In-Situ Nuclear Magnetic Resonance Detection and Visualisation of Hydraulic Fracture: A Proof of Concept

Abstract

ABSTRACT A novel approach to study hydraulic fracture initiation and propagation inside Nuclear Magnetic Resonance core flooding system (NMR) was examined on 38 mm diameter (1.5") outcrop sandstone rock plugs. A 3.4 mm diameter and about 35 mm long borehole (two-third of the plug's length) was drilled at the center of the plugs, and then a 3.17 mm diameter (1/8") PEEK tube was carefully inserted and glued. Dry plugs were assembled and loaded into the NMR confining system, and then hydrostatically pressurised to 500 psi (∼3.5 MPa). Potassium iodide brine was slowly injected into the borehole, while the increasing injection pressure was monitored throughout the experiment using an independent pressure sensor positioned near the borehole's inlet face. NMR-T2 relaxation and saturation profile measurements were conducted at different pore pressure stages to locate and quantify any pore size changes. Both X-ray CT images and NMR spatial T2 measurements were only acquired before and after hydraulic fracture was created to support its location, aperture closure and growth. The results proved that NMR core flooding system is sensitive to any porosity or pressure changes induced by hydraulic fracture development. INTRODUCTION The oil and gas industry has widely adopted Nuclear magnetic resonance (NMR) measurement for reservoir characterization and monitoring. NMR detects the relaxation times of hydrogen nuclei in a rock's pore spaces, providing valuable information on petrophysical properties such as fluid(s) volume, porosity and pore-size distribution. Both laboratory and field-scale studies have extensively employed NMR, and its reliability and accuracy have been well-documented in various petrophysical research (Kleinberg and Horsfield, 2003; Blümich et al., 2007). Hydraulic fracturing (HF) is a common technique used to increase the permeability of tight reservoirs that are uneconomical to produce otherwise. High-pressure fluids are injected into the reservoir to create fractures that facilitate the flow of hydrocarbons to the wellbore. Despite extensive research on hydraulic fracturing, the actual fracturing process remains poorly understood, particularly at a laboratory scale.