A laboratory pyrolysis study to investigate the effect of water pressure on hydrocarbon generation and maturation of coals in geological basins

Abstract

This study investigates the effect of water pressure on hydrocarbon generation and maturation of coals. Using a 25 ml Hastalloy pressure vessel, two high-volatile coals (Longannet, UK 0.75% Ro, and perhydrous Svalbard (Spitsbergen), Norway 0.68% Ro) were pyrolysed under non-hydrous, hydrous at 175 bar pressure, and high water pressure hydrous (500 bar and 900 bar) conditions at 350 °C for 24 h. The bitumen yield obtained during pyrolysis, together with the Rock-Eval S2, hydrogen index (HI) and vitrinite reflectance (VR) results from the pyrolysed coal residues indicated that water under relatively low pressure (175 bar) hydrous conditions promoted hydrocarbon generation and coal maturation in relation to non-hydrous conditions, consistent with previous work. However, under high water pressure (500 and 900 bar) conditions, a combination of the hydrocarbon gas (C1-C4) and bitumen yields, RockEval S2, HI, VR and solid state 13C NMR results demonstrated that the changes in reaction pathways occurring with increasing pressure resulted in both hydrocarbon generation and maturation being retarded. The observed effect of pressure implies that for Type III source rocks, hydrocarbon generation will be retarded in high pressure geological basins, with gas yields being proportionally reduced more than bitumen yields. Source rock maturation (or coalification) is also retarded, with the decreases in vitrinite reflectance and carbon aromaticity being relatively small but significant in terms of explaining retardation in geological basins.

High pressure water pyrolysis of coal to evaluate the role of pressure on hydrocarbon generation and source rock maturation at high maturities under geological conditions

“…The low pressure experiments at 380 °C (230 bar) and 420 °C (300 bar) were performed by adding 12 ml and 10 ml water respectively to the vessel, while the high water pressure experiments at both temperatures were conducted at 500, 700 and 900 bar. The experimental procedure used has been described in detail previously (Uguna et al, 2012a) and summarised below.…”

“…The sample studied is the same Svalbard perhydrous coal used by Uguna et al (2012a). The results obtained at 380 °C and 420 °C will be compared with the results obtained from the same coal at 350 °C (Uguna et al, 2012a) to assess the effects of pressure and temperature on gas generation, oil generation and cracking, and source rock maturation.…”

“…The experimental carbon mass balance obtained previously at 350 °C for 24 h (Uguna et al, 2012a) are presented in Table 4, while those obtained for pyrolysis at 380 °C and 420 °C are presented in Table 5. The extracted residual coals (carbon content determined using the residual TOC of the pyrolysed coals), hydrocarbon gases, oil and bitumen yields were used to determine these balances.…”

“…Hydrocarbon generation reactions occur under saturated, high water pressure conditions and according to chemical theory, reactions like hydrocarbon generation whose products occupy a greater volume than its reactant should be influenced by pressure. Previous pyrolysis studies on the role of pressure during hydrocarbon generation and source rock maturation have been reviewed in detail by Uguna et al (2012a), with the findings appearing to be in conflict, due to variety of pyrolysis methods used. Most high pressure pyrolysis studies performed under confined conditions using gold bags or gold capsules, in which the sample being pyrolysed is either not in contact or in contact with only a limited amount of water, produced little or no significant pressure effects (e.g.…”

“…Monthioux et al, 1985Monthioux et al, , 1986Freund et al, 1993;Michels et al, 1994;Huang, 1996;Knauss et al, 1997;Shuai et al, 2006;Tao et al, 2010). In contrast, under high pressure conditions in fixed volume vessels, in which the pyrolysed sample is in direct contact with water, pressure has been found to significantly retard both hydrocarbon generation and source rock maturation (Price and Wenger, 1992;Landais et al, 1994;Michels et al, 1995;Carr et al, 2009;Uguna et al, 2012aUguna et al, , b, 2013.…”