Produce high rate gas from poor CBM reservoir-Study on CBM resource types and “Window-Longtail Effects” of reservoir during delivering gas

It is found that there are two difficult problems in China CBM industry. The one is that not only the success rate of drilling and fracking is very low but also gas rates from more than 50% of those producing wells are far less than they are originally designed. The other is that almost all the production activities happen in a very concentrated small area,leaving more than 80% of China CBM resources far away from commercial potentials. A new model of CBM resource classification is established after a full discussion is made on how a soluid of coal-water-gas is built in the long geology history. And technologies applicable to produce gas from the 4 types of reserves in the model is according- ly recommended. Further studies in this paper concludes that CBM resources are different from other natural gas by three aspects. Firstly it is “Non-gas” in underground. Typical CBM is presented in four kinds of Soluids,in which coal- water-gas are coupled together driven by a combination energy of “ pressure” and “ stress”. Secondly gas has to be “Separated and Fostered” before it is drained to the surface. Actually a real independent gas flow cannot be normally delivered from seam until an effective process of “dewater” or “de-coal” is done. Thirdly there is a strong “ window- longtail effects” in poor CBM reservoir. No appreciable gas could be drained if there is no good engineering control over “time window” on reservoir stimulation,neither is there any sustainable gas flow if “ pumping longtail” is not well managed.