It brings severe challenges to drilling fluids when drilling high temperature deep well in environmentally sensitive sea area and facing strict environmental protection laws and regulation requirements meanwhile, the toxic value of drilling fluid must reach up the standard requirement of 30,000 mg/L.A new, environmentally safe water-based polymer system has been developed for drilling applications with temperatures resistance to 200°C and biological toxicity value LC50 more than 100,000 mg/L.
The new system consists of two basic polymeric components for high temperature rheology and filtration control, along with a special nano-plugging agent, glycol shale inhibitor, extreme pressure lubricant, and barite or formate weight material, providing superior performance for a variety of drilling environments. The system shows the base slurry is light colored and non-toxic to the marine environment, which can be discharged directly into the sea. The environmental friendly feature is a significant superiority over traditional high-temperature systems, such as sulfonated drilling fluid and oil-based drilling fluid which normally require the use of a large number of poisonous additives.
Experiments show that the new system has three important characteristics. Firstly, thermal stability time is more than 72h when aging at 200°C, HTHP filtrate loss is in the range of 12 to 25mL between temperature of 150 and 200°C, and sodium chloride and calcium chloride pollution resistance reach up to 200,000ppm and 5,000ppm, respectively. Secondly, the characteristic of excellent lubricity and inhibition can be comparable to oil based drilling fluids. Thirdly, the low biological toxicity also is one the most important characteristic, 96h LC50 semi lethal concentration of artemia is more than 100,000 mg/L, and EC50 median effective concentration of luminescent bacteria is more than 300,000 mg/L, which meet the biological toxicity discharge requirements for the first level sea area.
The extensive testing results of this new drilling fluid demonstrated its superiority characteristic and low biological toxicity to marine environment. Good results from field testing in Bohai offshore oil field are also presented, the deepest test well depth is 6066m, and the highest downhole temperature is 204°C.