ARMA 18 – 055 Nano-and MicroScale Deformation Behavior of Sandstone and Shale

Hydraulic stimulation and production-induced permeability alteration in stress-sensitive Monterey Formation rocks require an in-depth understanding of the deformation behavior of different rock types. Here, we evaluate and compare deformation behavior of two types of clastic rocks—shale and sandstone—across nano-to-micro scales using experiments and simulations. Using core samples of shale from the Monterey Formation and tight sandstone from the Dominguez Hills in California, we conduct nanoindentation, scanning electron microscopy and particle-based simulation to understand the difference in deformation behavior of shale and sandstone under different loading-unloading conditions. We quantify Young’s modulus, hardness modulus, and stiffness of the rocks using nanoindentation data. Our work provides important insights into grain-scale deformation behavior of two different rock types commonly found in petroleum reservoirs. Understanding of grain-scale failure mechanisms can inform development of new upscaled constitutive models for usage in continuum-scale field simulations, which cannot afford to resolve the grain-scale processes due to computational cost. choose the operational parameters of both hydraulic stimulation and pressure depletion. In this paper, we conduct nano-to-micro scale indentation experiments to obtain load-displacement behavior and elastic moduli of shale and sandstone core samples from California oilfields. We perform scanning electron microscopy (SEM) before and after the indentation to identify different types of minerals and their size distributions. We obtain elemental composition using energy dispersive X-ray spectroscopy (EDX). Finally, we build a particle-based peridynamics model to simulate the indentation experiment at high resolution and understand the process of obtaining a continuumscale constitutive model from nanoscale loading experiments. 2. FIELD GEOLOGY Our samples were obtained from rotary sidewall cores of a single well drilled deep into the Monterey section from a Los Angeles Basin oil field in California, USA which has been producing since the 1920s. The field is one of various fields along the Newport-Inglewood fault (the most notable feature of the basin), which is a part of the larger system of right-lateral strike-slip faults in the region (Figure 1). The Los Angeles Basin is the most productive basin by area in the world with the Monterey formation being its source rock (Hargreaves, 2012). The LA basin system is a prolific Miocene to Pleistone source rock with submarine fan and slope channel reservoirs. Its faulted anticlines make up the largest traps and an abundance of thick sandstone reservoirs. It consists of multiple stacked formations including the Monterey formation. The basin is an active petroleum system. Total Organic Content (TOC)-source rock analyses of the field show good-to-excellent oil potential and an increasing TOC with depth. Hydrogen/Oxygen index plots also show a Type II kerogen that indicates it is of a marine origin and oil prone. The Monterey formation is an extensive sedimentary deposition of Miocene age. It is known as the primary source rock for oil produced from other formations in California and estimated to trap about 15.5 billion barrels of recoverable oil. The huge potential to develop the resources locked in this oil rich deposit using hydraulic fracturing makes it a key resource play (EIA, 2011; EIA, 2014). Figure 1: LA Basin Oil Fields. (Photo from US Dept. of the Interior USGS) A sandstone sample (Figure 2) was recovered from a depth of 9783.25 ft. The sample is very coarse-grained and moderately well-sorted with no visible porosity. Measured porosity when saturated with brine is about 8.2% with a rock density of 2.65 gm/cc and a permeability of about 2.1 md. Figure 2: Slabbed core showing sandstone section photographed under white (left) and ultraviolet (right) light Shale sample was obtained from the Monterey section at a depth of 12,468.45 ft. The sample is very dark gray with hard slicken-sided bedding surfaces. There is a surface fracture visible on the edge of the core chip that is used for indentation. The interval is heavily fractured and brittle as can be seen in Figure 3. Figure 3: Slabbed core showing shale section photographed under white (left) and ultraviolet (right) light Table 1: Core Analysis and NMR (Nuclear Magnetic Resonance) Data of Samples Core Analysis Data NMR Data Saturation 100% Brine Saturation