Gas bearing system difference in deep coal seams and corresponded development strategy

The realization of large-scale development of deep coalbed methane(CBM),which is abundant and has huge development potential in China, is beneficial to establish a large coal measure gas industry, serving the increment of oil and gas reserves and ensuring national energy security. Based on the systematic summary of the exploration and development achievements of deep CBM in recent years, the differences of deep CBM bearing system models and corresponding development strategies are revealed by the combination of experiments and theoretical analysis. The results show that the deep CBM can be divided into over-saturated dry coal system, saturated to near-saturated wet coal system and under-saturated wet coal system due to different tectonic evolution and preservation conditions. In the dry coal system, when the reservoir pressure is higher than 10 MPa, the free gas can be compressed in the coals and exceeds the maximum content of adsorbed gas, which is easy to have a high production. The fractures or some macropores in the wet coal system are saturated by water, which needs the process of drainage, depressurization and desorption to produce gas. In the process of continuous hydrocarbon generation in coals, a large amount of free gas is generated from nanopores to form overpressure, which inhibits the formation water invasion. The adsorbed gas and free gas in the deep coals are in dynamic transformation. However, under the comprehensive influence of Van der Waals force, capillary force and buoyancy, the coals can form continuous-quasi continuous natural gas reservoirs, showing the characteristics of “source rock controlled reservoir”(continuous and stable coal seams influence reservoir quality) and “physical property control reservoir”(physical property affects sweet spots distribution),and thus the sweet spots can be found in stable high permeability coal seams with good sealing conditions. The deep coals are compacted densely with high in-situ stresses, which leads to the increment of compressive strength of coals and enhancement ordering of reservoir reconstruction fractures. Thus, the three-dimensional seepage network can be formed by different particle size proppants. For the successful production of dry coal system, a large-scale staged fracturing of horizontal wells can be adopted to expand the flow area, improve the gas production rate, and achieve the main purpose of volume fracturing and “fragmentation”. However, the wet coal system mainly adopts moderate transformation and “dredging”,and considers strategies such as an indirect fracturing of horizontal well roof. During the geological history, coal derived methane continues to migrate and accumulate in adjacent strata, forming a full gas bearing system. In the mature exploration areas, the basic geology and engineering understanding should be fully evaluated, making strategies for “overall deployment” and “categorized breakthrough”,to achieve a large-scale development of deep CBM and an overall recovery of multiple natural gases.