Effects of maturation on multiscale (nanometer to millimeter) porosity in the Eagle Ford Shale

Abstract

Porosity and permeability are key variables that link the thermal-hydrologic, geomechanical, and geochemical behavior in rock systems and are thus important input parameters for transport models. Neutron scattering studies indicate that the scales of pore sizes in rocks extend over many orders of magnitude from nanometer-sized pores with huge amounts of total surface area to large open fracture systems (multiscale porosity). However, despite considerable efforts combining conventional petrophysics, neutron scattering, and electron microscopy, the quantitative nature of this porosity in tight gas shales, especially at smaller scales and over larger rock volumes, remains largely unknown. Nor is it well understood how pore networks are affected by regional variation in rock composition and properties, thermal changes across the oil window (maturity), and, most critically, hydraulic fracturing. To improve this understanding, we have used a combination of small- and ultrasmall-angle neutron scattering (U)SANS with scanning electron microscope (SEM)/backscattered electron imaging to analyze the pore structure of clay- and carbonate-rich samples of the Eagle Ford Shale. This formation is hydrocarbon rich, straddles the oil window, and is one of the most actively drilled oil and gas targets in the United States. Several important trends in the Eagle Ford rock pore structure have been identified using our approach. The (U)SANS results reflected the connected (effective) and unconnected porosity, as well as the volume occupied by organic material. The latter could be separated using total organic carbon data and, at all maturities, constituted a significant fraction of the apparent porosity. At lower maturities, the pore structure was strongly anisotropic. However, this decreased with increasing maturity, eventually disappearing entirely for carbonate-rich samples. In clay- and carbonate-rich samples, a significant reduction in total porosity occurred at (U)SANS scales, much of it during initial increases in maturity. This apparently contradicted SEM observations that showed increases in intraorganic porosity with increasing maturity. Organic-rich shales are, however, a very complex material from the point of view of scattering studies, and a more detailed analysis is needed to better understand these observations.

Tensile-dominant fractures observed in hydraulic fracturing laboratory experiment using eagle ford shale

“…X-ray diffraction (XRD) data showed that calcite was the dominant mineral (96.3 per cent), followed by quartz (3.7 per cent), which likely explains the low anisotropy in the velocities observed for these samples. Previous studies also reported that Eagle Ford shale has a calcite-rich formation (Anovitz et al 2015). The tensile strength and permeability were measured in the directions perpendicular to the sedimentary plane, including the fracture propagation direction addressed in the present study (Section 3).…”

Effects of Ionic Strength, Salt, and pH on Aggregation of Boehmite Nanocrystals: Tumbler Small-Angle Neutron and X-ray Scattering and Imaging Analysis

“…Details of the neutron small-angle scattering approaches have been reported elsewhere − and are, therefore, summarized briefly. SANS and USANS were used to probe aggregate sizes from ∼100 Å to ∼15 μm.…”

Characterization and Analysis of Porosity and Pore Structures

“…However, in the latter cases specialized masks, such as rectangular, slit (cf. NavarreSitchler et al 2013), or annular (Anovitz et al 2015b) shapes can be used. The images on the left shows a sample ground to 150 m, then fl oated off the glass slide (originally glued on using super glue) and attached to the Cd mask.…”

“…Contrast matching experiments were used to demonstrate the presence of two phases, a zing-rich particle and a nanoporous carbon, the relative sizes of which were determined from the Q-dependence of the contrast curves. Such an approach (Anovitz et al 2015b) can also be used to analyze the pore surface mineralogy as a function of pore size, providing a link between porosity, overall mineralogy, and reactive mineralogy as a function of pore size and concentration. Figure 30.…”