Research On Well Pattern Optimization Of Low Rank Coal-Bed Methane Surface High Efficient Development In Dafosi Coal Field

Bin Chang mine is a typical low rank coal seam in Jurassic Period of the Ordos Basin,where geological reserves of coal resources and coal bed methane resources are rich,but it is a highly gas mining area,where gas transfinite and gas accidents happens occasionally.Because it is difficult to meet the current needs relying only on underground gas drainage,the ground gas extraction is more significant.In this paper,based on development geology,reservoir characteristics and resource conditions of low rank coal seam in Bin Chang mine,we carried out optimization and optimizing of low coal rank CBM ground development method and ground efficient extraction well pattern optimization in Dafosi coal field.The main research results of this paper are as follows:Numerical simulation software CBM-SIM was used for making history matching of well D3 in Dafosi coal field.By setting up a reasonable geological model,the sensitivities of well type parameters of different well type was analyzed.The results shows:two branches optimal branch angle of V-shape well is 60°.Multi-branches horizontal well works best when the branches are on different sides and asymmetrically,and the optimal branch angle is 60° between unilateral branch and main branch,while the best branch spacing is 250m to 300m;The target coal seams for CBM development of Dafosi coal field is coal 4top and 4,and ground vertical wells are used where single coal thickness is all thicker than 1 m.And then,taking five parameters into account,such as gas production,net present value,internal rate of return,invest recovery period and drilling risk coefficient using osculating value method,we optimized the multi-branches horizontal wells as the best horizontal well,used where coal 4 thickness is thicker than 2m and coal 4top thickness is thinner than 1m;Analysis of the well network optimization elements,combined with the geological characteristics of Dafosi coal field,rectangular well pattern was chosen.Based on geological analogy method,the economic critical well spacing,single well reasonable control reserves,fixed single-well productivity,economic limit reasonable well pattern density method and numerical simulation,we defined two kinds of primary pattern well spacing,those are the 400 m×350 m and 450 m×400 m;Pattern of well spacing of multi-branch horizontal was studied.The results shows:when main branch extension directions parallel to the main fracture direction,multi-branches horizontal wells productivity is higher;According to well spacing density of the vertical wells,two well spacing plans were defined.We carried out productivity prediction and economic evaluation,the plan 1st was chosen as the final well spacing plan in Dafosi coal field at last.