Numerical Research of Fracture Initiation in Hydrate Reservoirs Based on XFEM
Authors: Wang Jinshan , Zhang Yiqun , Hui Chengyu , Ren Yingqiao , Cui Xu , Jiang Yiquan , An Youkeren , Huang Haochen 
Published: August 2024

ABSTRACT:
Natural gas hydrate (NGH) has emerged as a potential clean energy source. The China Geological Survey successfully conducted two trial mining operations in the Shenhu area of the South China Sea in 2017 and 2020. However, the exploitation of clayey silt NGH reservoirs poses significant challenges, including low production. Hydraulic fracturing is considered a potential key technology for increasing the production of NGH reservoirs. Understanding the characteristics of fracture initiation and expansion is crucial in determining the efficiency of production enhancement. To address this, a numerical model of a hydrate reservoir was established using the extended finite element method. The study investigated the impact of reservoir properties and fracturing execution parameters on hydraulic fracturing of NGH reservoirs. The findings revealed that high hydrate saturation, an increase in horizontal stress difference and high fracturing fluid injection rate contribute to longer fracture length and higher fracture initiation pressure. Conversely, lower hydrate saturation and decreased horizontal stress difference facilitate an increase in fracture width. Additionally, the study highlighted the influence of stress interference on the formation of multi-cluster fractures with small spacing, resulting in deviations and steering. The study provides key insights for selecting a suitable multi-cluster fracture initiation scheme for NGH reservoirs.

1. INTRODUCTION
Natural gas hydrate (NGH) is a special crystalline compound formed by water molecules and light hydrocarbons (mainly composed of CH4) at low temperature and high pressure (Li et al., 2016).NGH is mainly found in seafloor sediments, permafrost zones and some deep lake sediments (Li et al., 2018). Natural gas hydrate resources are huge, and the global CH4 resources are estimated to be between 1015-1018 m3, which is equivalent to about 40 times of the total conventional natural gas resources, and twice as much as the known commonly used fossil fuels (Boswell & Collett, 2011; Moridis et al., 2011). Therefore, NGH is considered to be a promising alternative to traditional fossil energy sources in the 21st century (Jiang et al., 2012).
Due to the low enrichment degree and wide distribution of natural gas hydrate in sea area, it is very difficult to exploit natural gas. In the world, only two countries, China and Japan, have conducted test production of natural gas hydrate in offshore areas. In 2013, the first test production in Japan was terminated due to serious sand production problems in the production wells, and the gas production reached 119,500 m3 (Yamamoto et al., 2019). In 2017, the second test production project (Yu et al., 2019) lasted for 24 days, using two production wells, the first well was stopped due to sand production, and the second well lasted for 24 days with a total gas production of 22.3× 104 STm3 (Zhu et al., 2020). In the same year, China conducted its first test production of gas hydrates in the Shenhu area of the South China Sea, which lasted 60 days and used a single vertical well to produce 5,000 STm3 per day. In 2020, China conducted the second test production of NGH in the Shenhu area of the South China Sea. This test production adopted the depressurization method of horizontal Wellbores for the first time, which was 5.57 times the average daily gas production of the first test production (Wu, 2017). However, there is still an order of magnitude gap between the current production of test production and the threshold of commercial development.