Experimental and Analytical Studies on Permeability Reduction of a Coal Seam by CO2 Injection
Authors: Anggara Ferian , Sasaki Kyuro , Sugai Yuichi 
Published: March 2015

Permeability of coal seam have a strong effect on CO2 Enhanced Coal Bed Methane Recovery(CO2-ECBMR) project. In particular, qualification of permeability reduction due to coal matrix swelling is a key parameter to control CO2 injection and the project sustainability. Swelling experiments using visualization method were conducted. Crushed samples from Indonesia Low Rank Coal (LRC) were used and the experiments were carried out up to 10 MPa at 48° C temperature, resulting CO2 in the supercritical conditions that more appropriate for CO2 injection in the field. Coal swelling which was represent by upward surface movement of column was assumed as total expanded volume for each particle. With this assumption, crushed samples were modeled like block coal samples. For three coal samples, the maximum expanded volume due to the swelling by CO2 adsorption has been evaluated as 0.03 at 10 MPa pressure. Since the swelling results were very comparable to others, this method has more advantage in term of sample preparation and experiment work compare to block coal.

Based on the present swelling data, two analytical models proposed by Palmer and Mansoori (P-M) and Shi and Durucan (S-D) have been applied to Yubari field test. Permeability reduction (k/k0) ratio by CO2 swelling in the test has been estimated as 0.021 to 0.056 for the coal seam condition of 10 and 15 MPa of initial formation and injection pressures, respectively. Sasaki et al.15 proposed analytical radial flow model to evaluate CO2 swelling ratio (ß). The swelling ratio on permeability has been evaluated as ß = 0.020 to 0.044 and original coal permeability k0 ˜ 3 md by matching with monitoring data measured in the field. Both values of k/k0 and ß have shown extremely good agreements. Furthermore, initial permeability of the coal seam was also proved by fall-off data. Thus, it is concluded that the CO2 injection rate can be expected by the models showing relationship between porosity, permeability and swelling of coal.

Introduction

CO2 capture and storage (CCS) is one of the expected methods to reduce its emissions into the atmosphere. In fact, geological sequestration is most likely to be the only option that will allow storing CO2 in large enough quantities over long geologic period of time. Geological formations mainly used for CO2 storage are depleted oil and gas reservoirs, unmineable coal seams and particularly, saline formations (deep underground porous reservoir rocks saturated with brackish water or brine). CO2 injection onto coal seams is a very attractive option for geological storage, since CO2 is readily adsorbed by the coal. At the same time, the injected CO2 displaces adsorbed methane from the coal's internal surface. The coal seams can adsorb CO2 gas volume which is almost double of methane and even more.1 However, coal matrix swelling by adsorbing CO2 and consequently reduces its permeability is the main problem for this option.

Coal reservoir is naturally fractured reservoir. Being normal to the bedding plane and orthogonal to each other, the face and butt cleats in coal seams are usually sub-vertically orientated. Assuming coalbeds being exploited for methane are nearly horizontal with mild, if any structures, thus in-situ cleats of target coal are nearly vertical model.2 For the purpose of gas and water flow, coal bed can be conceptualized as a collection of matchstick as shown in Figure 1.