Study and application of drilling fluid system for strong plugging and collapse prevention in multi-branch wells of Zhangji Coal Mine

Abstract:

Zhangji coal mine multi-branch well is a drilling project of 1 straight 5 branch wells deployed to find out the geological （collapse column， thin coal belt and graben， etc.） and hydrogeological characteristics of No.3 abnormal area.The main target layers of T3 hole are 3_up and 3_down coal seams， which have low mechanical strength， developed fractures and cleats， poor homogeneity and high shear stress. The wellbore of coal seam section is extremely unstable， and it is easy to cause wellbore collapse， lost circulation， stuck or even buried wellbore during drilling. By analyzing the main technical difficulties of drilling fluid operation， the polymer drilling fluid system for strong plugging and anti-sloughing of multi-branch wells is developed. The main formula is： base slurry+0.5%KPAM+1%ammonium salt +1%potassium humate+1%filtrate loss agent+1% anti-sloughing agent+3%NS plugging agent， viscosity 35~50s， density 1.05~1.2g/cm³， water loss 4~6mL. One main hole and five branch holes were drilled， drilling quantity was 2506.23m， hole formation rate was 100%， core recovery rate was 92%， which ensured the requirements of construction.