| Kuznetsk Basin is the most coal bearing basin in Russia,and has underwent coal seam gas surface producing operation in the Naryksko-Ostashinksaya coal field.Surface test wells have been exploited in the in-situ area,and are also promised to be used in the gob.It is of particular importance to design the reasonable producing patterns in the in-situ area and gob,to optimize the producing connection between the two areas,and,additionally,to examine the producing availability.Therefore,this paper firstly analyzes the real producing data of the currently existing wells and then examined the reservoir properties including permeability,adsorption property,gas content and reservoir pressure using lab experiments.Based on the analyses,numerical simulation was used to optimize the producing patterns by building different geological models for the in-situ area and gob.Finally,the coalbed methane flowing mechanism is discussed and the results could be able to provide the available sequence producing patterns between the two areas,theoretically and technically.The main findings in this paper are as follows:1.The studied area develops eight coal seams,namely Nos.73-72,78-77,79,80,80a,81,82 and 86-84.The investigated coals have relatively thick thickness in the middle part of the studied area,but thin out gradually towards eastern,western and southern parts.In addition,coal seams are exposed in the northern part of the study area.The burial depth of coals is mainly controlled by Kyrgay-Ostashinskaya syncline and decreases gradually from shaft portion to both flanks of syncline.2.Lithotype of the investigated coals belongs to bright coal and maceral components are dominated by vitrinite,followed by inertinite,with trace amounts of exinite.The investigated coals have super-low moisture contents,but have super-low to low ash yields in the Nos.72-73 and 77-78 coal seams and intermediate ash yields in the Nos.80acoal seam.However,the Nos.79,80,81,82 and 84-86 coal seams have varying ash yields,mainly from from super-low,low to intermediate ash yields.The No.79coal seam is classified as intermediate-high volatile coal while the Nos.73-72,78-77,80,81 and 82 coal seams reach intermediate-high to high volatile level and Nos.80a and 86-84 high volatile level.The investigated coals have relatively low metamorphic level and belong to high-volatile bituminous.The investigated coals have 1.40 g/cm~3apparent relative density.During testing of gas contents in the investigated coals,there is a problem resulting in inaccurate testing data,and thus testing data are corrected.The gas content ranges from 9.97-21.29,6.72-21.38,10.91-18.24,7.55-16.19,8.78-14.58,9.00-19.05,9.47-19.10 and 8.75-18.79 m~3/t in the Nos.73-72,78-77,79,80,80a,81,82 and 86-84 coal seams,respectively.The V_L value of the Nos.78-77,80,82 and86-84 coal seams is 26.12,26.90,32.00 and 30.81 cm~3/g,respectively,and P_L is 1.17,1.28,1.24 and 1.39 MPa.Based on the height of dynamic liquid level on the first day of drainage,reservoir pressure gradient is almost equal to normal pressure gradient(9.8KPa/m).The initial reservoir pressure gradient of coals from low to upper ranges from2.65-10.67,2.00-10.00,4.19-7.48,1.68-7.47,5.16,3.89-7.16,1.50-9.04 and 3.39-8.66MPa.The investigated coals are under-saturated and gas saturation is 62.97%,73.33%,65.12%and 66.96%in the Nos.78-77,80,82 and 86-84 coal seams.BET specific surface area is 0.85,0.61,0.57,0.89 and 1.00 m~2/g in the Nos.78-77,80,81,82 and86-84 coal seams,respectively,and BJH volume is 0.0050,0.0038,0.0028,0.0043 and0.0039 cm~3/g as well as D-R specific surface area is 111.26,145.73,140.78,123.39and 112.63 m~2/g and D-A volume is 0.0488,0.0465,0.0551,0.0556 and 0.0521 cm~3/g.The cleats are well developed and in some cases,are filled by minerals.The cleats are interconnection but density is uneven,which is mainly controlled by lithotype.The permeability is 15.87,15.1,12.5,4.8,6.7,12.5 and 10.8 mD in the Nos.73-72,78-77,79,80,81,82 and 86-84 coal seams,respectively.3.Combined with coal thickness,burial depth,reservoir pressure,gas content,permeability and so on,this thesis provides simulation program regarding multiple seam fracturing completion of vertical well,barefoot completion,reaming completion along seams in in-situ area.The four exploring wells in in-situ area are simulated and associated results show that multiple seam fracturing completion can result in higher gas contents and recovery ratio than barefoot reaming completion,but multiple seam reaming completion along seams has better gas production.In addition,this thesis also provides simulation program regarding different well spacing,fractured fracture half length,permeability and well network of vertical fractured well in in-site area.This result shows that the well spacing with 250×250m has the highest gas production and recovery ratio and is the optimal well spacing;the half length of 50m fracture corresponds to higher peak of gas production and longer production time,as well as has relatively high cumulative gas production.When other reservoir conditions are consistent,the relatively high permeability corresponds to high gas production and recovery ratio.Based on study of simulation program of the rectangle,trapezoidal,five point type and quincunx well network,this paper shows that the spatial distribution of quincunx well network can guarantee the depression cones covering the whole drainage area,no pumping“blind area”and high gas production and recovery ratio as well as good drainage effect.Thus,quincunx well network is the best one.4.Combined with coal thickness,burial depth,tectonic condition,gas content and permeability,this thesis determines simulated region of gob.The No.86 coal seam,as target coal seam,has the burial depth of 600-800 meters by determination of absolute open flow.In addition,this paper compiles simulation program for horizontal well,horizontal well group and“V”with 60 degree angle in gob.The simulated results show that the three types of completion are applicable in this region.Based on the simulation of different permeability,the greater the permeability is,the quicker the gas concentration of gob can reduce,when other reservoir conditions are consistent.On the basis of the reservoir conditions,the No.82 coal seam is considered asbeing protection layer to increase the permeability by using pressure unloading.5.This paper systematically summarizes the coalbed methane exploitation mode,vertical well used in the in-situ area,completion typesincluding multiple seam fracturing completion and barefoot reaming completion,as well asquincunx well network.The“V”well and horizontal well are applicable in gob.Additionally,this thesis summarizes the mechanism of coalbed gas accumulation,migration and output in in situ and gob areas.Based on the simulation analysis ofinterwell interference,this paper discusses the difference of gas production from different well networks,and constructs the vertical permeability change pattern of multipleseam and crack barefootcompletion,and systematically analyzes changing mechanism of vertical creaks and analyzes strata parameters of multiple seam drainage.In addition,this paper simulates thevertical displacement and strain,and horizontaldisplacement as well as strainshear failure and tensile failure ofprotection layerwhen pressure is uploaded from maximum and minimum principal stress,which indicates that the vertical displacement and strain of protection layer are more obvious in directionperpendicular to maximum principal stress than thatparallel tomaximum principal stress. |
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