A model to describe hydraulic fracture conductivity decline curve in 3-stages –focus on Longmaxi formation

This study focuses on modeling fracture conductivity decline curve in 3-stages of slope. In the past few decades, advancement in hydraulic fracturing stimulation technology has greatly helped in the exploitation of hydrocarbon resources from unconventional shale formations. However, applying this technology to deeper shale gas formation (3500–4,500 m) requires a better understanding of deep unconventional formations due to their downhole conditions (high temperature and high pressure). During hydraulic fracturing, micro fractures are often unpropped due to their micro width and complexity. The micro fractures create their own conductivity via sheared displacement (2.5 mm average) of the surfaces in contact. Thus, a reason to investigate the effect of shear displacement in fracture conductivity. Several researchers have investigated shale rock for effect of rough fracture surface on fracture conductivity, however less research have investigated effect of shear-displaced rough fracture on conductivity (Longmaxi formation). Therefore, Longmaxi formation of Sichuan basin, China, was taken as a case study to analyze the characteristic effect of shear failure of rough fracture surface on fracture conductivity. XRD and FESEM tests were done and a 3D laser scanner was used to analyze the fracture surface roughness and a shear displacement of 2.5 mm was allowed before fracture conductivity test was performed (using API RP 61 standard procedure with nitrogen as test medium). JRC correlation was done for the root-mean square of the fracture surface asperity height using Yu/Vayssade and Tatton/Grasselli method. The fracture conductivity curve showed a 3-stage decline pattern. From the 3D laser scan, it was observed that the fracture perpendicular to the bedding plane was mainly rough in the lateral direction, and therefore the fluid flow channel was mainly a lateral groove perpendicular to the flow direction, so the fracture flow ability was poor. JRC result showed roughness for all samples. The 3-stage fracture conductivity model obtained from this work could assist researchers/engineers in the design of hydraulic fracturing operations for deep depth of Longmaxi formation of Sichuan basin. •Lack of adequate research was identified in area of fracture conductivity in rough fractures for Longmaxi shale deeper than 3,500 m.•The mechanism of fracture conductivity creation via rough shear fracture was presented and analyzed.•XRD and SEM results of Longmaxi formation shale