Coalbed Methane Reservoiring Process Under Condition Of Multi-Coalbeds Overlay

Coalbed methane(CBM) reservoiring process had particularity under multi-coalbeds condition, coal-bearing strata developed high frequency caycles, that influenced CBM reservoiring effect.Taking Bide-santang basin of Zhina coal field in western Guizhou province as the study object, CBM reservoiring character and geology mechanism under multi-coalbeds condition were emphatically discussed.The following major innovations were achieved.Firstly,based on trace element geochemistry annlysis of coal-bearing strata from single well, combined with other reprensentative boreholes data in study area,sequence stratigraphic framework of late Permian coal-bearing strata was amended.Four sequence boundaries were identified,and five third-order sequences were divided.Sequence structure was a binary combination including the TST and HST.Secondly,through synthetically using of relevant test methods, the changing law of porosity-permeability, adsorption character and diffusibility of coal along sedimentary sequence were firstly revealed and characterized.The results showed that: with coal bed horizon falling, pore structure of coal appeared that aperture decreased,pore volume fluctuated with a small range,while pore surface increased,that meaned that adsorption capacity of coal increasing and storage gas capacity changing little with coal bed horizon falling.Coal permeability presented three mutation semi-cycles with coal bed horzion changing. High permeability coal seam concentrated in the vicinity of the MFS and TST,which had higher consistency with third-order sequence. Langmuir volume and pressure of coal appeared heightening with coal bed horizon falling,but Langmuir pressure showed abnormal in some coal bed.Coal of sequenceâ…¢had the highest diffusion coefficient. Mudstone,siltstone,fine sandstone and limestone were good capping layer in suday area,with low diffusion coeffient which was far less than coal'. It was an important reason that underground fluid had poor relation between coal bed.Thirdly,coal reservoir property characterization based on modern nondestructive testing technique were perfected,and occurrence character of relavant parameters along sedimentary sequence was revealed.The study showed: with coal bed falling,the irreducible water saturation and T2c of NMR increased, while movable fluid porosity and gas permeability decreased,coal porosity-permeability generally become worse.Coates correction model of coal permeability was proposed.Using MIMICS software,the porosity CT formula of coal was established. The CT number deviation was introduced to quantify the heterogeneity of coal,and the heterogeneity of sequenceâ…¢was the strongest. Fourthly,for the first time,coupling analysis of stratigraphic sequence structure, gas-bearing property and properties, unattached multiple superposed CBM system and unified multiple CBM system were distinguished,and controlling-gas model of stratigraphic sequence structure was proposed. Sudies suggested that: the TST of third -order sequence was a relatively closed layer,and HST was a relatively open layer. Thus unattached CBM system corresponding with third-order sequence was developed, which leaded CBM content had mutation in the interface of sequence. Collocated relation in space of two type CBM system was analyzed. The paper thought that Shuigonghe, Santang and Zhuzang syncline developed more than 4 unattached multiple superposed CBM systems, while Agong and Bide syncline deverloped unified multiple CBM system,they co-existed and configured in space.In addition,based on the relation of coal heavy hydrocarbon abnormity,gushing water drilling and leaking water drilling of coal-bearing strata, the paper preliminary thougt that heavy hydrocarbon abnormity had the denotative role of CBM reservoiring effect,and the area of heavy hydrocarbon abnormity had the possibility of overpressure in coal reservoir.