Anti-Sloughing Drilling Fluid Technology for Deep Sidetracking Wells in the Tahe Oilfield

The deep Bachu and Sangtamu Formation in the Tahe Oilfield are composed of an illite-montmorillonite mixed layer or hard brittle shale formations dominated by illite with strong water dispersion and development of micro-fractures.During drilling operations, wellbore instability can be encountered due to shale hydration.To eliminate these problems, a coordinated anti-collapse program with three elements was developed:sealing and consolidating the side well, inhibiting surface hydration and providing the effective stress support by reasonable drilling fluid density.In addition, anti-collapsing drilling fluids were developed.The evaluation results showed that the newly developed three-element anti-collapse drilling fluid had a temperature tolerance of 170℃, preferable contamination resistibility (5%NaCl, 0.5%-1% CaCl2, and 8% poor clay) strong anti-collapsing capacity (5.05% of shale swelling ratio and 91.33% rolling recovery rate) and favorable well-sealing performances (4 MPa of pressure-bearing capacity for 400 μm simulated fractures).Successful field applications were carried out in the Tahe Oilfield more than 20 wells.There were no downhole problems induced by wellbore instability.The average expansion rate was reduced by 63.4% compared with completed adjacent wells in the same block.The construction period was shorted by 4.3 d.Results showed that the wellbore instability problems of deep sidetracking wells in the Tahe Oilfield can be overcome through the effective deployment of a three-element anti-collapse drilling fluid technology.