The Relationship Between Rock Properties and Fracture Conductivity in the Fayetteville Shale

Abstract

Abstract Fracture conductivity in shale is largely dependent upon the shale rock properties and the formation conditions. This paper aims to explore two different vertical intervals of the Fayetteville Shale, FL2 and FL3, by comparing laboratory fracture conductivity measurements. Fayetteville Shale outcrop cores were artificially fractured, preserving the surface roughness for the conductivity measurements. Controllable parameters, such as proppant size, concentration and type, were kept consistent between the two zones. An initial unpropped experiment was run, followed by two 30:70 mesh propped experiments at 0.03 lb/ft2 and 0.1 lb/ft2 concentrations. The FL2 consistently recorded higher conductivity values than the FL3 at closure stress up to 3, 000 psi. Therefore, each zone was evaluated using mineralogical composition, thin-section analysis and surface-roughness scans to identify rock property differences. The FL2 and FL3 rock analysis identified that the two zones are very different. Although the FL2 and FL3 samples used in the conductivity experiments had similar clay content, the FL2 contained more quartz and the FL3 contained more carbonate. Additionally, the FL2 samples were less fissile and had larger surface fragments break along the fracture surface, whereas the FL3 samples broke parallel to the bedding plane and had flaky dust fragments break along the fracture surface. The FL2 had higher conductivity values at low closure stress due to the rearrangement of bulky surface fragments and larger voids created on the surface when fragments were lost. The difference between the FL2 and FL3 conductivity decreases as the concentration of proppant increases because the voids causing unpropped conductivity differences become filled with proppant. Finally, production data was used to confirm laboratory results by providing evidence that wells in the FL3 produce less than the wells in the FL2 formation.