Investigation of the Effects of Near-Wellbore Pressure Loss and Pressure Dependent Leakoff on Flowback during Hydraulic Fracturing with Pre-Existing Natural Fractures

Abstract

It is generally believed that the incomplete return of treating fluids (flowback) is a reason for the failure of hydraulic fracturing and it is associated with poor gas production. Capillary effects and fracture face skin are known to be the main parameters that limit flowback in tight gas sands. However, near-wellbore pressure loss during the main operation of hydraulic fracturing and its effects on flowback are not well understood. In the case of pre-existing natural fractures in a reservoir, near-wellbore pressure loss is high mainly because multiple fractures with tortuous paths are often created. Also, it is believed that this tortuous path causes shear dilation in natural fractures and opens the closed fractures. As a result, there is a large amount of pressure dependent leakoff. Therefore, pressure dependent leakoff has both positive and negative impacts. It can increase rock permeability and at the same time it can cause a high near-wellbore pressure drop. Hence, near-wellbore pressure needs to be reduced in order to have better proppant placement and maximize the fracturing fluid flowback. For this research, several hydraulic fracturing treatments in the Patchawarra formation in the Cooper Basin, South Australia have been studied. There are some pre-existing natural fractures in our case study. The bottom-hole treating pressure has been analysed with 600 psi of near-wellbore pressure loss causing a low percentage of proppant placement. We constructed a 3D hydraulic fracturing model coupled with multiphase flow simulation for the prediction of flowback of water and gas production. Several injection fall-off tests were first interpreted and found that the leakoff is pressure dependent. Our simulation has shown that high near-wellbore pressure loss has an impact on the effectiveness of the fracture treatment. This results in a shorter fracture length and low pressure distribution inside the fracture that leads to a low recovery of fracturing fluid. As a result, we have successfully provided recommendations for best practice to reduce near-wellbore pressure loss. Copyright 2015, Society of Petroleum Engineers