Title: Coal Bed Methane Content Determined with Mudlogging Methods
Speaker: William Sterling Donovan, Donovan Brothers Inc.,
Date: November 16, 2001
Publication: The Outcrop, November 2001, p. 5, 8
As the pace of Coal Bed Methane (CBM) exploration has increased, the need for an effective and economic method of formation evaluation has become apparent. Currently four methods are used as CBM formation evaluation tools. Two of these evaluation techniques rely on coring while a third involves pilot production tests. A discussion of the relative merits and the reasons for conflicting gas content results from the four evaluation techniques will be presented. Mudlogging as a method of CBM formation evaluation is reliable, quick and inexpensive. The mudlogging gas content method presented is currently being used to effectively evaluate CBM potential. Other mudlogging techniques from the use of raw data to the other simple normalization techniques are shown to be inadequate.
Data comparing initial gas and water production data to mudlogging gas content values is presented. Not all coals contain methane and large lateral variations in gas content exist in individual coal seams. 75% of the coal bed methane production in the San Juan Basin comes from 35% of the wells on 5% of the acreage. Thus, even in areas with relatively well known CBM potential, location-specific data is essential.
The desorption isotherm is used to determine the rapidity of desorption from cuttings. The size of the cuttings affects the desorption rate and can be determined by the drilling parameters. Bit type, rotary speed and weight on bit can be controlled to produce optimal cutting size and results. How to calculate gas content for both air and mud-drilled holes is discussed. The influence of both adsorbed gas and “free gas” on gas content measurement will be discussed. Simple methods to determine the relative contribution of adsorbed gas or free gas are presented.
Methods to optimize the data gathered either from air drilling or mud drilling will be explored. Choosing to mud drill or air drill depends on whether the presence of gas or permeability is the key exploration parameter. Some examples of the gas readings in units for both air and mud systems will be presented to indicate the difficulty of using raw mudlogging gas data. Raw mudlogging gas readings are affected by the drilling fluid, hole size, drilling rate, compressor or mud pump rate, flow line gas loss, gas trap efficiency and gas sensor non linearity. The importance of using carbide lags to normalize these factors and other vagaries in the measurement system will be stressed.
Common operational difficulties and their solutions will be discussed. Problems include under-sized pumps and compressors, lost circulation, erratic mud pumps, balled bits, high mud viscosity and high mud gas. Generally, good drilling practices with properly sized pumps will alleviate any problems. Drilling with water or very lightly treated mud and circulating while the mud is thinned can also overcome the problems.