| Compared with conventional drilling fluid, micro-foam drilling fluid has distinct advantages which are shown as follows. Because of its well protective effect on reservoir and effective prevention of shale sloughing, the stable water-based foam drilling fluid is propitious to hole stability. As a result of high penetration rate and its low density, underbalanced drilling can be used in some low pressure area. Due to the low filter loss of the drilling fluid, the formation damage can be reduced. On account of high cuttings-carried ability, the bottom-hole cuttings can be effectively remove, the drilling rate can be improved and the service life of drill bit can be prolonged. By use of the drilling fluid, we can prevent downhole fires and explosion. Because the foam drilling fluid has mud cake characteristic, low filtration and viscoelasticity, using the drilling fluid can effectively reduce filtration loss, limit liquidity of free water and permeation of mud cake, and buffer pressure of circulation fluid which can play a role in anti-sloughing to a certain extent. Therefore, foam drilling fluid has anti-sloughing ability in unstable formation, and can prevent and eliminate leakage loss in permeable and fractured formation. In addition, the flow quality and density invariability of foam fluid, especially the rheological and density on the condition of reservoir pressure and temperature, have an important influence on practical drilling tools.Under the indoor condition of given arrangement, the 3 micro-foam fluids prepared of different initial density are stable. From the analysis together with microscope, we can see the bubble become small, the number of which is increase, and is going uniformity varying with churning time. Generally after the churning time up to 2~3 hours, the bubble distribution is stable, the diameters of which mainly range from 0.003~0.016 mm. The stability of micro-foam fluid is improved and the proportion of small micro-foams increases with the increase of foam complexing agent. Within 8 hours after preparation, the 3 micro-foam drilling fluids of different initial density are stable, and the stability of them is further improved with the increase of foaming agent. When hold up time is over 12 hours, the upside of foam, the initial density of which is 0.7 g/cm3, appears obviously unstable and nonuniform.The rheological of micro-foam drilling fluid is mainly depended on temperature and pressure. Studies have shown that micro-foam fluid which appears laminar flow is propitious to cuttings-carried, purifying borehole and reducing the wall erosion of a well, which is propitious to the hole stability. Simultaneously, micro-foam drilling fluid has the properties of low filtration, strong repression, anti-pollution, thermal stability, etc.The simulated experiment of mass hole stability have shown the results as follows. Micro-foam drilling fluid can obviously interfere and cushion pressure, so that the increase of mud shale pore pressure can be alleviated. In the penetration process, micro-foam is captured in the pore throat, and the flow resistance from micro-foam drilling fluid to formation can be greatly increased caused by the superposition of Jamin effect. As a result of low filtration, the fluid can reduce the immersion of mud filtrate to wall of a well, thus, the hydration of mud shale is decreased, etc., which demonstrate micro-foam drilling fluid has a good property of anti-sloughing. |
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