Research On In-situ Stress Logging Evaluation And Engineering Application In Coalbed Methane Wells

In-situ stress is not only closely related to the adsorption/desorption,migration,and accumulation of coalbed methane,but also an important factor affecting the stability of well wall and fractures propagation in coalbed methane wells.As the coalbed methane development in China shifts to deep and soft coal seam areas,it has become an urgent need for safe and efficient extraction of coalbed methane to determine the state of in-situ stress in advance.Previously,in-situ stress test of coalbed methane wells mainly used hydraulic fracturing to obtain the minimum horizontal principal stress at a certain depth in the formation.The range of test data is limited and dispersed.In-situ stress logging interpretation can obtain continuous vertical principal stress and horizontal principal stress information,and realize advanced interpretation of in-situ stress profile.This paper is based on coal geology,logging,coal petrology and elastic mechanics to analyze physical and numerical theoretical,an in-situ stress logging interpretation model is established to analyze the effects of vertical principal stress,maximum and minimum horizontal principal stress on the fracture pressure,collapse pressure,and hydraulic fractures propagation in coalbed methane horizontal wells.The optimization technology of coalbed methane horizontal wells based on modern in-situ stress information is formed.The results of the paper are as follows:(1)The rock deformation coefficient is introduced and the coal-measure effective stress coefficient logging prediction model is constructed according to the coal structure.A coal-bearing strata horizontal principal stress logging calculation model is established by using effective stress coefficient of coal measures to improve the combined spring in-situ stress model.The interpretation accuracy reaches 88%.(2)Based on the improved combined spring model and coal-bearing formation density integration method,a set of coalbed methane well in-situ stress logging interpretation automatic processing software has been developed.(3)Based on the formation vertical principal stress,maximum and minimum horizontal principal stress,reservoir pressure,rock tensile strength,rock internal friction angle,rock cohesion,effective stress coefficient of coal measures,horizontal wellbore azimuth,and well circumference angle,the models for predicting the collapse pressure and breakdown pressure of the coalbed methane horizontal well are established.(4)Taking the safety density window of drilling fluid,the coefficient of coal body structure firmness,and the characteristics of formation clay minerals as evaluation indicators,a comprehensive evaluation method for the stability of the lining of horizontal wells along coal seams and horizontal wells with coal roofs is constructed.(5)It is revealed that when the horizontal principal stress difference is large,the main fracturing fractures along the direction of the maximum horizontal principal stress are formed;when the horizontal principal stress difference is close,the hydraulic fractures of the hard coal seams form network fractures.In the initial stage of hydraulic fracturing,the soft coal seam does not produce cracks,but is squeezed around the wellbore.After squeezing to a certain degree,the coal seam begins to crack and propagation;when the horizontal principal stress difference is close,the soft coal is produced Reticulated joints;when the horizontal principal stress difference is large,the compression fracture produces two dominant cracks along the direction of the maximum horizontal principal stress of the soft coal seam and the direction at an angle of 45° with the direction of the maximum principal stress.(6)Based on the coalbed methane enrichment area of the field,a safe and efficient coalbed metane development technology is formed by using in-situ stress profile of coalbed methane wells to optimize drilling horizon in horizontal wells,design well trajectory and drilling fluid system,and optimize wellbore position in horizontal section through "shale content curve" and "relative position curve of well trajectory and coal seams".In addition,the large discharge volume of "tubing + annulus" hydraulic jet along the hard coal seam horizontal well is staged fracturing,the "large scale and discharge volume,medium sand ratio,slugged sanding" of horizontal well roof in hard coal seams is hydraulic fractured,and the "high pre-liquid ratio,large scale and discharge,medium sand ratio,slugged sanding" of horizontal well roof in soft coal seams is hydraulic fractured.