Optimization and application of drilling fluid in the shallow extended-reach horizontal well in Dongfang 1-1 Gasfield

When the extended-reach horizontal well with vertical depth of 1 350 m is drilled in Dongfang 1-1 Gasfield, mud balls tend to occur in the surface layer (i.e., Ledong Formation and Yinggehai Formation). During the drilling of the horizontal section in the second Member of Yinggehai Formation, the torque and drag is high and the pressure is depleted, so the mud is lost very easily. Besides, the reservoir is shallow with poor diagenetic property, so the borehole tends to be instable. To deal with these drilling difficulties, a series of exploration and practice have been conducted for many years. The previous operation mode of drilling fluid in non-reservoir intervals is transformed and sea-water polymer system is exploratorily used to ensure the sufficient hydration of mudstone in non-reservoir intervals. Nano-scale deformable blocking agent PF-Greenseal is introduced into reservoirs to optimize the shielding-temporary blocking property of solid free drilling fluids, and combined with film blocking agent PF-LPF and efficient lubricant PF-Greenlube, the following drilling difficulties are solved successfully, such as mud ball, circulation loss, high torque and drag and borehole instability, and a set of drilling fluid operation mode for the shallow extended-reach horizontal well in Dongfang 1-1 Gasfield is established. This operation model is applied on site in 3 adjustment wells. It is shown that the rate of penetration (ROP) in non-reservoir intervals increases to 85 m/h from 41-64 m/h, and the cost of drilling fluid is reduced by 50%. Furthermore, the borehole in reservoirs is stable with hole enlargement ratio less than 3% and low torque. In addition, reservoir protection effect is good and the skin factor of the three wells is close to 0. The research results can be used as the technical reference of drilling fluid for the drilling operation in similar oil and gas fields.