Abstract

The paper presents the first stage of experimental model research on relations between pressurized fracturing of rocks and related surface displacements_ Models, made of polyurethane resins, simulate the assumed linear elastic response of a rock mass to local hydraulic fracturing. The specific objectives of the present research are:

  1. to test predictions of various analytical and numerical models used in fracture geomechanics;

  2. to produce empirical data on displacements, strains and stresses to be used co further develop reliable analytical and numerical solutions;

  3. to develop new experimental procedures to be used for solving practical problems in mechanics of geological materials and structures.

Three independent experimental techniques were adopted and expanded to determine surface displacements above the pressurized cracks;

  1. classical holographic interferometry;

  2. newly developed shearographic interferometry:

  3. infrequently used Fizeau fringes interferometry.

In the next stage, threedimensional isodyne stress analysis will also be used.

Experimental models have the form of cubes, 300 × 300 × 300 mm. Ellipsoidal cracks of diameter 25 mm are located 50–100 mm beneath the surface. Both the hydraulic pressure inside cracks and the volume changes of cracks are measured.

Theories and techniques of the three experimental procedures were tested by measuring surface deformations of special test specimens. Those specimens are beaker-Size specimens and circular polyurethane cylinders, with pressurised ellipsoidal cavities beneath the surface.

Results will be used to assess the reliability of the analytical and numerical procedures used for hydraulic fracture analysis, by testing the predictions of the underlying physical models and by testing the admissibility of the underlying simplifying physical and analytic assumptions.

For example, fracture solutions often contain a singularity which strongly affects near-field predictions of stress and strains, but which has a vanishingly small effect on far-field results. The physical simulations are intended co test the robustness of these simplifying assumptions.

Introduction
General Remarks

It is strongly stated in pertinent literature that the needed advances in energy resource recovery regarding oil and gas mining include necessary further development in rock mechanics. (Carrol, 1985) The involved physical processes are extremely complicated (Franklin and Dusseault, 1989), therefore some drastic, but rational, simplifications are needed to develop acceptable and useful models of the pertinent responses, Results presented in this paper pertain to research whose aim is to contribute to further development in modeling quasi - static hydraulic fracture. The particular aim is to establish reliable empirical functional relations between the growth in size and volume of the sub-surface ellipsoidal cracks (cavities) and the related changes in magnitude of selected parameters of mechanical responses of geological structures, such as the surface displacement fields (upheaval), and the stress and strain fields between the sub-surface fracture and surface. Knowledge of those fields is necessary for testing the physical admissibility of the analytical and numerical solutions and procedures which relate directly volumetric changes of the sub-surface cavities to the basic geometric parameters of the surface upheaval such as vertical surface displacements and the slopes of the deformed surface.

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