Discrete element modeling of rock fracture behavior: Fracture toughness and time-dependent fracture growth.

摘要

Understanding the mechanics of fracture is important in oil and gas reservoirs. Applications range from the characterization of natural fractures that enhance fluid flow to the prediction of fracturing around a wellbore that can affect its integrity and stability. Two parameters that are of particular importance in the fracturing process are fracture toughness and subcritical index. There is a fair amount of experimental data on different rock types for these parameters but it is not well-known what petrographic properties control their magnitude. Also, because of sample preparation difficulty, fracture mechanics testing of weakly cemented sandstone is very challenging.; In order to better understand the micro-mechanics of fracturing of clastic rocks (sandstones of various cementation), a numerical study was performed using the Discrete Element Method (DEM). DEM was employed in order to model laboratory test behavior, by assessing individually the sensitivity of results to volume of cement, time-dependent cement properties, grain/cement mineralogy, temperature, and confining pressure. The micro-mechanical properties of DEM (stiffness and friction of grains and stiffness, strength, and volume of cement) were determined using macroscopic uniaxial and triaxial compression tests. The time-dependent properties of subcritical crack growth were implemented by incorporating stress corrosion of inter-particle bonds. The stress corrosion rate was quantified by the activation energy and volume of quartz. The fracture toughness and subcritical index of Berea sandstone was measured and the results were extended to weaker rock by reducing the cement volume. The DEM results generally agree with laboratory experiments. As intergranular cement volume is reduced, fracture toughness and subcritical index decrease. Based on this relationship, the fracture mechanics properties of weak rocks, which are difficult to measure in the laboratory, can be predicted.; Using the DEM model constrained by laboratory results, the importance of subcritical crack growth in wellbore stability problems was investigated. Wellbore instability in shale can be an immediate result of stress redistribution and increasing formation pore pressure following the removal of the rock mass in the wellbore. Additionally, because of large clay content and the potentially high chemical reactivity with drilling fluids, shale can be susceptible to time-dependent failure. Previous studies (mostly based on continuum modeling using poroelasticity) have concentrated on predicting the onset of failure. However, the use of DEM makes it possible to evaluate the progression of failure over time by tracking the propagation of the damage zone boundary.