An Analysis of the Effect of CO2 Injection on the Recovery of In-Situ Methane From Bituminous Coal: An Experimental Simulation

A set of experiments is described in which CO2 is injected into large cores of CH4- and water-saturated bituminous coal at elevated pressures. CO2 at Pressures up to 800 psig [5516 KPa] is used to simulate the enhanced recovery of in-situ CH4 from coal beds. CO2 injection increases the recovery of CH4 by a factor of two to three times that achieved in simple desorption by pressure drawdown and atmospheric diffusion. In pressure drawdown and atmospheric diffusion. In general, higher CO2 pressures achieve greater CH4 recovery. The presence of even small amounts of N2 in the injection gas greatly reduces the CH4 recovered. CO2 at 500 to 800 psig [3447 to 5516 kPa] is shown to be capable of completely demethanating integral coal samples. This was confirmed by tests run on crushed cores. CO2 consumption by permanent adsorption is quite high vis-a-vis the CH4 recovered and may preclude its use as an enhanced-recovery energy process. Its primary function would appear to be as a means of safely primary function would appear to be as a means of safely demethanating coal beds before mining. Introduction This paper represents an extension of the work by Fulton, et al. to higher CO2 pressures. A rather complete literature review is presented in Ref. 1 and is not repeated here. This paper describes a series of laboratory tests run on Pittsburgh seam bituminous coal from West Virginia. Pittsburgh seam bituminous coal from West Virginia. Large coal cores were injected with CH4 to various equilibrium pressures and saturated with water. The CH4 then was vested and allowed to desorb at atmospheric pressure. This procedure is called "natural production." CO2 was injected until a predetermined production." CO2 was injected until a predetermined equilibrium pressure was reached. The pressure then was released either rapidly or slowly until atmospheric production was negligible. The gas quality and quantity production was negligible. The gas quality and quantity were analyzed and the CO2 adsorbed determined by material balance. Variations on this basic procedure included (1) the exclusion of the natural production cycle, (2) the speed of CO2 pressure drawdown, (3) the number of CO2 cycles that constitute the simulated recovery process, (4) the use of N2/CO2 mixtures as the injection gas, (5) variations in injection pressures from 200 to 800 psig [1379 to 5516 kPa], (6) subsequent exposure of crushed samples to CO2, and (7) the determination of the total CH4 in place (MIP) by successive injections of CO2 at 800 psig place (MIP) by successive injections of CO2 at 800 psig [5516 kPa] after the process cycles and regardless of the CO2 pressure employed in the latter. Experimental Procedure The experimental procedures and equipment descriptions are essentially the same as those described in Ref. 1. Briefly, the same size coal samples were used (3 1/2-in. [8.9-cm] diameter) and the pressure vessels were replaced with high-pressure stainless steel cylinders with O-ring seals. A new gas chromatograph was used and the collector system remained essentially unchanged. The coal was stored under water with a bactericide added, until cored. The cores were dried at 158 deg. F [70 deg. C] under vacuum for 30 to 70 days. The cores were subjected to CH4 adsorption until an equilibrium pressure was established at 200 psig [1379 kPa] (800 pressure was established at 200 psig [1379 kPa] (800 psig [5516 kPa] in the case of Sample 22). The cores psig [5516 kPa] in the case of Sample 22). The cores were permitted to imbibe water treated with a bactericide for several days, after which the immersed cores were subjected to a CH4 pressure equal to the adsorption pressure to achieve maximum water saturation. pressure to achieve maximum water saturation. The excess water was drained from the vessels and the porosity computed from the volume of water remaining porosity computed from the volume of water remaining and the assumption of 100% saturation of the coal fractures and matrix pores by the water. Following this the coal was allowed to desorb CH4 at atmospheric pressure until the produced CH4 was negligible. This lasted from 5 to 15 days and was proportional to the adsorption pressure. The natural production cycle was not included pressure. The natural production cycle was not included in Runs 14 to 16. After desorption, CO2 (CO2/N2 in the case of Run 20) was injected until some specified equilibrium pressure was established. These pressures, which are pressure was established. These pressures, which are listed in Table 1, ranged from 200 to 800 psig [1379 to 5516 kPa]. SPEJ P. 521