Technology of compound salt weak gel drilling fluid for long horizontal wells in deep shale oil reservoirs

When drilling long horizontal wells in the deep reservoir of Block Jixin 2 in Tuha Oilfield, several challenges were identified, including the instability of shale wellbore, difficulty in hole cleaning as well as challenges in reducing friction and resistance. This prompted the need for advanced drilling fluid technology with high performance in inhibiting properties, sealing and collapse prevention, cutting carrying capacity and lubrication. Therefore, a study on drilling fluid technology was conducted. By compounding NaCl, KCl, sodium formate and ZNP-1, the drilling fluid system exhibits strong inhibition, effectively addressing the wellbore instability issue. The system employed "asphalt powder + acrylic resin plugging agent + wall cementing agent" to achieve full plugging of pores and fissures of micron, sub-micron and nano size. Additionally, a combination of low-fluorescence lubricant and encapsulated friction reducer was used to enhance the lubrication capability of the drilling fluid. A weak gel shear thinning agent was incorporated to improve the carrying capacity of the drilling fluid. The formulation of the compound salt weak gel drilling fluid system is finalized as follows: water+4% bentonite+0.2% caustic soda+15%NaCl+5%KCl+15%NaCOOH+1%ZNP-1+1%DRGJ-1+0.5%LV-CMC+4% asphalt powder+2% resin plugging agent+2% solid wall agent+0.3% weak gel+2% low-fluorescence lubricant+3% encapsulated friction reducer+barite. The performance of this system was evaluated, and the results showed a linear expansion rate of 7.21% of mud shale after 16 hours, a dynamic shear force of not less than 11 Pa, a filtrate viscosity coefficient not exceeding 0.0612, a drilling fluid activity of not more than 0.76, a high-temperature and high-pressure filtrate loss less than 10 mL, and resistance against 10% drill cuttings and 5% CaSO₄ contamination. The technology was applied in 5 wells in Block Jixin 2, and when compared to neighboring wells such as Jixin 2 and Jixin 2-2, the drilling cycle was reduced by 33.05%, the average wellbore diameter enlargement rate decreased by 68.49%, and the average complexity rate decreased by 68.19%. This technology provides a technical support for safe and efficient drilling of similar deep long horizontal wells in oilfields.