A combination of physical laboratory analysis and digital rock analysis (DRA) tests were conducted on plug samples from two wells in the Delaware Basin. The samples tested were taken from the Second Bone Spring (2BSPG) and Wolfcamp A (WCA) formations. The purpose of these tests was first to improve overall reservoir characterization and evaluation. The second purpose, and subject of this publication was to compare and contrast data on porosity, total organic content (TOC), and mineralogy from traditional physical laboratory experiments and from DRA methods.
Physical laboratory tests on these samples included pyrolysis, TOC, X-ray diffraction (XRD), Dean-Stark extraction, helium porosity, and DRA. The DRA method included argon-ion milling, SEM imaging, image processing, and quantitative image analysis. Rock properties determined from physical tests were water saturation (Sw), oil saturation (So), helium porosity, and mineralogy. Properties determined from DRA were effective porosity, porosity associated with organic matter (PAOM), total porosity, and solid organic matter volume (OM). Similarities and differences between and among these properties are shown and discussed in this paper.
SEM-based porosity as reported in some publications has been shown to be less than helium porosity (Walls et al., 2016; Capsan and Ramirez, 2016). In some cases, the disparity appears to be related to clay content. This is supported by Dean-Stark and XRD data that show bulk volume water (BVW) is directly correlated to clay volume. Reasons sometimes cited for these differences include insufficient image resolution and damage to organic material caused by ion milling. This work suggests that another factor may be more important, namely adsorbed or bound water on clay minerals. Adsorbed water can be removed by Dean-Stark and retort methods, and thus counted as part of total porosity. Because SEM images do not resolve bound water, porosity from DRA tests will be less than helium porosity and more comparable porosity available for free water and hydrocarbons. If the volume of clay bound water is taken into account, porosity from traditional helium methods and DRA methods compare favorably.
Full diameter core samples were obtained over zones that included 2BSPG and WCA formations. Fig. 1 shows the approximate location of the two wells called Well N and Well S. Tests on these whole cores included X-ray computerized tomography (CT) imaging, spectral gamma scanning, representative sample selection, mineralogic analysis, plug sample porosity, and Dean-Stark fluid saturation. Fig. 2 shows log displays of the two wells.
