 29225 and 29687 )

The authors conducted fracture propagation experiments on blocks of cement paste, cement paste with sand, and a tight outcrop sandstone rock. A novel acoustic monitoring system was developed for measuring the fracture profile and radius during the tests. Results of laboratory tests on cement agreed with numerical simulations based on elastic rock deformation and linear elastic fracture mechanics. Tests at confining stresses of 20 and 23 MPa gave higher pressures than predicted.

Impact of Rock Plasticity on Hydraulic Fracture Propagation and Closure

We performed scaled laboratory experiments of hydraulic fracture propagation and closure in soft artificial rock and outcrop rock samples. We also performed numerical simulations of the fracture behavior in plastic rockswith independently measured rock properties. The simulations aided in interpreting the measurements and extrapolating the results to field scale. Compared with elastic rock, plasticity induces a larger width for a given net pressure. However, the pressure to propagate fractures is only marginally increased and, in the case of the laboratory tests, was actually lower than expected from elastic behavior. The most dramatic effect of plasticity is that closure is much lower than the confining stress because of strong stress redistribution along the fracture.

Numerical modeling of crack reorientation and link-up

The FRANC3D/BES software system has been used to simulate the reorientation and link-up of hydraulic fractures in three-dimensional (3D) problems. The adopted technique only needs to discretize the body surface and the crack surface. The crack propagation direction is determined using the minimum strain energy criterion. Crack propagation amount is calculated using the mode I stress intensity factor. In hydraulic fracturing, the number of multiple cracks for a given number of perforations depends on the resulting interaction of the cracks. The interaction may be expressed by the fracture stiffness which has been obtained for 3D problems in this paper.

Active seismic monitoring of hydraulic fractures in laboratory experiments

In scaled laboratory tests, we perform acoustic measurements in a time-lapse sequence to separate the fracture response from the background signal. Using both compressional and shear waves (that are very sensitive to fluid filled fractures) we can, not only detect the hydraulic fracture, but also characterize its shape and geometry during its growth. We show the application of the technique to propagation and reopening of hydraulic fractures. During fracture growth the acoustic waves excite diffractions at the tip of the fracture. Depending on the acquisition geometry, we detect many events related to surface waves propagating along the fracture. Also, we observe that shear waves detect the migration of the fluid front during reopening of a pre-existing hydraulic fracture, in contrast with the compressional waves which are insensitive to the fluid front during reopening.

Thermo-mechanical properties of roof rock of coal for underground gasification

The mechanical stability of the cavity formed during underground gasification of coal is very important. Triaxial tests were performed at high temperature (up to 800 °C) and high confining stress (up to 15 MPa) to investigate the rock properties at in-situ conditions. The tested samples were taken from roof rock of a coal layer from a mine in Belgium. The thermal and mechanical properties of the tested rock types will be used for modelling. In this paper the possible consequences of these results for the stability and growth of underground cavities are discussed.