LAUSR

Abstract The gas content test for coalbed methane (CBM) reservoirs consists of measurements of lost, desorbed and residual gas, and it is a crucial parameter for evaluation of such reservoirs and optimization of favorable zones. The desorbed and residual methane contents are non-simultaneously determined by on-site desorption test, which is both time-consuming and susceptible to a volume effect – leading to a large variation in the results. Conventionally, adsorbed methane content is indirectly estimated by a volumetric method, which has the shortcoming of being unable to directly measure in-situ adsorbed gas content. Thus, it is important to develop an effective and convenient method for quantitative characterization of multiphase methane in coals using a single technique. In this study, we performed a series of nuclear magnetic resonance (NMR) adsorption measurements on 15 sub-bituminous coals and anthracites. The transverse relaxation time (T2) distributions of methane are measured for three peaks: P1 (T2 = 0.1–8 ms), P2 (T2 = 10–300 ms), and P3(T2 = 300–2000 ms), corresponding to adsorbed methane in the coal matrix, free methane within larger pores, and lastly free methane in the space between the sample and the sample cell, respectively. In this study, the NMR calibration coefficient of free methane (Kfree-methane) is set to 3.44 × 10−1 cm3/p.u., regardless of the coal rank. In contrast, the NMR adsorbed methane calibration coefficient (Kadsorbed-methane) varies substantially in coals of different ranks, but show little difference in coals with the same coal rank. We suggest the Kadsorbed-methane of 3.2 × 10−2 cm3/p.u., 2.3 × 10−2 cm3/p.u. and 3.6 × 10−2 cm3/p.u., corresponding to low-, medium- and high-rank coals, respectively, for estimating the adsorbed methane content. Using these calibration coefficients, multiphase methane can be quantitatively estimated using the NMR measurement alone. The method is applicable both in the laboratory and in the field, for desorbed and residual gas quantification.