The purpose of pumping out water from CBM well is to depressurize the coal seam to initiate desorption of hydrocarbon gases, mainly methane. Determination of accurate liquid level is required to calculate gas flow rate and well performance, to optimize pump sizes for maximum possible liquid withdrawal, for optimizing gas production, determination of pump setting depth for its optimum performance in view of NPSH and simultaneously to ensure water level to go down below the last coal seam. However, in CBM wells once critical desorption pressure is achieved, the water level monitoring gets complicated due to violent degassing and foaming. The liquid column in casing/tubing gets aerated by the gas produced from the coal seams.
This paper presents a case study done in Raniganj coal block, West Bengal, India, which is one of the major CBM block in India. The total CBM acreage of the Raniganj CBM gas field is about 500 Sq Km.
The study was done to calculate foam free liquid level with the use of mathematical formulae and portable surface acoustic liquid level measuring instrument which is quite simple and cost effective method to calculate accurate liquid level in wells that are producing considerable amount of gas, without any downhole intervention and on wells where installation of permanent gauges is not economical.
The study was done in different wells where the calculated foam free liquid level was compared with results obtained from permanently installed pressure gauges and the results were found to be quite consistent. Hence, an accurate study of liquid level can be done in CBM wells which could be further used to optimize the production without compromising on accuracy of data with minimal cost, effort and resources.