Jon E. Olson
The objective of the proposed study is to develop methodology for predicting strength and failure of particulate material, especially weakly cemented sand and gravel, from small-strain stiffness as measured with the velocity of stress waves. The project will be especially concerned with phenomena such as soil liquefaction in earthquakes and sand production in petroleum wells. The objective will be approached through the development of an improved, fundamental understanding of how particle cementation affects both the strength and the stiffness of cemented granular material.
We will attempt to quantify the relationships between the degree of cementation and bulk mechanical strength, as well as the degradation of the cementation with accumulated strain. The program of studies will have two concerns:
How can the degree and nature of particle cement be determined from properties such as wave velocities and static moduli?
How can this characterization of cement be related to such engineering behavior as aggregate strength, liquefaction potential, and sand production?
In engineering problems involving geologic materials, strength is often the primary design parameter. Civil and petroleum engineers frequently work with poorly cemented granular materials, ranging from weakly cemented natural sand and gravel to more strongly cemented sandstone and even cement-stabilized construction aggregates. For example, subsurface soil deposits can be improved by introducing cement or grout with various site improvement techniques. Artificially cemented gravel can be used to construct stiffer sub-grade base courses for pavements. Very weak, naturally occurring cementation can be sufficient to prevent the liquefaction of saturated sand in an earthquake. In the petroleum industry, the inflow of sand into an extraction wellbore can be a significant problem for poorly consolidated or weakly cemented reservoirs. Preventive measures include reducing allowable flow rates to minimize fluid drag on near-wellbore particles and introducing artificial cementing agents to the reservoir to increase the rock strength. Correlation of wave velocities to strength would facilitate the characterization of cemented geologic materials in all of these applications.
A concurrent numerical and experimental investigation is planned. Experiments on cylindrical test specimens will be conducted in a triaxial test apparatus, while the numerical work will involve Distinct Element Method (DEM) simulations using commercially available software. The parallel pursuit of laboratory tests and numerical simulations is a powerful approach to developing fundamental insight into the behavior of geologic materials. Experimental data permit immediate calibration and verification of analytic results, while numerical simulations can be helpful in identifying underlying mechanisms responsible for observed behavior and in designing useful laboratory tests. Two- and three-dimensional DEM simulations will be used to compute bulk material strength and elastic wave velocities as a function of particle size distributions, intergranular cement and material porosity. Behavior observed from these simulations will be used to devise appropriate experimental tests and test conditions that are most likely to produce meaningful data. Samples for experimental testing will include naturally occurring granular material of varying degrees of cementation and strength, as well as artificially created samples. The artificial samples should allow for more controlled sensitivity analysis of particular rock properties and how they affect mechanical behavior.
Jon E. Olson
Center for Petroleum and Geosystems Engineering
1 University Station C0304
The University of Texas at Austin
Austin, Texas 78712-0228
Phone: (512) 471-7375 FAX: (512) 471-9605