The high temperature stability and wellbore cleaning problems of oil⁃based drilling fluid have to be solved when it is used to drill extended reach wells in the KQT gas field located in the East China Sea. For this reason, the key additives of oil⁃based drilling fluid including the emulsifier, wetting agent, rheology modifier and organic clay were selected using the interfacial tension test, emulsifying efficiency test, microscope observation, emulsion viscoelasticity and rheological property tests. After that, the oil⁃based drilling fluid for extended reach wells was developed, and the comprehensive properties were tested. The results show that the drilling fluid has good stability after hot rolled at 180 °C. The selected rheology modifier can enhance the structure strength of oil⁃based drilling fluid, and effectively increase the gel strength and low shear rate viscosity, thus increasing the wellbore cleaning effect. Meanwhile, it has good filtration property and contamination resistance property. This drilling fluid has been used in 4 wells in the KQT gas field, and good field application results are obtained.

Water sensitive damage and liquid trap caused by waterbased drilling fluid invasion are very prominent problems in the process of shale gas drilling and exploitation, which seriously affect drilling safety and production of shale gas well. According to the shale gas reservoir characterization and reservoir damage factor, based on nanoemulsion theory and basic reservoir protection principle, a novel nanoemulsion agents named SDME2 were developed by employing Gemini hyamine surfactant GTN, plus Tween 80, namyl alcohol and noctane as major raw materials. The results showed that the novel nanoemulsion agent could effectively reduce the surface tension of external fluid, increase contact angle, decrease water saturation in core, and increase recovery of low permeability reservoir permeability. The novel nanoemulsion agent was also conducive to shale gas desorption. Taking the newly developed shale gas reservoir protection agent as one of primary treatment agents, a new type of waterbased drilling fluid for shale gas reservoir protection was developed and its performance was good.

The effect of the drilling fluid components on sag performance was studied. Experimental results show that the type of base oil, oilwater ratio, type of internal phase, particle size and type of weighting material have great effect on the sag stability of the system. The greater the viscosity of the base fluid, the lower the oilwater ratio, the better the sag performance is. The oilbased drilling fluid system with NH4Ca(NO ₃) ₃ as the internal phase shows better sag stability after static aging than the system with other internal phase such as formate, calcium chloride and sodium chloride. The smaller the particle size, the better the sag stability is. The oilbased drilling fluid system with Mn ₃O ₄ as the weigh material shows the best settling stability under static and shear conditions.

5# mineral oil and 3# mineral oil were used separately to prepare the basic mud and the oilbased drilling fluid system at room temperature. PIBSI（polyisobutylene succinimide） with different concentration were added to investigate the effect on properties of the basic mud and oilbased drilling fluid system under different temperatures. The results showed that, PIBSI can improve sag stability and viscosity of the basic mud changes with the concentration of the PIBSI under low temperature(4 ℃). As for oilbased drilling fluid system, PIBSI can decrease the viscosity of the system under low temperature while showed little effect on the viscosity and the gel strength under room and high temperatures if the concentration of PIBSI was in the appropriate concentration (for the 5# mineral oil based drilling system, the appropriate concentration was 2%, while for the 3# mineral oil based drilling system, the appropriate concentration was 0.2%). The larger the concentrations of the PIBSI, the lower the HTHP filtration loss will be.

The significant formation and aggregation of hydrates would lead to blockage of BOP stacks during deepwater drilling. The process of gas hydrate formations in the presence of common additives used in deepwater drilling fluids were investigated in the simulating condition of deepwater by utilization of a simulating device. The hydrate inhibition properties of additives were analysed according to the time of significant hydrate formation. The results show that polyglycol(SD301) could delay the significant hydrate formation noticeably, and it performed better than kinetic hydrate inhibitor(PVCap). Polyanionic cellulosic polymer(PACLV), biopolymer(XC), carboxymethyl cellulose(CMCLV) and kinetic hydrate inhibitor(PVP) could also control the hydrate formation to some extent. While the inhibitions of partially hydrolyzedpolyacrylamide(PLUS), polyamine(SDJA) and sulfomethal phenolaldehy resin(SD102) could be ignored.

The formation mineral composition, micro structure and physicalchemical properties of representative shale samples of deep complex shale formation in Yuguo area of Tuha oilfield were analyzed by Xray diffraction, Scanning Electron Microscope, swelling and dispersion tests, specific surface area capacity measurement, specific water capacity and other experimental methods the mechanisms of wellbore instability in deep complex formation were made clear. A three cooperation principle for stabilizing wellbore by improving the performance of drilling fluid was put forward. This method includes the technologies of sealing and consolidating side wellbore physicchemically, enhancing inhibition against hydration and providing the effective stress support of reasonable drilling fluid density. The responding antisloughing drilling fluid which is suitable to deep complex formation in Yuguo area was developed by selecting all kinds of additives and optimized formulation. The site trial indicates that this drilling fluid has great capacity to antisloughing and sealing layer. The drilling fluid is easy to manage and maintain. Using this drilling fluid can improve drilling velocity and save drilling cost.