ABSTRACT:

A compact tool has been developed to estimate cohesion from thin disk shaped samples of shale. The tool was designed to achieve fast and inexpensive testing on small samples. In conjunction with the continuous wave technique (CWT), cohesion and ultrasonic velocity can be determined, both important parameters in the mechanical description of a fine grained material as shale. Finite element simulations - using ABAQUS with a constitutive model for isotropic linear elastic perfectly plastic material with associated flow rule - show that a zone of pure shear stress is localized in the vicinity of the punching pistons. A laboratory study on a reference material (PMMA), two outcrop shales and a field shale showed linear trends for all materials when plotting peak force versus sample thickness, in general agreement with expectations. The calculated cohesion from the punch test was found to be higher than the values found from conventional low confinement compression tests. This is to be expected, since the punch test predetermines the location of the failure plane whereas the failure plane in a triaxial compression test will occur at the dominant plane of weakness. The displacement rate was varied from 1.5 mm/min to 0.015 mm/min and a small strengthening was observed with faster displacement rates. This can presumably be ascribed to creep, as consolidation is not likely to play a significant role in a state of pure shear.

1. INTRODUCTION

Drilling problems in shale remains a costly problem for the petroleum industry. The problems are related to the mechanical properties of the shale, and better knowledge about these properties, such as the rock strength, would be very useful for optimization of the drilling procedure. Rock strength is usually measured in standard uniaxial or triaxial test on core plugs. For shales, however, preserved core material is rarely available. Shale cores are often heavily fractured, so that standard size core plugs are not easily obtained. Moreover, even if the material is available, the tests have to be run very slowly due to the low permeability of the rock so that the tests become quite expensive, and hence only few such tests - if any at all - are normally done. In response to these problems, we have for several years been developing a number of techniques for testing on small shale samples [1]. The use of small shale samples is beneficial because the tests can be run on rock fragments - in some cases even on drill cuttings. Also, the time needed to obtain pore pressure equilibrium is largely reduced for small samples so that tests can be run much faster. One of the techniques developed for this purpose is the Continuous Wave Technique (CWT) [2], which is used to measure acoustic velocities on mm-thick disk-shaped samples of fine-grained material. The very small and easily prepared samples required by this tool makes it well suited for studies that require a large number of quick and simple tests, such as rig-site measurements on drill cuttings, or studies on rock anisotropy or sensitivity to fluid exposure.

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