A Research On Plugging Theory And Technology Of Water-Based Drilling Fluid

The main reason of wellbore instability is that water gets into the shale formation or dirty formation near wellbore zone. Plugging to prevent water get into the formation and prevent the pressure transmission from wellbore to formation are important technical methods to stabilize borehole wall. However, it is required that the sizes and shapes of plugging material should match the pores or microfractures of target formaton. The common used blocking agents can hardly meet the requirements of drilling construction because of their low softening point, or strong fluorescence, or other disadvantages. Through the analysis of the anti-sloughing plugging process, this study comes up with a new idea of finding plugging material that making thermoplastic nanometer silicon dioxide polymer composite material serve as new material of drilling fluids. Besides, with special emphasis on the anti-sloughing plugging’s evaluation methods, performance and mechanism as well as the plugging agent, the study discussed the anti-sloughing plugging technique and theory of water-base drilling fluids.Based on analysis of the drawbacks of the existing drilling fluids plugging mechanism evaluation methods and guided by the principle of the simulation, this essay developed an anti-sloughing plugging tester working at high temperature and high pressure and then established an evaluation method of plugging ability of drilling fluids. This method uses high-temperature and high-pressure filter press which is frequently-used, in drilling fluid experiments to simulate the high temperature and high pressure downhole. Meanwhile, the stirring of filter press simulates the flow of drilling fluids, gaps between plate metals simulates microfractures (20～100μm),and filter papers serve as micropores. Thus, we can evaluate plugging capability according to the water loss within a certain period of time. The experimental results show that the evaluation method proposed has a good feasibility, repeatability and stability.This essay discussed the factors such as NCO/OH ratio, IPDI, PPG and DMPA feeding mode, as well as the dosage of the nano silicon dioxide, which will influence the properties of NAZ-1. And optimized the optimum synthesis conditions based on an inorganic/organic composite material(blocking agent NAZ-1) using the in situ polymerization, Analyzed and then characterized the morphology of composite material, particle size distribution, softening point and fluorescence intensity based on environmental scanning electron microscope, fluorescence spectrophotometer, softening point tester and laser particle size instrument. Evaluating the properties of NAZ-1 when it was singly added in base drilling fluid. Experimental results show that NAZ-1’s compositing compatibility and high temperature plugging property are remarkable, which is superior to those of the common sulfonated asphalt and emulsified asphalt.Composite material NAZ-1 being the main agent, this research developed a new kind of water-based drilling fluid (recipe:4%bentonite+4%NZA-1+3%QCX-2+3%SMC+ 3%SMP-2+1%SPNH+0.1%CMC) based on orthogonal experiment method, which have a good performance in anti-sloughing and sealing in high temperature. Experiment results show that the rolling recovery rate of the rock debris, in a good inhibitory range, is 63%; water loss is 15 ml at 150℃ for 40 microns microfractures, which indicates it has a good sealing performance at high temperatures as well as a good lubrication performance.The applications of pressure transmission and the micro experiment clarify the plugging and anti-sloughing mechanism of NAZ-1 system:under certain temperature and pressure, becoming softer and with deformation, NAZ-1 flows into the near wellbore formation, and then be adsorbed and piling up in pore. Pore surface then become hydrophilic, preventing water flowing into formation as well as blocking water and chemical potential.