Abstract
The tight sandstone oil reservoirs characterized by the low porosity and permeability must be hydraulically fractured to obtain the commercial production. Nevertheless, the post-fracturing production of tight oil reservoirs is not always satisfactory. The influence mechanism of various factors on the fracture propagation in the tight oil reservoirs needs further investigation to provide an optimized fracturing plan, obtain an expected fracture morphology and increase the oil productivity. Thus, the horizontal well fracturing simulations were carried out in a large-scale true tri-axial test system with the samples from the Upper Triassic Yanchang Fm tight sandstone outcrops in Yanchang County, Shaanxi, China, and the results were compared with those of fracturing simulations of the shale outcrop in the 5th member of Xujiahe Fm (abbreviated as the Xu 5th Member) in the Sichuan Basin. The effects of the natural fracture (NF) development degree, horizontal in-situ stress conditions, fracturing treatment parameters, etc. on the hydraulic fracture (HF) propagation morphology were investigated. The results show that conventional hydraulic fracturing of the tight sandstone without NFs only produces a single double-wing primary fracture. The fracture propagation path in the shale or the tight sandstone with developed NFs is controlled by the high horizontal differential stress. The higher stress difference (<12MPa) facilitates forming the complex fracture network. It is recommended to fracture the reservoir with developed NFs by injecting the high-viscosity guar gum firstly and the low-viscosity slick water then to increase the SRV. The low-to-high variable rate fracturing method is recommended as the low injection rate facilitates the fracturing fluid filtration into the NF system, and the high injection rate increases the net pressure within the fracture. The dual-horizontal well simultaneous fracturing increases the HF density and enhances the HF complexity in the reservoir, and significantly increases the possibility of forming the complex fracture network. The fracturing pressure curves reflect the fracture propagation status. According to statistical analysis, the fracturing curves are divided into types corresponding to multi-bedding plane (BP) opening, single fracture generation, multi-fracture propagation under variable rate fracturing, and forming of the fracture network through communicating the HF with NFs. The results provide a reference for the study of the HF propagation mechanism and the fracturing design in the tight sandstone reservoirs.
Highlights
The tight sandstone oil reservoir originally has the low production due to the low porosity and low permeability and must be hydraulically fractured to achieve its effective development (Chen et al, 2019; Chuprakov et al, 2014; Wang et al, 2016; Wei et al, 2017; Yang et al, 2016)
The results provide a reference for the study of the hydraulic fracture (HF) propagation mechanism and the fracturing design in the tight sandstone reservoirs
The fracturing effect of the tight oil reservoir is closely related to the reservoir stimulation degree (Ante et al, 2018; Guo et al, 2015; Lai et al, 2017), which is affected by the natural fracture (NF) system, the in-situ stress conditions, the fracturing treatment parameters and techniques etc
Summary
The tight sandstone oil reservoir originally has the low production due to the low porosity and low permeability and must be hydraulically fractured to achieve its effective development (Chen et al, 2019; Chuprakov et al, 2014; Wang et al, 2016; Wei et al, 2017; Yang et al, 2016). Guo et al (2014) performed HF propagation simulations in shale outcrops, observed the HF morphology through high-energy CT scanning, and investigated the effect of the horizontal in-situ stress, horizontal differential stress coefficient, injection rate, fracture fluid viscosity, etc. The HF propagation morphology in the tight sandstone was studied in terms of the NF development (with or without NFs), in-situ stress condition (different horizontal differential stress), treatment parameters (injection rate, fluid viscosity), multi-well simultaneous fracturing, and well completion methods. The perforated hole is placed along the rH direction, and the perforation hole axial plane is placed along the rh direction
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