Numerical Investigation on Field-Scale Fracture Propagation of Multi-Layered Formations Based on Unconventional Fracture Model
Authors: Jing Meiyang , Yang Ruiyue , Huang Zhongwei , Cong Richao , Chen Jianxiang , Wen Haitao 
Published: September 2023

ABSTRACT
The investigation of fracture propagation in multi-layered formations can assist in determining the optimal hydraulic fracturing pumping schemes for unconventional tight gas reservoirs. The option of multilayer-commingled fracturing or separate-layer fracturing technologies, based on geological and engineering conditions, is crucial for the commercial development of oil and gas in multi-layered formations. This paper presents a field-scale interbedded sandstone-mudstone fracturing model that employs an unconventional fracture modeling method based on logging interpretation data of the Upper Shihezi Formation in the Ordos Basin. The study examines the effects of in-situ stress, interlayer spacing, injection rate, and fluid viscosity on the fracture propagation of layered formations. The research findings suggest that the in-situ stress difference among multiple layers and interlayer spacing are the primary factors affecting the stimulated reservoir volume (SRV). A larger interlayer spacing and in-situ stress difference can lead to a greater SRV in separate-layer fracturing. On the contrary, the multilayer-commingled fracturing technology is suggested. In this scenario, higher fluid viscosity and lower injection rate are recommended. Based on numerical results, a separate-layer fracturing/multilayer-commingled fracturing selection chart was established for layered formations. The key findings are expected to provide a basis for selecting separate-layer fracturing/multilayer-commingled fracturing in tight layered reservoirs.

INTRODUCTION
With the sustained growth of China's economy and society, the consumption of fossil fuels has increased, resulting in a steady annual increase of CO2 emissions. In 2020, China's CO2 emissions reached approximately 10.3 billion tons, with coal, oil, and natural gas accounting for over 90% of the total emissions, at 9.5 billion tons. At the 75th United Nations General Assembly, China pledged to achieve peak CO2 emissions by 2030 and carbon neutrality by 2060 (Jin et al., 2023), representing a positive step towards China's global climate governance and fulfilling the Paris Agreement. Thus, the low-carbon utilization of fossil fuels is vital to ensuring energy security and achieving the "dual carbon" goal. Tight gas, a clean and efficient energy source with vast reserves, presents an opportunity to alleviate energy shortages and environmental pressures. Accelerating the development and utilization of tight gas resources can help China achieve the "dual carbon" goal sooner.
Unlike the thick sand bodies found in the United States, tight gas reservoirs in China typically exhibit thin thickness and multi-layer characteristics. Multi-layered reservoirs often contain multiple sets of vertically stacked gas-bearing strata, which exhibit low permeability and strong heterogeneity, resulting in lower single-layer production (Wang, 2017). The Ordos Basin, China's second largest gas-bearing basin, is a key area for developing layered tight gas reservoirs, which are characterized by low reserves abundance, multiple sets of thin interbeds in the longitudinal direction, and common occurrences of sand-sand and sand-mud interbeds. The number of gas-bearing layers is usually between 9 and 11, with single-layer thickness ranging from 3 to 5 meters. The single-layer production capacity is low, and the contribution of each layer to gas production varies significantly. The sand body is generally small in size (Yang et al., 2016 and Li et al., 2012).