An Integrated Study of Probabilistic Formation Evaluation and Geomechanical Analysis to Investigate the Potential of Hydraulic Fracture Stimulation in Najmah Unconventional Shale Reservoirs

Abstract

Abstract The Najmah Shale, an organic-rich marl, is generally considered the primary source rock for hydrocarbons in Kuwait’s Jurassic and Cretaceous reservoirs. The purpose of this study was to estimate the original hydrocarbons in place (OHIP) and the geomechanical properties of the Najmah reservoir to aid in the design of a hydraulic fracture stimulation program in West Kuwait. An integrated petrophysical evaluation utilized conventional and sidewall core measurements, and standard and advanced open-hole logs were used to estimate net pay, porosity, oil saturation, and geomechanical properties. Formation evaluation of the Najmah Shale as a potential unconventional reservoir posed numerous challenges. These challenges included the ambiguous effects that high total organic carbon (TOC) has on conventional porosity logs and resistivity logs and the associated shale volume estimations. In this study, a probabilistic multi-mineral model was developed to more accurately assess the TOC of the rock and the associated porosity, saturation, and clay volume. Advanced well logs, including spectral gamma ray and elemental spectroscopy logs, were used to improve the mineralogical model of the complex formation. Routine core analysis, programmed pyrolysis, and X-ray diffraction (XRD) analyses were used to verify and calibrate the multi-mineral model results. Since a dual-porosity system was present in the formation, the Simandoux saturation equation was used to evaluate the fluid saturations. Anisotropic horizontal stress profiles were developed for specific wells based on analysis of dipole sonic logs, resulting in a greater regional understanding of the target interval. Based on the results of the multi-mineral modeling, the average TOC of the Najmah Shale varies from well to well throughout West Kuwait, with values as high as 14.8%. The effective porosity of the Najmah Shale ranges from 1 to 8%. Water saturation is low for these organic-rich formations. Water zones may occur above or below the organic-rich interval depending on the location. The geomechanical properties of the Najmah Shale are conducive to hydraulic fracture stimulation, by analogy to proven productive shale plays. The Sargelu interval, below the Najmah Shale, exhibits distinctly higher minimum horizontal stress gradients while the limestone above the Najmah Shale presents a weaker stress barrier. The results of the probabilistic formation evaluation of the Najmah Shale indicated that a significant volume of hydrocarbons is present in the formation. The geomechanical properties of the Najmah and adjacent units are conducive to successful hydraulic fracture stimulation. The evaluation of water-bearing zones adjacent to the target formation is critical to the investigation of the formation’s stimulation potential.