J-C. Roegiers

Poroelasticity considerations in in situ stress determination by hydraulic fracturing

Abstract The impact of poroelasticity effects on the fracture closure pressure p foc , breakdown pressure p b , and reopening pressure p re , and their influence on the interpretation of in situ stress are reviewed in this paper. Using solutions of a borehole and a fracture in an infinite poroelastic domain, it is shown that: (i) the breakdown pressure can be significantly higher than the elastic prediction (valid for impermeable rock); and (ii) the fracture closure pressure is generally larger than the far-field normal stress, the difference being dependent on the injection time.

shale reactivity test: A novel approach to evaluate shale-fluid interaction

Abstract The oil industry has been studying rock-fluid interaction not only in the drilling phase but also in completion, stimulation, and in enhanced oil recovery projects. Correct evaluation on how the rock reacts when in contact with a certain fluid is crucial for the success of several operations. Shales are especially troublesome rocks and a great deal of attention has been devoted to these rocks. A new approach to evaluate rock-fluid interactions is proposed. The test evaluates the reactivity of a rock when in contact with a fluid using a triaxial cell to reproduce downhole in-situ stresses. Although used, so far, to test shale reactivity, the methodology can be used for any type of rock, with a significant clay content (shaly sandstones or limestones). The paper describes the test procedures, sample preparation, and results obtained with dried and well-preserved downhole cores. The results show that this test provides much more detailed information about the rock behavior than the standard swelling pressure tests already in use today by the industry. The test can be run to evaluate the effect of the chemical potential alone, or in conjunction with the hydraulic potential.

Effects of packer on hydraulic fractures initiated from highly-deviated and horizontal boreholes

Abstract This paper presents observations and modeling results on the effects packers have on fracture initiation and near wellbore fracture geometry for a set of laboratory hydraulic fracturing experiments. An interesting observation is that significant discrepancies in fracture initiation pressures between the experimental results and theoretical predictions, which was ascribed to packer effects, occurred only for highly deviated boreholes. 3-D FE simulations of the laboratory tests, with emphasis on the packer structure and its response to the pressurization of the sealed-off interval, were subsequently conducted in trying to explain the mechanisms associated with the observed phenomena. The packer design was found to be the major cause of the discrepancy. Based on the observations and analyses conducted, significant effects of packer on hydraulic fractures initiated from highly inclined and horizontal boreholes were delineated, and conditions that may favor/impair such effects are discussed.

On Fracture Development in Rock Subjected to Compression

This paper describes an experimental study involving laser-induced cracking of transparent blocks made of PMMA and polyester casting resin. Disc-shaped cracks were introduced into the cylindrical and rectangular specimens by focusing a Nd-YAG laser beam. The PMMA samples were frozen in a dry ice-bath at a temperature of -72 °C or in liquid nitrogen (-198.5 °C) to make them brittle. Similarly, the resin samples were cooled in the freezer, at -20 °C, prior to testing. Besides being transparent, such a material also exhibits elastic and strength properties similar to rocks. The samples were subsequently loaded uniaxially to observe internal 3-D crack propagation in compression. The whole process took no more than 2 minutes in order to maintain the frozen condition of the samples, which did not exhibit any noticeable residual strain. This technique was then expanded to induce several cracks of given sizes and at given locations, allowing the study of 3-D crack interaction in compression.