The Study Of Polyethylene Glycol Driling Fluid System For Drilling In Oceanic Gas Hydrate Bearing Strata

The study of natural gas hydrate is very important and popular in modern geosciences and energy industry. With the rapid development of economy, China consumes more and more energy resource, the major constituent of which is coal. This will lead to severe pollution. We should make efforts to develop new type of alternative energy resources to keep sustaining development of energy. Natural gas hydrate is considered to be the potential new energy resource in the 21st century.Enormous gas hydrate reservoir exists in seafloor sediment, and drilling is a direct means to study the hydrate. The heat generated in the process of drilling, the release of ground stress and the change of phase equilibrium condition caused by inhibitor could induce the dissociation of gas hydrate. The gas released in these processes enters the drilling fluids, decreases the density of the fluid, and sequentially decreases the hydro static pressure at the bottom of borehole. This would speed up the dissociation of gas hydrate around the wellbore and at the bottom. At last, a large amount of gas hydrate dissociates, and the accident of hole size expansion, blowout and collapse of well, the deformation of casing, the settlement of ground would happen. Moreover, the inburst of the gas may cause the pressure of upper part of drilling system increase, which probably turn the gas into hydrate again. When drilling through gas hydrate bearing strata, the dissociation of gas hydrate and reformation of hydrate within fluid circulating system are the main factors that affect drilling safety. Therefore, drilling fluids which can inhibit gas hydrate dissociation and reformation, maintain wellbore stability, and ensure wellbore safety are important for smoothly drilling through the hydrate strata.Currently the relevant experiment and field study on drilling fluids for gas hydrate are rarely reported, the majority of which concentrated on the inhibition of drilling fluids, i.e. the chemical inhibitor can inhibit the formation and dissociation of gas hydrate. However, the characteristics of gas hydrate formation decide that besides the inhibition of drilling fluids, we should also consider the parameters that effect the stability and well security such as the density, rheology, hydrate inhibition, the filter cake and PH value etc. At the moment of drilling strata, the gas hydrate of the wellbore and beneath the bit would dissociate with the sudden change of temperature and pressure in spite of the application of inhibitor. At this point, we should pay more attention on the performance of prevent collapse of drilling fluids. After all, the continuous and safe drilling is the chief goal of drilling. Further, since there is little research on drilling fluids for permafrost region, it is necessary to study on the synthetic performance of drilling fluids. This will provide a foundation of theory and technology for the exploration and exploit of gas hydrate in the future.Based on the NSFC program "the Research on the Mechanism of inducing and inhibiting Dissociation of Natural Gas Hydrate During Drilling", the author firstly analyzed the properties of gas hydrate and discussed the characteristics of gas hydrate formation and problems encountered during the process of drilling. Then, the author studied the requirement on the performance of drilling fluids, and analyzed the inhibitor in the fluid and its mechanism of inhibition. To drill in the gas hydrate formation, it is not only required us to control the formation and dissociation of gas hydrate, to ensure the keepgoing of drilling process is more important. Therefore, the author synthetically analyzed the entire performance of drilling fluids. Based on this analysis, combining with the previous discussion on gas hydrate formation and theory of drilling fluids, especially the low temperature of permafrost strata, the author put forward the method of drilling fluids experiment: conduct study on the aspect of the density, the inhibition performance on gas hydrate, the performance of building wellbore, the filtration amount, the rheological behavior and stability at low temperature of drilling fluids. In this way, the author selected the drilling fluids system which is favorable for wellbore stable and well security, and established an experimental foundation of design theory of drilling fluids system when drilling in the gas hydrate strata. This is the preparation for the future large scale gas hydrate exploration and exploit in permafrost regions and ocean, which is also the aim of the dissertation.In order to reach the above goals, combined with the earlier investigation and the relative study done by the host project, the relative components of the drilling fluids of natural gas hydrate have been selected by the principles of the drilling fluids used in the deep sea and permafrost drilling and the relative experience. The selection mainly included the suitable processors, such as inhibitors and additives. And the influence of each component of the drilling fluids to the natural gas hydrate inhibition and the stability of the drilling well has been considered. Then, the experimental research has been carried on for the different systems of the drilling fluids, including the rheological property, the dehydration and so on. At the same time, in view of the low temperature characteristic of the permafrost, the related performance experimental research has been carried on under the low temperature, and the further research has been done for the drilling fluids formulations according to the test result. Finally, the generalized analysis has been done for the experimental research carried on, and the appropriate drilling fluids system have been selected, which provides the experimental foundation for the further research of the host project.After a great deal of experiments, the paper proposed a basic polyethylene glycol drilling fluid formula for Deep Ocean drilling. For the purpose of inhibiting gas hydrate dissociation and reformation, a variety of chemicals and inhibitors were added, and their capability evaluated by the natural gas hydrae integration and simulant mini-drilling experimental system.The study illustrates that the viscosity and shearing force of polyethylene glycol drilling fluid with clay increase rapidly at low temperature, while for the same drilling fluid without addition of clay, its rheological parameters don't change much. At a condition of -4℃and 18MPa, the inhibition ability of PVP(K90) is better than PVP(K30), an addition of 1% of PVP(K90) will effectively inhibit gas hydrate formation in the polyethylene glycol drilling fluid. Therefore, the polyethylene glycol drilling fluid with PVP(K90) has excellent performance on inhibition of gas hydrate formation.