A Study of Natural Gas Hydrate Reservoir Stimulation by Combining Radial Well Fracturing and Depressurization
Authors: Hui Chengyu , Zhang Yiqun , Zhang Panpan , Wu Xiaoya , Huang Haochen , Li Gensheng 
Published: September 2022

ABSTRACT:
Gas hydrates are widely distributed and abundant in nature, and are regarded as an essential clean energy for the future. Improving the development mode and promoting production efficiency is an important way to realize the commercial application of natural gas hydrate. Hydraulic fracturing has an impressive application effect in developing traditional oil and gas resources. This study proposes fracturing of radial horizontal wells to exploit oceanic hydrate reservoirs. Based on the trial production data of SH7 site in the Shenhu area, a 2D model was developed to simulate the hydrate decomposition process using fracturing of radial horizontal well. Results indicate that fractures can effectively improve the production efficiency of natural gas hydrate, and the fractures can provide high conductivity channels and effectively increase the pressure drop propagation range, thus facilitating the decomposition of hydrates. Horizontal and vertical fractures boosted gas production by 306% and 550%, respectively. Vertical fractures are more conducive to the heat replenishment from the boundary layer, thus increasing gas production more efficiently. This study provides a theoretical basis for the potential applications of radial horizontal well fracturing in field trials of gas hydrate development.

1. INTRODUCTION
Natural gas hydrate is an ice-like crystalline compound composed of natural gas and water molecules under high pressure and low temperature environment, which has the characteristics of high energy density and abundant reserves(Sloan, 2003). In nature, gas hydrates are abundant in permafrost and shallow submarine sediments. Global assessments suggest that gas hydrates carbon reserves reach 1015-1018 m3, about twice as much as proven conventional fossil fuels(Yu et al., 2019; Zhang et al., 2020). The effective development and utilization of natural gas hydrate has significant implications for the world’s energy landscape, climate change and other issues(Cui et al., 2019; Li et al., 2016).
Conventional methods of gas hydrates extraction include: depressurization, thermal stimulation, CO2 replacement and inhibitor injection methods(Tupsakhare et al., 2017; Yang et al., 2016; Yuan et al., 2013). Based on previous research and field trials, the depressurization method is the most promising way to achieve commercial exploitation of gas hydrates, which is simple and more efficient than other methods(Terzariol et al., 2017; Yang et al., 2016). In 2013, Japan was the first to use the direct well combined with the depressurization method for hydrate test recovery in the Nankai Sea. China conducted gas hydrates test production twice off the southeast coast in 2017 and 2020, with the second using the horizontal well combined with the depressurization method. The production process lasted for 30 days and the average gas production rate reached 2.87*104 m3/d and set a new world record(Ye et al., 2020). Nevertheless, the gas production from the test recovery is still far from the threshold of commercial extraction(2*105 m3/d)(Chen et al., 2021). Therefore, increasing the recovery rate of gas hydrate becomes a key issue to achieve commercial extraction.