Experimental research into fracture propagation of complex lithologies in fractured tight oil reservoirs
ZHANG Ran, LI Gensheng, GUO Jianchun
1 State Key Laboratory of Petroleum Resource and Prospecting, China University of Petroleum-Beijing, Beijing 102249, China 2 State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Southwest Petroleum University, Chengdu 610500, China
In the Cangdong Depression of the Dagang Oilfield, the lithology of tight oil reservoirs in the Kong-2 member of the Kongdian Formation includes shales, sandstones, carbonate rocks and transitionalmigmatites. Its lithologyis complex and variable, with strong heterogeneity. The reservoirs are mainly middle-porosity to low-porosity, and extra-low to ultra-low permeability, which have developed certain stratification and natural fractures. Previously, the concept of network fracturing was introduced. This led to the effective implementation of combined fracturing fluids and multiple sand fracturing techniques, which achieved some progress in tight oil fracturing. However, due to the heterogeneity of reservoir rocks, there is a lack of understanding on the ability of each layer to form fracture networks. This has resulted in significant differences in the effectiveness of different well fractures. In view of this, downhole rock specimens from Cangdong tight oil reservoirs were used for triaxial mechanical testing, mineral composition testing, and CT scanning. Based on these tests, the mechanical parameters, mineral composition and natural fracture distribution of the reservoir rock specimens were analyzed. Rock specimens with either horizontal, low-angle, high-angle or complex natural fractures were selected for true triaxial fracture testing. In addition, CT scans were performed to monitor the fractures. Our aim was to investigate the patterns of fracture initiation and propagation in the tight oil reservoirs of the Cangdong Sag. The experimental results show that: (1) Based on the effectiveness of fracturing, the fractures can be categorized into three forms: single hydraulic fractures, initiation along natural fractures, and complex fractures. (2) The primary factor controlling the pattern of hydraulic fracturing is the natural fractures inthe Kong-2 member ofthe Kongdian Formation. (3) In the experiments, when hydraulic fractures encountered natural fractures, there were three propagation modes: opening along natural fractures, penetration through natural fractures (direct penetration or penetration after changing direction), and obstruction by natural fractures. The difference in principal stress determines the mode for fracture propagation. (4) The development degree of natural fractures significantly influenced the fracturing pressure and fracture-fluid loss. These results can serve as a reliable basis to understand the fracture patterns of tight oil reservoirs in the Cangdong Depression of the Dagang Oilfield, and to assess the effectiveness of network fracturing techniques in reservoirs.
