Prospects for the development of the theory and technology of non-hydration penetration enhancement in soft coal seams

In the future quite a long historical period, China will still adhere to the basic national conditions of coal. The efficient exploitation of coal, as a guarantee energy source, is crucial to China's energy security. However, more than 70% of coal seams in our country are low-permeability coal seams, and gas extraction is difficult and long. The mining imbalance caused by low gas extraction rate is the primary factor to curb the release of coal production capacity. Especially with the depletion of shallow resources, the complicated occurrence environment of deep resources will make this problem more prominent. So penetration enhancement of low-permeability coal seams is an effective means of realising efficient gas extraction, solving the imbalance between mining and excavation, and then increasing coal production. Hydraulic technology such as hydraulic punching, water jet cutting and hydraulic fracturing is the most widely used and effective penetration enhancement technology due to its mature technical equipment and flexible application. However, when applying hydraulic technology in soft coal seams, the drilling holes are prone to collapse, which leads to problems such as drill holding, hole blowing, and gas overlimit. Compared with hydraulic technology, non-hydraulising technology avoids the softening of coal strength by water from the source. The use of non-hydraulising technology to increase the penetration of soft coal seams enables the gas channel to be kept open for a long time, and has the ability of efficient extraction, which is in line with the development concept of new quality productivity. Therefore, non-hydraulic technology is a feasible way to break through the bottleneck of high-efficiency gas extraction technology in soft coal seams. There are many kinds of non-hydraulic technology, but none of them has been applied or promoted in engineering. To seek the development direction of non-hydraulic technology in the field of gas extraction, promote the development of non-hydraulic technology, and break through the bottleneck of high-efficiency soft coal seam gas extraction technology, this paper systematically combs through 10 non-hydraulic penetration enhancement technologies, such as mechanical cutters, controllable shock waves, and abrasive air jets, and classifies them into three categories: mechanical penetration enhancement technology, electromagnetic wave/mechanical wave penetration enhancement technology, and gas-related penetration enhancement technology. Their technical principles, penetration enhancement mechanisms, advantages and bottlenecks are elaborated in detail, and feasibility suggestions for the development of non-hydraulic technology in soft coal seam gas extraction engineering are put forward. The following main conclusions are formed: the non-hydraulic technology crushes coal rock in the form of mechanical force, impact or thermal stresses, forming pressure relief areas and fissure networks, promoting gas desorption and seepage, and then improving the gas extraction rate. Subject to the constraints of energy density and force transmission method, most non-hydraulic technology has a small pressure relief range compared with hydraulic technology. This is the main factor that curbs the engineering application of non-hydraulic technology. In addition, the maturity, operability and applicability of the technology and equipment are also important factors that inhibit its application. In the application of low-pressure abrasive air jet in soft coal seam, it is found that under the condition of gas pressure less than 1 MPa, the radius of reaming can still be more than 1 m, which can realise the uniform pressure unloading in a wide range of coal seam. Combining the energy transfer efficiency and the maturity of the technology and equipment, the low-pressure abrasive air jet has the potential to be applied and promoted under the condition of further improving the safety guarantee technology.