| Coalbed methane (CBM), which is also called coal seam gas, is a kind of unconventional natural gas. It develops with the process of coal forming and occurrence in the coal seam. The main component of Coalbed methane is methane. Coalbed methane is considered a harmful gas in the early stage of coal exploitation. Enrichment region not only often causes coal mine explosions and other disasters, directly discharged into the air will also cause20-30times higher than that of the greenhouse effect of carbon dioxide. With the development of coalbed methane industry, its beneficial valve has been found. The methane content of CBM is more than70%, some even as high as98%. Methane is a new type of clean energy which is of high value, high quality, and products of combustion has no pollution to the atmosphere. The reasonable and effective development of CBM can not only solve the problem of energy shortage, optimize energy structure and improve energy utilization level, but also reduce air pollution, reduce greenhouse gas emission, and promote the harmonious development of economy and environment. In addition, the development of CBM can also reduce the coal mine gas explosion, which can ensure the safe production of coal mine. In a word, the development of CBM has multiple functions.Coal reserves are abundant in our country, the total is among the highest in the world, which has considerable reserves of CBM resources. Although the development history in our country has been decades of years, it has not received enough attention, supporting technical facilities are not perfect, so it develops slowly and has not yet reached the level of market and system. At present, the major water drainage and gas recovery method is still rod pump production, although it has obtained the certain achievement and established a series of theories system, there are still many inevitable problems and the effect is not very ideal. For example, during the production process of the CBM, there are often have braise, sand and other solid particles, which will easily cause the solid particles sticking and the eccentric wear of the sucker rod and other issues. Then results the service life of equipment shorter and often needs to stop the pump well to workover. Workover process not only wastes time and energy, but also affect the development and quality of the CBM. In addition, the rod pump production produces lower pressure drop and smaller pressure drop radius, which causes single well controlled reserves cannot liberate very well. It will result a phenomenon that low permeability coal seam gas production peak may appear, but the stable production period keeps not too long time.Most of the coal reservoirs in china has the characteristics of low pressure, low porosity and low permeability. The pressure gradient in the formation is0.8-1.0MPa/100m. The average porosity is about10%, and most pore not communicated with each other and become invalid porosity. The original permeability of most coal seam is very low (the permeability of matrix is less than10-3mD, the fracture permeability generally between0.1～100mD). Based on the existence of these objective factors, gas has low driving ability by the way of drainage exploitation. Plus to the finite ability of existing production equipment to reduce the bottomhole pressure, all these reasons will led to coalbed methane desorption degree and migration rate is low, resulting in the phenomenon stable production period is not long and the final yield is not high. The method of getting efficient development and stable production period is to innovate the way of drainage. To seek the drainage equipments which can cause more pressure difference. Do our best to expand the drop radius. In this way the critical desorption pressure will spread farther, the range of CBM desorption will be wider, and finally enhance the recovery ratio of CBM.Hydraulic jet pump is a kind of rodless oil extraction equipment. Common sucker rod pump must have mechanical energy and fluid energy transformation when it works. Unlike this type of pump, hydraulic jet pump use the high-speed fluid as the working power to transmit energy. The whole lifting process is completed through the conversion of pressure energy and kinetic energy. Because of its no moving parts, and also has a series of unique characteristics such as simple structure, high delivery capacity, easy maintenance and low cost and so on, it has obvious advantages and irreplaceable in many processes and the harsh environment. For example, it has a good adaptability in the directional well, horizontal well and offshore field. In addition, because of the availability of thermal and chemical properties of the power fluid, hydraulic jet pump can also be used for high pour point oil, heavy oil and high waxy well and so on. With the development of science and technology, the advantages of hydraulic jet pump have become increasingly prominent and more and more attention has been taken. At present, the application of drilling and drainaging conventional gas has already existed, as well as the research of coal mining using hydraulic jet pump. Besides the application in petroleum industry, hydraulic jet pump also play the advantage in other industries, application field becomes increasingly wide.In this paper, firstly, the situation at home and abroad of coal seam gas and jet pump is carried out, and then analyzed the rule of coal seam gas in coal seam gas reservoir about adsorption, desorption, influent and the rule of pressure propagation in coalbed methane, and also introduced the jet pump production system, took some research in its working principle, essential parameter, character and so on. Based on the analysis of the characteristics about the coalbed methane and jet pump advantage, with the jet pump technology from abroad as reference, the paper discusses a new technology to drainage the coalbed methane using jet pump in our country. This technique is to use the jet pump high speed jet to cause a powerful vacuum effect, greatly reducing the bottom hole pressure, letting the bottom hole pressure drop to OMPa、-1MPa、-5MPa、 -10MPa or even lower. So that we can increase the production pressure difference exponentially, gain greater pressure difference than the conventional production technology, get a greater pressure drop radius to improve the coalbed methane desorption more fully and wider. By this way, the coalbed methane drainage means can range from "point" to "surface", namely area of drainage and finally achieve the stable production of coalbed methane and enhanced oil recovery effect. This study put forward the image concept such as "area of drainage" and "negative pressure". This paper also analysis the design ideas, design technique, technical requirements, attention and the outstanding advantages. It have determined the feasibility of this technology.In addition, we have designed and optimized the jet pump. A detailed analysis of the main design steps of the coalbed methane production using the hydraulic jet pump. The main design content including the bottom hole flowing pressure, pump depth, calculation of pressure and temperature system, determine the lift height combined with the pump dimensionless characteristics, hydraulic power system analysis and operation parameters of the ground. Additionally, due to the different production stages, the seepage phase states in the coal are also different. Therefore, when determine the bottom hole pressure and in the analysis of inflow performance of the coal seam, it has been discussed in stages, which aim is to determine the bottom hole flowing pressure when the production is designed. When talking about the optimization of ejector pump, it has studied the spray nozzle, throat and diffusion bell and other key components, analyzed the diameter and length of the components. In addition, we also presented a new method to calculate the ejector pump parameter equation. This method starts from the conservation of energy, aiming to achieve the optimum pump efficiency. This study has positive significance for the research and application in the future. |
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