Inhibition of Mud Balls Formed in Drilling the Shallow Soft Mudstone Formation in an Dongfand Gas Field

The shallow gas reservoirs in the 2nd Segment of the Yinggehai formation is developed using the Z platform in offshore western South China Sea. The non-reservoir formations such as the Ledong formation, the 1st and the 2nd segments of the Yinggehai formation above the reservoir are soft and have high argillaceous contents. In the early development stage of the reservoir a strongly encapsulating PLUS/KCl or PEM polymer mud was used to inhibit the long section of mudstone formations encountered, but this drilling fluid behaved poorly in cleaning the hole and many mud balls were formed during drilling, resulting in high torque, high pump pressure, difficulties in tripping operation and reduced operational time efficiency. By analyzing the cause of mud ball formation, inhibitive drilling fluids were abandoned and a whole dispersed drilling fluid was adopted to drill the non-reservoir formations. During drilling with the whole dispersed drilling fluids, special high viscosity pills were used to clean the hole and the mud was converted to a drilling fluid with excellent lubricity. With these measures, mud ball formation and hole cleaning problems in drilling the top section were successfully resolved Five wells drilled on the Z platform had obviously elevated rate of penetration (ROP); the average ROP of the φ311.2 mm was 141.32 m/h, 62.69% higher than the fastest well drilled previously, creating a record in extended reach horizontal drilling of shallow gas reservoir in the eastern area of west South China Sea.