Experimental study on fracture height propagation during multi-stage fracturing of horizontal wells in shale oil reservoirs

Key words:  Jimsar  shale oil  horizontal well  multi-stage fracturing  CO₂-gel hybrid fracturing  fracture height propagation

Abstract:

In order to solve the problem of low degree vertical stimulation in shale oil reservoir of Lucaogou Formation in Jimsar Sag, the advantages of fracture height propagation using CO₂-gel hybrid fracturing over conventional water-based fracturing and supercritical CO₂ fracturing were investigated by conducting laboratory tri-axial hydraulic fracturing experiments. A multi-stage fracturing experimental method for horizontal wells with real rock samples was firstly established. The fracture geometry induced in the multi-stage fracturing was comprehensively determined by a rock sample splitting technique along with computed tomography (CT) scanning and acoustic emission monitoring. The experimental results indicate that the fracture height formed using a low viscosity "slick-water" was restricted, though the performance of the "slick water" was better than supercritical CO₂. High-viscosity gel fracturing fluids can effectively increase the fracture height propagation, but with fewer bedding planes activated. During CO₂-gel hybrid fracturing, the blocking effect of gel can effectively reduce the leak of CO₂, so that CO₂ can break through the confinement of bedding plane on the fracture height. Meanwhile, high compressibility of CO₂ can release a large amount of elastic energy during fracturing, which can promote shear failure of the bedding planes and natural fractures and generate a complex fracture network. Increasing the injection rate of CO₂ during CO₂-gel hybrid fracturing, the fracturing pressure can be increased over 10.1%, and the shear failure event can be increased by 4.2%, which can induce a more complex fracture network. Consequently, a novel CO₂-gel hybrid fracturing method for stimulating multi layered shale oil reservoirs was proposed and verified. Firstly, high-viscosity gel is injected into the wellbore to initiate fractures vertically, and then CO₂ with a large injection rate is pumped to further increase the stimulated reservoir volume.