Study Of Water-based Drilling Fluid Film Technology

All kinds of techniques and measures must be taken to prevent water from going into the formation under the condition that water based drilling fluid is adopted. Water entering into mud shale formation near borehole wall is one of the main reasons of hole collapse, the entry of water will bring the increase of pore pressure and decrease of effective confining pressure, and then cause the serious change and deterioration, besides, after the absorbing of water, the intensity of mud shale will decrease, and then cause the instability and slumping.If there is semi-permeable membrane on the surface of borehole wall rock, we can utilize the permeable pressure difference between two PHases on the membrane to control the flow direction(flow towards formation rock or wellbore) of solvent medium in the fluid, in this way, the instability PHenomena causes by the water absorbing of mud shale can be prevented, and works as a capability of anti-sloughing. However, studies from both home and abroad indicate that the crude mud shale is non-ideal membrane, that is to say the membrane is permeable and can not prevent the solute from permeating the membrane. As far as how to improve the non-ideal membrane of the mud shale and transfer them to ideal or sub-ideal membrane is a big and long-term technical problem in studying and settling wellbore instability problem.On the basis of formation theory and mechanism studying of the semi-permeable membrane, this paper presented a set of theory methods and established the mechanism to improve the semi-permeable membrane and transfer them to ideal or sub-ideal membrane, which is enhance the charge density in the thin zone of mud shale and reduce the pore size of the mud shale. This require the molecular of the treating agent injected must be small enough to enter into the pore of the mud shale and deposit in shale's pores, and then made the inner mud cake formed in the framework of mud shale around the wellbore, and this will lead the permeability of the inner mud cake is lower than that of mud shale. If there are hydroPHobic groups on the molecular chain, the idealism of semi-permeable membrane will be further strengthened.This paper establishes some evaluation methods of the effect of semi-permeable membrane. Through analyzing the mechanism of semi-permeable membrane effect, this paper designs and manufactures a semi-permeable membrane efficiency measuring device which can simulate the bottom hole's high temperature and pressure conditions. The result of semi-permeable membrane effect tested by the device is credible which has been proved by experiments.