Design and Application of Differential Staged Fracturing in Continental Shale Oil: A Case Study of Well X from Fuxing Area, China
Authors: Wang Xinliang , Mou Jianye , Gao Budong , Zhang Lufeng 
Published: August 2024

ABSTRACT:
Fuxing continental shale is characterized by poor physical property, strong heterogeneity, low brittleness index, and poor compressibility. Based on the concept of integrated geological and engineering, differential staged fracturing technology of horizontal well was proposed to unlock the production potential and enable commercial productivity. This paper takes well X as an example to perform fracturing design and field application. Firstly, the basic parameters of well X, such as porosity, gas-bearing property, mineral composition, rock mechanic, crustal stress and so on were introduced. Then, a series of numerical simulation was conduct to investigate the effect of pre-fluid, injection rate, natural fractures by using unconventional reservoir model (UFM). Finally, the field application of differential staged fracturing was performed and post-fracturing effect was evaluated by using micro-seismic monitoring technology. Laboratory experiments and logging interpretation show that the porosity of the target layer is 2.85∼6.66%, Young's modulus is 22-34 GPa, Poisson's ratio is 0.2∼0.3, and the horizontal stress difference is 6-10 MPa. Numerical simulation results show that the preferred single fracturing section length is 42-72m, the number of clusters is 6-8, the clusters spacing is 7-9m, the viscosity of pre-flush fluid is 50-100mpa.s, and the discharge rate is 18-20m3/min. Micro-seismic monitoring shows that fracture size is basically formed when the fluid strength exceeds 35 m3/m. This study provides a theoretical basis and reference for the efficient stimulation of continental shale oil.

1. INTRODUCTION
China's continental shale oil resources are abundant. Sixteen sets of shale formations have developed in basins such as Songliao, Ordos, Junggar, Bohai Bay, Sichuan, and Qaidam(Li et al., 2022; Sun et al., 2023). The rejuvenation area of the Sichuan Basin is an important area for exploration, increase, and production of shale oil and gas in China. The Fuling, Changning, Weirong, and Yongchuan shale oil and gas fields have been successively discovered. The proved oil and gas reserves exceed 2×1012m3. Continental shale oil differs from marine shale oil. It is characterized by low total organic carbon(TOC), low formation pressure, high clay content, strong plasticity, and low brittleness(Wei et al., 2022).There are significant lateral variations in sedimentary facies, and strong heterogeneity exists. These characteristics determine that the experience of transforming marine shale in North America cannot be simply replicated for continental shale oil. Typically, this type of reservoir utilizes horizontal well closely spaced multi-stage fracturing technology to remodel the target reservoir, aiming to expand the scale of fracture control and achieve the goal of increasing and stabilizing production(Huang et al., 2022; Zhang, 2021). Conventionally, geometric well completions are employed, which evenly distribute stage clusters along the horizontal well section. Statistical results from multi-stage fracturing of horizontal wells in the United States indicate that conventional geometric well completions result in preferential fracturing of clusters corresponding to low stress within the section, with over half of the clusters failing to achieve successful fracturing, leading to insufficient transformation of the target formation and consequently reducing production and increasing construction costs(Waters et al., 2006). 70% of oil and gas production comes from perforation clusters corresponding to low stress in each fracturing stage (Miller et al., 2011). Tang et al. (Tang et al., 2023; Wang et al., 2022; Chen et al.) conducted fracturing design on continental shale oil reservoirs in various regions and found that the main construction parameters affecting reservoir transformation were cluster number, cluster spacing, the viscosity of pre-flush fluid, and the discharge rate.