The Multiple Bedding Planes (BPs) Effect on Fracture Height Propagation: Experimental Study and Field Data Analysis

Abstract

Abstract Geological structures such as pre-existence of multiple bedding planes (BPs) significantly influence the hydraulic fracture height propagation in shale gas and shale oil reservoirs. Field engineers have also proved that fracture height is much lower than designed via monitoring such as microseismic and DTS/DAS test. In this paper, hydraulic fracturing simulation is studied by injecting fluids into a 1m×1m×1m rock-block sample with multiple thin BPs developed at the in-situ stress condition, and during the operation process, acoustic emission (AE) monitoring is used to obtain the fracture propagation events. After the fracturing physical simulation, we cut the rock sample from three different perspectives and take visual observation of the fracture geometry via fluorescence irradiate. Secondly, we carried out series field data analysis such as fracturing curve history matching, flow-back data analysis and production data analysis, aiming to find the correlation between fluids leak-off rate, flow-back salt concentration, fracture height, fracture complexity and the number of BPs. Results showed that fracture propagation is a competition controlled by stresses and rock BPs. Fracture propagation is generally perpendicular to minimum principal stress while is locally controlled by the rock BPs. It always initial to be vertical fractures near the wellbore area, while as the vertical fracture penetrates the BPs, fracturing fluids would leak into the BPs and form horizontal fractures, lead to slow propagation in height direction and change the final fracture geometry. Fluorescence irradiate also showed fracture propagation can be divided into three modes, directly penetrate the BPs, arrested by the BPs or bifurcate into BPs, and these are mainly depending on the high-angle natural fractures. The results of this study suggest that a significant portion of fracturing fluids could be retained in BPs, which in turn is the reason of fracture height propagation obstruction. In field data analysis, we find that fluids leak-off rate has positive correlations with BPs number, fracture complexity and hydrocarbon production. Flow-back salt concentration (salinity) also has positive correlation with BPs number while has negative correlation with fracture height. Ratio of fluids leak-off and fracture height (Vleak/Hf) or ratio of flow-back salt concentration and fracture height (Cf/Hf) can be used as evaluating indicators for the number of BPs that fracture penetrates through. Experimental study and field analysis of the BPs effect on fracture height propagation can provide constructive guide for the hydraulic fracturing of BPs developed reservoirs.