Characteristics Of In-situ Stress Of Coal Reservoir And Its Effect On Productivity Of Coalbed Methane Wells In Liulin Block

As a clean and efficient unconventional natural gas,coalbed methane(CBM)has become one of the important energy sources to make up for the lack of conventional natural gas.Due to the characteristics of low porosity and low permeability of coal reservoir,the development of coalbed methane usually requires fracturing,and the key factor affecting the fracturing effect of coal reservoir is in-situ stress.Coalbed methane(CBM)resources are very rich in Liulin block in the eastern margin of Ordos Basin,but the productivity of CBM Wells in this area is seriously restricted due to the lack of understanding of the distribution law of in-seam stress field.Therefore,this paper studies the distribution characteristics of in-situ stress field of the main coal seam in Liulin block and the influence of permeability under stress field on productivity by using logging,well test,experimental test and dynamic production data,and obtains the following main results:(1)Five logging parameters with good correlation degree with the minimum horizontal principal stress were selected by using the gray correlation model,and then the in-situ stress prediction model based on support vector machine regression was established.Based on the calculation of this model,we further reveals the three principal stress in the plane distribution characteristics,summed up as "the southern and western stress value is high,the north and east low stress value",and discusses the stress field in vertical zoning characteristics,points out that with the increase of buried depth of coal seam in the study area,the stress field is changed from dynamic field type to static field type.Correspondingly,the lateral pressure coefficient decreases from high to low.(2)It is illustrated that the permeability of the main coal seam in the study area presents a distribution characteristic of "high in the north and low in the south" on the plane,and presents a change rule of "reduce-increase-decrease" with the increase of the buried depth in the vertical direction,which shows a change trend opposite to the in-situ stress on the whole.The correlation between permeability and in-situ stress and lateral pressure coefficient is further analyzed,it is pointed out that permeability decreases with the increase of horizontal principal stress,and there is a good negative exponential relationship between permeability and lateral pressure coefficient.Combined with the structural characteristics of deep coal seam,the control mode of in-situ stress field on permeability in the study area is established.(3)It is found that the coal bed methane production in the northern area is better than that in the southern area on the whole,and the coal bed methane production in Shanxi Formation is higher than that in Taiyuan Formation.Based on the analysis of the correlation between average gas production and three principal stresses,reservoir pressure and permeability,it is pointed out that when permeability is small,the average gas production increases faster with the increase of permeability.When permeability increases to a certain extent,the increase rate of average gas production decreases and tends to be stable gradually.Combined with the relationship between insitu stress,permeability and productivity of coalbed methane wells,three change models of permeability under the influence of in-situ stress on productivity are constructed.By carrying out the above research work,the geological conditions of coal seam and the development performance of coalbed methane in Liulin block are further clarified,which provides a certain theoretical basis for determining the high permeability zone of coal reservoir in the study area,improving the single well production of coalbed methane and rolling development of coalbed methane.