Recovering Elastic Properties From Rock Fragments

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)

OnePetro 

ABSTRACT

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.

INTRODUCTION

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.