Tight sandstone gas accumulation mechanism and dev

Authors: ZhenXue Jiang Zhuo Li Feng Li XiongQi Pang Wei Yang LuoFu Liu FuJie Jiang

Publish Date: 2015/10/19

Volume: 12, Issue: 4, Pages: 587-605

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Abstract

Tight sandstone gas serves as an important unconventional hydrocarbon resource and outstanding results have been obtained through its discovery both in China and abroad given its great resource potential However heated debates and gaps still remain regarding classification standards of tight sandstone gas and critical controlling factors accumulation mechanisms and development modes of tight sandstone reservoirs are not determined Tight sandstone gas reservoirs in China are generally characterized by tight strata widespread distribution areas coal strata supplying gas complex gas–water relations and abnormally low gas reservoir pressure Water and gas reversal patterns have been detected via glass tube and quartz sand modeling and the presence of critical geological conditions without buoyancydriven mechanisms can thus be assumed According to the timing of gas charging and reservoir tightening phases the following three tight sandstone gas reservoir types have been identified a “accumulation–densification” AD or the conventional tight type b “densification–accumulation” DA or the deep tight type and c the composite tight type For the AD type gas charging occurs prior to reservoir densification accumulating in higher positions under buoyancycontrolled mechanisms with critical controlling factors such as source kitchens S regional overlaying cap rocks C gas reservoirs D and low fluid potential areas P For the DA type reservoir densification prior to the gas charging period GCP leads to accumulation in depressions and slopes largely due to hydrocarbon expansive forces without buoyancy and critical controlling factors are effective source rocks S widely distributed reservoirs D stable tectonic settings W and universal densification of reservoirs L The composite type includes features of the AD type and DA type and before and after reservoir densification period RDP gas charging and accumulation is controlled by early buoyancy and later molecular expansive force respectively It is widely distributed in anticlinal zones deep sag areas and slopes and is controlled by source kitchens S reservoirs D cap rocks C stable tectonic settings W low fluid potential areas P and universal reservoir densification L Tight gas resources with great resource potential are widely distributed worldwide and tight gas in China that presents advantageous reservoirforming conditions is primarily found in the Ordos Sichuan Tarim Junggar and TurpanHami basins of centralwestern China Tight gas has served as the primary impetus for global unconventional natural gas exploration and production under existing technical conditionsThe field of tight sandstone gas exploration has witnessed global breakthroughs since the resource was first discovered in the San Juan Basin of the USA in 1927 Tight sandstone gas belonging to unconventional gas reservoirs is mainly found in North America the Asia–Pacific region Europe and the Middle East with total proven reserves of around 210 × 1012 m3 IEA 2009 Recently it has served as the main source of global natural gas reserve and production growth Zou et al 2011a Dai et al 2012 Pang et al 2013Given the considerable resource potential of tight sandstone gas a series of studies have focused on development conditions accumulation mechanisms and type classifications Accumulation mechanisms involving relative permeability sealing diagenesis sealing force balance sealing and lateral fault sealing have been proposed Masters 1979 Gies 1984 Jiang et al 2000 2006 Pang et al 2003 Jin et al 2003 However accumulation controlling factors modes and mechanisms of tight sandstone gas are poorly understood Based on a review of previous studies of tight sandstone gas and through a detailed case study of typical tight sandstone gas reservoirs found in China this paper discusses accumulation mechanisms type classifications accumulation controlling factors development modes and resource potential of tight sandstone gas Such efforts will play a significant role in enriching natural gas geological theories and in advancing the exploration and development of tight sandstone gasTight sandstone gas is natural gas contained in tight sandstone reservoirs with porosity of 10  and in situ permeability of 01 × 10−3 μm2 which belongs to unconventional gas reservoirs Tight sandstone gas is mainly found in densified reservoirs with micronano pores and throats having generally limited or no natural productivity that is typically less than the lower bound of industrial gas flows The industrial gas production can be obtained only under specific economic and technical conditions hydraulic fracturing reform measures or horizontal and multilateral wells Zou et al 2011a The tight sandstone gas has become an important field of natural gas exploration and development in recent years owing to its great resource potential Dai et al 2012 Zou et al 2013

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