Research on Environmentally Safe Temporarily Plugging Drilling Fluid in Water Well Drilling

Abstract

Abstract In China, the drilling industry has had numerous environmental restrictions on some materials it uses and the methods of the disposal. Both the governors and the drilling engineers welcome the use of environmentally safe drilling fluid. However, there is a contradiction between maintaining borehole stability and protecting the permeability of the reservoir in water well drilling. The mud invades in the porosity in a large radius near the wellbore. With the flushing work, some of the invaded mud is cleaned. However, the majority of the mud remains in the porosity. The residual mud not only plugs the porosity but also is a kind of contamination to the aquifer. It becomes a barrier to recover the permeability of the promising aquifer and greatly decreases the yield of water. The paper proposes the use polymers such as Na-CMC, guar gum and HEC to develop temporarily plugging drilling fluid, combining with the bio-degradation principle in modern biology. This fluid is marked as self-degrading, harmless to the reservoir and can be widely used in oil, gas and water well drilling. Distinguishing from traditional temporarily plugged agents, it does not need additional unplugging technology which elementarily improves the unplugging technology. This paper analyses in detail the action mechanism of temporarily plugging drilling fluid and the enzyme degradation mechanics. Meanwhile, it presents the lab tests of viscosity and permeability recovery and the field trial in Zhangye city of Gansu province of North West China. The results of lab and field trial confirms the performance of temporarily plugging drilling fluid that it can be self-degrading, harmless to the reservoir, effectively maintains the borehole and plugs the leakage while remarkably recovers the permeability of the reservoir. Therefore, it has a promising future. Introduction Protection of our environment from pollutants has become a serious task. Now, at the beginning of 21st century, the restrictions are becoming more stringent and restraints are becoming worldwide issues 1. Numerous drilling fluids are used, but for water wells, a suspension of bentonite or similar clays in water is commonly used. The desirable properties obtainable with bentonite are high viscosity, jelling strength, and relatively low solids content in the mud 2. However, in the process of drilling, the porosity and permeability of the natural formation on the vicinity of the well increases 3. The mud invades in the porosity in a large radius. With the flushing work, some of the invaded mud is cleaned. However, the majority of the mud remains in the porosity. The residual mud not only plugs the porosity but also is a kind of contamination to the aquifer. It becomes barrier for recovering the permeability of the promising aquifer and greatly decreases the yield of water well. Therefore, there is a contradiction that maintaining borehole stability and recovering the permeability of the promising aquifer in drilling work. To solve this contradiction, the author and his colleagues develop a kind of temporally plugging drilling fluid. It is auto-degrading, toxicless and harmless to the local environment. Compared with conventional temporarily plugged agent, it elementarily improves the unplugging technology and the productive efficiency. We can control the rheological performance by changing the compounds to meet the needs of temporarily plugging. That is to say, it can maintain the borehole stability and its viscosity reduces after the completion of drilling work, which can recover the permeability of the promising aquifer and as a whole improve the yield of water well.