Quantifying the micromechanical effects of variable cement in granular porous media

Co-PI’s: Tom Buchheit and Ben Cook (Sandia National Labs)

Laurel Goodwin (University of Wisconsin-Madison)

Abstract:

Understanding the fundamental micromechanical processes that result in the macroscopic deformation of granular porous media is of paramount importance to the DOE, as these processes control a variety of behaviors that impact both contaminant transport and the distribution and production of hydrocarbons. It is generally known that the amount, type, and location of intergranular cements are a primary factor influencing the strength of porous sediments and sedimentary rock. Small amounts of intergranular cement can have profound effects on bulk mechanical such as rock strength, elasticity and permeability. However, the microscale interactions between individual constituents of the grain-cement network subjected to differential stress are poorly understood, inhibiting the development of a physically based link (and mathematical models describing the link) between microscopic phenomena and macroscopic behavior. An understanding of how cement influences the properties of a grain-cement-pore system has, until recently, been limited to qualitative statements such as “cement strengthens rock by preventing sliding and rotation of grains.” This qualitative understanding leads to improperly idealized and overly simplified constitutive relations in models used to simulate geomechanical problems with significant economic impact, e.g. wellbore failure, formation collapse, and sand production. Clearly, one important benefit of a detailed experimental characterization would be improved constitutive models for the predictive modeling of geomechanical phenomena.

The proposed research furthers the fundamental understanding of grain-cement micromechanics through appropriately small-scale experimental deformation and structural characterization studies of both synthetic grain-cement systems and natural sandstones. Our approach extends micro and nanomechanical experimental methods originally developed within the field of materials science for characterizing the properties of thin films and nanoscale materials. Nanoindentation will be used to probe the mechanical properties (such as elastic modulus, hardness and plasticity) of grains and cements in both synthetic and natural samples. Micromechanical measurement techniques will be developed from the nanoindentation method to conduct a detailed characterization of both elastic and inelastic behavior of grain-cement and grain-grain contacts and interfacial cohesion. For example, the high resolution force-displacement probing arrangement will be reconfigured into a pulling arrangement to allow the measurement of nanoscale tensile properties of specific micro-scale geological structures. Additionally, two mechanically distinct sandstones will be used to explore the micromechanical properties of natural granular porous rock using techniques similar to those described above. The results of our novel microscopic experiments on natural and synthetic rock will produce an extremely detailed micromechanical dataset of grain-cement interaction. These data represent a first, important step in building a quantitative mechanical link between grain-scale and macroscopic behavior.