Experimental investigation of competitive gas and water sorption in organic matter-rich marine, lacustrine and terrestrial sedimentary rocks of different thermal maturity

Unconventional gas resources such as CBM (Coalbed Methane) or gas shales are not only interesting scientific and engineering challenges but also attractive alternatives to conventional gas production in recurring periods of high oil and gas prices. The property to store significant amounts of gas makes coals as well as shales interesting targets in terms of domestic independence from oil and gas imports, as well as reducing world-wide greenhouse gas emissions by CO2 sequestration into deep not mineable coal seams. Within this broad framework, this thesis was initiated in order to answer specific questions concerning efficiency of gas storage capacity in different coals and shales at different moisture, temperature and pressure conditions and to evaluate the most important factors governing the gas sorption characteristics of samples with different depositional environments, mineralogy and maturity. This thesis is divided into three major parts, each dealing with a different major research question: 1. The first part deals with the investigation of competitive sorption of CH4, CO2 and H2O on natural coals of different thermal maturity. An improved version of a manometric sorption setup was used for mixed gas experiments conducted with CO2 and CH4 at 318K with pressures up to 10 MPa. Three coals (subbituminous-, high volatile bituminous coal and anthracite) were examined for their gas sorption properties (sorption capacity, selectivity) at different moisture conditions to evaluate their suitability for enhanced coalbed methane production. 2. The second part investigates the methane sorption capacities of commercially produced American gas shales from the Bossier and Haynesville Formation. The impact of water and temperature on these marine gas shales is examined by the means of methane sorption experiments at two different temperatures (318K, 348K) and at varying moisture conditions. In addition, the contribution of the organic and mineral constituents of the shales are evaluated, regarding their individual water as well as methane sorption capacity. Their relative contribution is calculated by using literature data and comparison to our experimental results. 3. The third part focuses on lacustrine shales, which until now were not in the focus of detailed examination. Three different samples from the West Lothian Oil-Shale Formation in the Midland Valley Basin, Scotland were analysed for their water and methane storage capacities to evaluate their potential for future shale gas production. Our results are used to critically discuss a recent reservoir estimation for these shales published by the British Geological Survey. In addition, comparisons to other European potential gas shales are drawn and the impact of kerogen type and maturity, as the most important factors for methane sorption, are discussed on the basis of available data. The main results of these three studies can be summarised as follows: Part 1: • Pre-adsorbed moisture significantly decreases the total gas sorption capacity of coals (≈50%). The sorption capacity for CO2 is about twice as high as that for CH4. • At all times CO2 is preferentially sorbed compared to CH4. The selective sorption of CO2 over CH4 decreases with an increased amount of pre-adsorbed water for lower rank coals but is essentially unaffected by moisture content in the case of anthracite. • Usage of Langmuir parameters from pure gas sorption isotherms in the extended Langmuir formula expression is suitable as a first approximation of the total sorption capacity under competitive sorption conditions. Part 2: • Pre-adsorbed water decreases the methane sorption capacity of shales significantly (up to ≈80%). • The slope of decreasing methane sorption with increasing amounts of pre-adsorbed water is a function of rock composition. In overmature marine rocks, clays almost exclusively control the moisture content of shales and influence the reduction of sorption capacity. • The residual methane sorption capacity after full water saturation can be interpreted purely as a function of organic matter content in overmature marine shales. • A temperature increase of 30K in experimental conditions is negligible, when compared to the negative impact of small amounts of water. Part 3: • Sorption capacity of these low mature, organic-rich, lacustrine shales and carbonates is controlled by TOC content and water sorption capacity that is controlled by clay content. • Water sorption of type I immature to early oil zone organic matter is mostly negligible in the presence of clay minerals and there is no significant restricting behaviour concerning access of methane and water to their respective sorption sites. • Methane sorption isotherms are negatively affected by pre-adsorbed water. Shales lose between ≈20 to 80% of initial dry sorption capacity upon full moisture equilibration (97% relative humidity) until the critical moisture point is reached. The non-linear decrease of methane sorption capacity is controlled by TOC and clays. • Lacustrine type I kerogen shows differences in methane sorption characteristics compared to type II marine kerogen. Kerogen type II has an overall higher methane sorption capacity compared to the investigated type I kerogen making lacustrine shales less favourable for shale gas exploitation