Integrated Reservoir Characterization for Successful Hydraulic Fracturing in Organic Rich Unconventional Plays – A Case Study from UAE

Abstract

Several challenges are associated with reservoir characterization of organic-rich, unconventional plays, most significantly with estimating producible hydrocarbons and identifying sweet spots for horizontal wells and subsequent stimulation. This paper illustrates the data integration approach from the Shilaif member and the important factors for the hydraulic fracturing simulations and execution. The Shilaif member consists of a succession of argillaceous limestone, mostly fine-grained packstones and wackestones with subordinate calcareous shales in the lower part. The complex carbonate lithology and fabric, combined with low porosity and the requirement to evaluate total organic carbon, presents a challenge to conventional logs and evaluation. Low permeability and productivity dictate the requirement to stimulate the wells effectively. Thorough integration of advanced and conventional log data (resistivity, neutron/density, dielectric, advanced acoustic, spectroscopy, nuclear magnetic resonance (NMR), and images) with core data and mud logs plays a critical role in the evaluation and development of these organic-rich reservoirs. Extensive data acquisition was planned with a wireline suite that included resistivity/density/neutron/spectral gamma ray; acoustic logs; acoustic image; NMR; advanced elemental spectroscopy; and dielectric technologies to characterize the hydrocarbon potential of organic-rich rock within the Shilaif member. The same suite of logs are critical for hydraulic fracturing simulations and play a heavy role when executing and pressure-matching the fracture geometry. Lithology and porosity from neutron/density logs are refined with NMR and spectroscopy to enable accurate evaluation of total organic carbon (TOC) and volumes. The advanced elemental spectroscopy data provided the mineralogy, the amount of carbon in the rock, and consequently the associated organic carbon within the Shilaif member. The NMR technology provided lithology-independent total porosity. The difference between the NMR and the density techniques provides accurate information about organic matter. NMR technology in this present case study was used to identify and differentiate the organic matter and hydrocarbon presence within the Shilaif member. Acoustic and image logs were used to evaluate the geomechanical properties that enable stimulation design to maximize the drainage while remaining within the boundaries of the reservoir. Accurate calibration of the stress profiles from core data assured the stimulation design was operationally achievable within pressure specifications and bounding formations. Detailed knowledge of natural fracture networks was critical to building an accurate geomechanical model. A complete workflow from formation evaluation to selection of interval to stimulate the Shilaif formation will be presented and used for future well development. The data integration work illustrated in the paper is a key for unconventional reservoir characterization that enabled identification of the sweet spots for horizontal wells and the successful hydraulic fracturing in the organic rich rocks of the Shilaif member.