Studies The Interaction Mechanisms And Application Of Ultra-Low Permeable Drilling Fluid

With the rapid development of petroleum industry, the formations being drilled are more complicated and the open hole is much longer, which has required the drilling fluids higher performance. Based on the partial studies of technologic project of Sino-Pec Cooperation, this paper has investigated the interaction mechanisms of ultra-low permeable drilling fluids in different down-hole environment, and then a new kind of ultra-low permeable drilling fluid agent YHS-1 has been approved successful application in different drilling projects (about 203 wells) in different formations.Ultra-low drilling fluid is a system that can form a thin and compact membrane with ultra low permeability by surface chemistry interaction to prevent the filtration of drilling fluids entering formations, this sealable membrane can be reversible. Under negative pressure, a sealable membrane can leave off the rock surface, which can reduce the negative effect of oil production. The development of this kind of drilling fluids is for resolving the technologic problems including loss circulation, drill pipe sticking and wellbore collapsing coexisting in complicated multi-pressure formations.This paper has developed a new kind of ultra-low permeable agent, which has sphere micelle architecture with relatively even diameter 610-660 nm, this kind of micelle polymer has showed higher pressurization and plugging performance. Then, the inertia materials and active minerals has been optimized for the compounding agents, which has further enhanced the formation pressurization performance of ultra-low permeable agent and guaranteed the formation of ultra-low permeable complex membrane with higher pressurization performance and the safety during the drilling operation.The interaction mechanisms of ultra-low permeable drilling fluids had been approved by different experiments including visible sand-bed penetration, pressure transmission, adsorption, mud cake microstructure observation and pressurization: (1) protection of oil-gas formation mechanisms: ultra-low permeable drilling fluids agents can form ultra-low permeable thin sealable polymer micelles, which can effectively seal the permeable formation and micro-fracture mud shale formation , and form a protecting membrane around the wellbore, which make the"zero"permeability easily realized, avoiding the mobilization of formation clay particles. In the same, the sealable polymer micelles layer locating in the rock surface also can be easily removed, which can be favored for reducing the formation damage to protect oil-gas formation; (2) Anti-collapse mechanisms: on the one hand, when the ultra-low permeable drilling fluids began to penetrating shale rock, the polymer micelles rapidly spread on the rock surface and formed a ultra-low permeable sealed membrane, which can further prevent the filtrate of drilling fluids from penetrating shale rock and contacting with the internal of the shale rock, thereby, reducing the hydration swelling and dispersion of wellbore and shale cuttings, and then made the wellbore more stable; on the other hand, even if some of drilling fluids entered and contacted with shale rocks, the filtration could not induce apparent dispersion and swelling of shale particles for the benign shale inhibition of this system. As a result, the drilling with ultra-low permeable drilling fluids can effectively prevent the formation from collapsing; (3) Anti-leaking off mechanisms: ultra-low permeable drilling fluids can effectively enhance the fracture pressure gradient of formations. On the one hand, the ultra-low sealed membrane of micelles polymers adhering to the wellbore can effectively isolate the pore fluids and the wellbore fluids, the drilling fluids is difficult to enter the wellbore formation, the pore pressure would not be widely enhanced as the common drilling fluids, the effective pressure would not be reduced greatly, the fractures would not created easily, thereby, the occurrence of leaking off can be eliminated; on the other hand, if the fractures are created in the weak bonding formations, the micelle polymers of ultra-low permeable drilling fluids can form a sealed membrane into the fractures, which can prevent the hydrostatic pressure further propagating into the deep formation and effectively prevent the formation from occurrence of leaking off.A Sand-Bed Filtrate Loss Instrument (GA) was developed for determining the filtration of ultra-low permeability drilling fluids, which can monitor the penetration of filtrate into the formations in real time, which can sustain pressure high up to 3.0MPa; A Multi-functional Core Flooding Instrument also has been developed to study the permeability of formation before and after contacting with drilling fluids, in order to find the pressurization and reservoir protection mechanisms; Besides the two advanced instruments, the High Temperature and High Pressure (HTHP) Well-bore Stability Test Instrument (WSM-01) also has been developed to study the anti-collapse performance and anti-collapse mechanisms.In view of different formation conditions, we have developed different ultra-low drilling fluid systems used in different oilfields and have applied in 203 wells for field test. The results showed that, all the wells successfully drilled and no any accidents caused by leakage, wellbore collapse induced by formation pressurization happened. The diameter of wellbore is so regular and the protection of oil-gas formation is more prominent. The well yield of the wells drilled by the ultra-low permeable drilling fluids is enhanced in different degree than adjacent wells in other block and has gained a good economic benefit of exploration. The results of application showed that, this kind drilling fluid system is mainly suit for the formations including multi-pressure layer systems, easily collapsing formation, unconsolidated leakage sandstone reservoir and micro-fracture formation, and so on. The benign economic benefit and society benefit have been gained in different oilfields.