In recent years, results of several investigations of the effects of stress environment on physical properties of sandstones, limestones, and granites have been published. Few data, however, are available on the elastic properties of shale. It is the purpose of this paper to describe an experimental technique for measuring the elastic properties of shales under earth-like stresses and to present preliminary results.
Equipment described consists of a pressure vessel which contains the sample and allows application of simulated overburden pressure; axial stress is imposed with a hydraulically-powered piston. Stress and strain measurements are made with resistance strain gauges. Deformation is measured by a strain gauge attached directly to the sample. Signals from the strain gauges are displayed as a function of time on a two-channel oscilloscope. From photographs of these traces, elastic moduli are computed. The equipment contains a gas-powered free piston and quick-operating solenoid valve to allow loading rates from 60,000 psi/sec to 1,200,000 psi/sec. Results obtained from a few samples generally show an increase in elastic modulus with increasing confining pressure and increasing rates of loading.
In recent years there have been published results of several investigations of the effect of stress environment on the physical properties of the limestones, sandstones, and granites encountered in oil well drilling. Few data, however, are available on the physical properties of shales, particularly in the moderate st ranges that are encountered in the dril line of wells. This may seem surprising when one considers that a large percentage of the oil well drilling is in shale formations. Shale properties are especially important in the Gulf Coast region where thick sections of shale cause problems involving lost returns, hole enlargement, tight hole, and poor drilling rates. A better understanding of why shales react as they do should come about by investigating their physical behavior under conditions that simulate subsurface environment.
Shale is a complex mixture of clays, sand, feldspars, carbonates, and detrital materials. With a material of such complex nature, it is difficult to ascribe observed behavior in a well bore to a single physical or chemical cause. Knowledge of the modulus of elasticity, i.e., the force required to produce unit deformation of a solid, may ultimately allow one to drill a shale more economically. Immediately, the knowledge is needed for (1) formation fracturing computations where the modulus enters directly into the calculations of fracture dimensions and indirectly into calculations of the pressures required to initiate fracturing, (2) lost returns problems in which a knowledge of the elastic modulus may increase our understanding of the inadvertent fracturing of shale formations by excessive mud weights or by pressure surges when running pipe into the borehole, (3) hole instability problems in which it is hoped that differences in the elastic modulus for different shales may be correlated with hole enlargement.
The present paper describes the equipment, sample preparation, and experimental procedure for obtaining Young's modulus data on shales. Young's modulus was obtained at confining pressures that simulate a subsurface stress environment and at loading rates that approach those delivered by a rotating rock bit. Measurements of Young's modulus were made at room temperature. This report presents preliminary results obtained from shales from West Texas, Louisiana, and the Texas Gulf Coast.
The apparatus used in this work permits measurement of a dynamic Young's modulus while an impulse force is being applied to a test specimen. The apparatus is shown in Fig. 1.