Research On Biodegradable Drilling Fluid For Horizontal Well

The technique of horizontal well has become one of the key techniques of oil and gas production, while the drilling fluid called "blood of well drilling engineering", especially for horizontal well drilling is the focus and also the problem in the area of drilling fluid technology. Related to the vertical well and conventional directional well, horizontal well has strict and severe requirements for drilling fluid techniques, such as hole cleaning ability, reduction of friction drag, well-bore stability, leaking prevention and reservoir protection. At the same time, with the enhancement of environmental protection in the world, including China, the environmental problems caused by drilling fluid are emphasized day by day, especially in the area sensitive to environmental change, such as in sea, lakes and inner rivers. The corresponding laws and constrains on drilling fluids are more and more strict. Therefore, it is imperative to develop and research on drilling fluid technique which could meet the requirement of both environment protection and horizontal well drilling.On the basis of CNOOC technical development project " Technical Study on Drilling Fluid for Horizontal Well " and Natural Fund Project Sponsored by Hubei Province "Research on Hydration Expansion Characteristics of Shale and Mechanism of Expansion Prevention",in allusion to the key techniques used in drilling fluid of horizontal wells , the key treating agents for drilling fluid in horizontal wells were developed based on drilling hydraulic theory and theory of drilling fluid rheology. On the basis stated above, a drilling fluid system used for horizontal well is developed, and the rheological characteristics, lubricating properties, filtration and wall-cake-building characteristics, inhibition properties of drilling fluid for horizontal well were studied and analyzed. A method of biologic-chemical oxygen consumption and a method of luminescent biacteria are used to study the biologic degradation and biologic toxicity of the horizontal well drilling fluid and its additive. The technique of biologic enzyme degradating drilling fluid and factors influencing its degradation effect are analyzed and studied and analyzed. The mechanism of biologic enzyme degradating drilling fluid is studied.The follow technical achievements are obtained in the study:1. Based on theory of horizontal wellbore cleaning and theory of drilling fluid rheology, the key additives for drilling fluid in horizontal wells were developed, a solid-free and weak gel drilling fluid system was established based on throry of reservoir protection. the properties of drilling fluid are analyzed and studied ystematically.(1) Based on the theory of salt resistance of water soluble polymer, a polysaccharide thickness agent VIS with good thickening effect and salt resistance is developed.(2) According to the theory of collochemistry, it is proposed that polyalcohol JLX with "clout point'' effect is used as a anti-collapse lubricant in drilling fluid for horizontal well(3) According to the theory of reservoir protection, a solid-free and weak gel drilling fluid system was estblished including: water+0.1%NaOH+0.2%Na₂CO₃+0.7%thickener VIS+2.0%fluid loss additive DFD+3%lubricant JLX, soluble salt is used to adjust its density.(4) The drilling fluid has unique rheological characteristic with higher yield point-plastic viscosity ratio(YP/PV) and higher low shear rate viscosity(LSRV). Its YP/PV is as high as 1.5, its LSRV(0.3round/min) is 55000 mPa.s, the effect of temperature on rheology is less.(5) Drilling fluid has excellent salt resistance. There is no effect of salinity on rheological property. Its pH adaptability is wide, properties of drilling fluid are stable with the range of 6-12(pH).(6) It has strong inhibition ability, its lubricity is equal to oil based drilling fluid almost.2. Methods of bio-chemical oxygen consumption and luminescent bacterial are used to study the biodegradation and biological toxicity of the drilling fluid system and its additives.(1) Method of bio-chemical oxygen consumption is used to evaluate the biodegradation of the solid free weak gel drilling fluid and its additives. The results show that the drilling fluid system and its additive are all biodegradable. The biodegrading rate of starch loss control agent DFD is the highest, next is the thickening additive VIS, the last is lubricant JLX.(2) Method of luminescent bacterial is used to analyze the toxicity of the solid free weak gel drilling fluid and its additives. The results show that the toxicity of drilling fluid system can meet criterion of discharge, the toxicity of its additives including thickener VIS, fluid loss additive DFD andlubricant JLX are toxicity-free.(3) The results of biological degradation and toxicity evaluation indicate that the solid free weak gel drilling fluid is environment friendly.3. The effect of reservoir protection of the solid-free weak gel drilling fluid is studied, the mechanism of formation damage is analyzed. The degradation of polysaccharide polymer by biological enzyme is tested; the factors influencing the effect of enzyme degradation are discussed; biological enzyme and other oxidizing agent degrading polymer are compared. The mechanism of formation protection of biological enzyme is analyzed.(1) The result of experiment on reservoir protection of solid-free and weak gel drilling fluid shows that blocking by polymer is the major cause of formation damage.(2) The feasibility and methods of using the biological enzyme degradating polysaccharide drilling fluid are studied. The study result indicates that biological enzyme is a high efficient gel breaker for polysaccharide drilling fluid with low dosage and high activity. Factors of influencing the gel break of enzyme are studied. The results show that 0.10% of enzyme SWM-2 has a gel break rate 98.4%; there is no influence of salinity on the gel breaking effect, a more proper gel breaking condition is: pH value is within 6-7;temperature is lower than 90℃; the reaction time is not less than 2h.(3) The study result of reservoir protection mechanism indicates that the solid-free weak gel drilling fluid and the biological enzyme gel breaker fluid have a good compatibility with reservoir rocks.