Combined Network Fracturing Technology with Variable Viscosity Injection Procedure: Simulation and Field Application in Shale Oil Reservoir, China

Abstract

ABSTRACT: To maximize stimulated reservoir volume (SRV), an emerging fracturing technology, combined network fracturing technology is proposed and applied in Shengli shale oil reservoir. This technology aims to form a complex fracture network consisting of main fractures with higher conductivity and complex branched fractures by variable fluid-viscosity injection. In this paper, integrated with the finite element method (FEM) with the discrete fracture network (DFN) model, a complex fractures propagation model coupled with flow-stress-damage was established. The accuracy of the model was verified by the comparison with a two-dimensional analytical model. The effects of fracturing fluid viscosity on fracture propagation were simulated. Compared with conventional fracturing, variable viscosity injection procedure can improve the growth of main fractures and branch fractures, and then increase the complexity of the fracture network. Finally, a field application has been implemented in a typical shale oil well in Shengli Oilfield. The peak oil production of the well after fracturing treatment reached 171t/day, and the average oil production can maintain at about 28t/d after half a year of production, which significantly improves shale oil production. The novel fracturing technology proposed in this paper can provide a reference for the optimization of the fracturing fluid viscosity in combined network fracturing. 1. INTRODUCTION Jiyang Depression, located in the northeastern Shandong Province and the south eastern Bohai Bay Basin, is extremely rich in shale oil resources and has become one of the main development blocks of Shengli Oilfield. The shale oil reservoirs there have obvious laminated structures, which results in variable in-situ stress and interbedded layer properties. Staged fracturing in horizontal wells has played a prominent role in increasing production in this block. To maximize the stimulated reservoir volume, it cannot be satisfied with the fracturing only in fracturing layer. Due to the special structure of shale reservoirs, connecting multiple production layers through fracture growth in the height direction is the most effective way. To realize the purpose, some relevant measures have been tried such as increasing pumping rate as much as possible. However, even so, the height of the main fractures was much lower than the expected value. Therefore, we propose an emerging fracturing technology called combined network fracturing. The technology aims to form a complex fracture network consisting of main fractures with large height and complex branched fractures by variable viscosity injection procedure. Until now, there is few reports on fracture growth in layered shale oil formation under variable fluid viscosity. It is essential to study the propagation mechanism of hydraulic fractures under layered formation with bedding planes, so as to choose an optimal treatment scheme of combined network fracturing.