Study On The Mechanical Parameters Of Coal-bed Methane And The Stability Of Wellbore In WW Basin

WW Basin has proven 94.4 million square CBM resources.The CBM resources is buried in the shallow of 1500m.It has great potential for exploitation.However,the exploration and development of coalbed methane resources in WW basin is in the initial stage.the engineering geological knowledge is low.It has not carried out coal macro-micro mechanical properties test and research.It is short of understanding of the physical and mechanical properties of coal seam and lack of researching of collapse mechanism of coal seam.The safety of coal seam drilling can not be guaranteed.In order to solving the technical difficulties faced by coalbed methane mining in WW basin,this paper firstly studies the mineral composition,physical and chemical properties and microstructure of coal rock,and analyzing the main causes of coal wall collapse.Secondly,according to the experiment of coal and rock,combined with field logging data,we can get the mechanical parameters of coal rock,coal seam porosity and permeability parameters as well as the stress parameters of coal seam.Thirdly,based on the study of coal seam stress,combined with the failure characteristics of coal weak plane,the calculation model of coal seam collapse pressure in WW basin is established.We can use computer to program to research many influencing factors on the collapse pressure of coal seam in WW basin.It considers influence of the attitude of weak plane,strength of weak plane,well trajectory,stress heterogeneity and the seepage of drilling fluid.Finally,the accuracy of the calculation model of the collapse pressure is verified by the example of a accident well.Based on the analysis of the influencing factors of the coal seam collapse pressure and the results of the field examples,we can put forward a series of measures to prevent the collapse of coal seamd and evaluate the effect of those measures.These research show that:(1)The main coal seam in the Taiyuan Formation of the Carboniferous in WW Basin is large in thickness,rich in gas content,shallow in burial depth,so it has the potential to develop coalbed methane on a large scale.(2)A large number of weak plane is developing in coal rock,which is main reason of the collapse of coal seam.(3)Through the analysis of influencing factors of the collapse pressure of coal seam,it is found that compared with the straight well,the horizontal well collapse pressure face relatively large impac.The collapse pressure of low well angle is generally higher and the collapse pressure of horizontal well is the lowest.When the horizontal section is drilled along the horizontal maximum stress direction,the collapse pressure is the lowest,which is favorable to the wellbore stability.Stress heterogeneity is stronger,the higher the collapse pressure.(4)Based on the calculation model of the collapse pressure,we can calculate an example of an accident site,and the calculation results are consistent with the actual situation in the field,which verify the accuracy and adaptability of the collapsing pressure calculation model.(5)According to the analysis of influence factors of collapse pressure,combined with the calculation results field examples,a series of measures are put forward.It includes optimizing the drilling orientation,controlling the reasonable drilling fluid density,enhancing the drilling fluid of wall building properties,inhibition properties and rheological properties,strictly controling the drilling speed,reducing drilling on wall collision.These measures ensure the borehole stability of coal seam.(6)Based on the application effect of seam collapsing prevention measures,it is indicated that the seam collapse prevention measures proposed in this paper have high applicability in the Carboniferous Taiyuan formation of WW basin,and these measures deserve promotion.Through the comprehensive analysis of the field technical data,the experimental study of the macroscopic and microscopic mechanical parameters of coal rock,the model research and numerical calculation of coal seam property and mechanical parameters and combined with the blocks in the scene actual situation,this paper obtains a set of the basic parameters of coal rock mechanics and forms analysis methods wall stability of coal bed methane drilling borehole and the countermeasures of sidewall instability which it suitable for the block.It is of great practical significance to study the efficient development of coal bed methane.