The Gas Research Institute (GRI) conducted pioneering work on measuring shale petrophysical properties in the 1990s, however, despite growing interest in shales, there are still no set standards with respect to obtaining core petrophysical measurements due to the inherent complexity of shales. Core cleaning is one aspect of this problem.
The objective of this study is to shed some light on the shale core-cleaning conundrum. The study shows the cleaning impact of different solvents on samples from different maturity windows and having different in-situ fluids. It also compares the cleaning efficiency between plug and powdered samples. Different cleaning apparatus, such as the high-pressure extractor (HPE) and the Soxhlet extractor, are also compared.
Different measurements, such as source-rock analysis (S1 and S2 values); gas chromatography-mass spectrometry (GC-MS) extraction analysis; Brunauer-Emmett-Teller (BET) surface area and pore-size distribution help to understand the dynamics of core cleaning. This study was carried out on samples from the Wolfcamp and Eagle Ford formations.
Cleaning has a major impact on various petrophysical properties like porosity (increases up to 50%), S1 (decreases up to 90%) and surface area (increases by 450%). This study showed that cleaning time is a function of maturity and sample state. Samples in the oil-maturity window are much more difficult to clean compared to the samples in the gas-maturity window. Similarly, plug samples are more difficult to clean compared to the crushed samples. Our study shows that toluene, dichloromethane (DCM) and chloroform have similar cleaning efficiencies but n-heptane is less efficient.
Coring is an integral part of any exploration program. The planning for a coring program, coring fluids and corehandling procedures at the wellsite are all very important for preserving the core and getting accurate measurements in the laboratory.
Dang, Son (University of Oklahoma) | Gupta, Ishank (University of Oklahoma) | Chakravarty, Aditya (University of Oklahoma) | Bhoumick, Pritesh (University of Oklahoma) | Taneja, Shantanu (University of Oklahoma) | Sondergeld , Carl (University of Oklahoma) | Rai, Chandra (University of Oklahoma)
Mechanical characterization of an isotropic rock requires the measurements of at least two elastic constants. Dynamic constants are obtained using ultrasonic techniques and static constants are obtained from the stress-strain response of the rock; both techniques can be used at elevated pressures and temperatures. These methods typically involve the use of cylindrical plugs; however, the existence of natural fractures or fissility of shale formations precludes the extraction of cores. The challenge is to improve reservoir characterization by measuring elastic properties using irregular, but ubiquitous smaller rock samples. We propose measuring two elastic parameters, i.e., Young’s modulus and bulk modulus through nanoindentation and mercury injection capillary pressure (MICP) experiments, respectively. With these two constants and the assumption of isotropy, all other isotropic elastic constants can be derived. The idea is to infer Young’s modulus (Enano) using nanoindentation and estimate bulk modulus (KMICP) using MICP data; neither measurement requires core plugs and can be carried out on irregularly shaped rock fragments. We assume the fragments are representative of the formation of interest; confirmation comes from establishing statistics. We measured Woodford, Haynesville, Eagle Ford, Wolfcamp, Bakken, Utica and Green River shale core samples. These values are compared to values obtained in ultrasonic-pulse transmission experiments. Ultrasonic values of K measured at 5,000 psi confining pressure agree well with the values of KMICP at 5,000 psi. Similarly, Enano shows a 1:1 correlation with ultrasonically derived Young’s modulus at 5,000 psi confining pressure. At a confining pressure of 5,000 psi, the influence of cracks is reduced.
The ubiquitous use of hydraulic fracturing to stimulate unconventional reservoirs drives the need for improved methodologies to compute the mechanical properties of rock. Mineralogical variability (Rickman et al., 2008; 2009; Passey et al., 2010) in shale should be considered in the decision of the placement of laterals. Ductility is a function of mineralogy, TOC richness and in-situ stress profile. Within a stimulation zone, where principle stresses are minimally varied, mineralogical variability directly affects elastic properties (Al-Tahini et al., 2006), brittleness and ductility (Bai, 2016): High concentrations of clay make shale more ductile, while the predominance of quartz is associated with brittleness. Jarvie et al., (2007) related brittleness directly to mineralogy.