The production of hydrocarbon from a porous media such as sandstone can cause damage to the rock. When the hydrocarbon is produced under increasing levels of stress, the mechanical damage caused may lead to sand production. Sand production plays a detrimental role in oil and gas production, and can cause tremendous financial drawbacks. A series of perforation collapse tests, involving mechanical and hydraulic loading, were conducted to gain a better understanding of the mechanism leading to sand production. We used numerical modeling and the computer code, FLAC , to assist in addressing the objective. One form of damage observed in the perforation collapse tests, showed localized helicoidal shear bands, which developed around the borehole wall. We modeled this behavior numerically using a strain softening Mohr-Coulomb model, in which the cohesion property was reduced with increasing plastic shear strain. The damage pattern offers a close resemblance to the one observed in the perforation collapse tests. It is shown that, for the conditions investigated, flow of oil increased the extent of shear damage, and, as expected, so did an increase in confinement (external boundary stress). On the other hand, using a small amount of cohesion hardening reduced the extent of damage.
The production of hydrocarbon from a porous rock such as sandstone can cause damage to the material. When hydrocarbon is produced under increasing levels of stress, the mechanical damage caused may lead to sand production. Sand production plays a detrimental role in oil and gas production, and can cause tremendous financial drawbacks. A series of perforation collapse tests were conducted to gain a better understanding of the mechanism leading to sand production. The purpose of this work is to use numerical modeling to contribute to this understanding.
In this paper, we use the word 'damage' to qualify noticeable unrecoverable deformation at the scale of the sample in the laboratory tests, and to refer to the occurrence of plastic deformation for a Mohr- Coulomb material in the numerical analyses.
The scope of the numerical study involves a sequence of tasks, which include a) concentrate on the impact of fluid flow and identify, for the laboratory perforation collapse tests, a range of parameter values for which damage occurs, b) model qualitatively the damage pattern observed in the laboratory tests, and investigate some of the parameters influencing the shape and extent of the region of failure, c) calibrate the numerical model in an effort to reproduce quantitatively the laboratory results, such as extent of failure and cavity deformation prior to sand production, d) develop a model for sand production and investigate the parameters controlling the shape and extent of the 'hollowed region', and e) calibrate the numerical model to reproduce the laboratory results quantitatively, such as volume of sand produced. The effort documented in this paper concentrates on fulfilling some of the objectives of the first two tasks.
Localized damage, observed in perforation collapse tests on the Salt Wash North and MacField Sandstones, takes the form of helicoidal shear bands. Slip lines of this shape have been identified analytically as the characteristic lines for axisymmetric, elasto-plastic problems governed by hyperbolic equations (see e.g. Kachanov ).