# The Principle and Downhole Testing of Water Injection Exploration in Depleted Reservoirs
## 1 Introduction  
Mine water hazards are one of the five major natural disasters in coal mines, and among these, water hazards from depleted areas pose the highest risk of causing casualties [1, 2]. Depleted water, originating from either the mine itself or neighboring mines, accumulates in the goaf or old mine workings during the mining process. Depleted water hazards are characterized by sudden large water volumes and are highly prone to causingcasualties [3]. Statistics show that in recent years, the number of casualties caused by depleted water in mine water hazard accidents in China has been the highest. The occurrence of depleted water differs from layered groundwater aquifers and fractured aquifers, with complex distribution, various types, and poor regularity [4]. Therefore, the exploration of depleted water presents significant challenges. In recent years, many geophysical methods have been applied to explore depleted areas, including surface electrical methods, surface transient electromagnetic methods [5; 6], 3D seismic methods, and underground methods such as downhole electrical methods and downhole transient electromagnetic methods. However, the ambiguity of geophysical methods inevitably affects their accuracy, which has yet to be confirmed by authoritative data. The resolution of geophysical methods and their ability to accurately detect smaller goals (such as old tunnels) still need further discussion. [PERSON] [7] mentioned that traditional geophysical exploration techniques have low resolution, making it difficult to effectively identify deep and small-scale ore body structures, and are poorly adapted under complex geological conditions. For instance, the major depleted water hazard accident at Wangjialing Coal Mine occurred because only geophysical exploration was conducted without drilling [8], which failed to detect the depleted water. In Europe and Russia, hydraulic fracturing with high-pressure water injection is mostly used to weaken the rock mass to prevent rockbursts [9]. But in regions such as Shanxi and Inner Mongolia of China, water injection tests can be used to probe old empty areas. Considering the complexity of depleted water exploration, it is required to conduct \"exploration for every excavation\" during tunneling, and drilling exploration is mandatory. According to the \"Coal Mine Water Prevention and Control Regulations,\" when conducting advanced exploration for unclearly located depleted water, the final borehole distance should not exceed \"3 m apart on the horizontal plane\" [10] to prevent accidental exposure to old tunnels and subsequent depleted water hazards. However, in practice, this regulation is difficult to comply with due to the extensive drilling requirements, which is one of the reasons for the occurrence of depleted water hazards [11]. To address this issue and achieve effective exploration of depleted areas (especially old tunnels) with minimal drilling (even just one borehole), the author and their team proposed a water injection exploration method for depleted areas based on the principle of stress-seepage coupling, and applied for relevant patents (patent number ZL2014 1 0454295X). This method involves injecting water and observing changes in water pressure and flow rate to explore potential depleted areas. This paper will elucidate the principles of this method and introduce the underground experimental conditions. Theoretical Analysis of Water Injection Exploration Method for Depleted Areas Based on Stress-Seepage Coupling Principle  
Fractures are widely distributed in various coal-bearing strata, and the surrounding coal and rock formations in depleted areas primarily consist of fissure media. The permeability of fractured rock masses is related to the spatial distribution of the rock fractures, including factors such as fracture density, orientation, and connectivity, but it primarily depends on the permeability of each individual fracture [12; 13]. The permeability of fractures in their original state is determined by factors such as fracture width.