The Study On Drilling Fluid For Drilling In Marine Natural Gas Hydrate Bearing Formations

Natural gas hydrate is an ice-like crystalline compound, formed by water and natural gas molecules under certain temperature and pressure conditions. It is also called clathrate hydrate or "flammable ice". Since the gas forming hydrate is mainly methane, it is can also be called methane hydrate. Natural gas hydrates distribute widely in marine continental margin sediments and permafrost environments, and the overwhelming majority of hydrate reservoir discovered existed in the former. Currently the research of gas hydrate has become hotspot of the development of the geosciences research and the energy industry. The research involves in many fields, such as exploration of new energy resource, greenhouse effect, global carbon cycle, climatic change, paleo oceangraphy, oceanic geohazard, natural gas storage and transportation, the flow assurance in oil and gas pipeline etc., and it may also have profound influence on the development of geoscience, environmental sciences and the energy industry. So many countries in the world have launched natural gas hydrate research project broadly from their aspects of interest respectively. Especially for the severe shortage of energy supply and high gasoline and natural gas price in recent years, many countries pay more attention to the energy potential of gas hydrate, which has the properties of high energy density, large reserves and wide-range distribution. As for China, the limited domestic oil and gas resources cannot meet the requirement of fast growing economy development, and the propotion of imported crude oil has increased fastly. Therefore, from the aspects of safety of energy supply and the demand of sustainable development, the scale of exploration and research of gas hydrate resources should be expanded.In order to explore and exploit gas hydrate formed in certain depth underground, drilling operations are indispensable approach. Because of the equilibrium characteristics of gas hydrate, drilling operation in gas hydrate bearing formations is quietly different from well drilling in routine oil and gas sediments, and also makes the wellbores confronted with difficult problems. Firstly, during the process of drilling, the stress of strata near borehole wall and the bottom of hole will release, and the pressure of the formation will reduce. At the same time, the heat energy generated by the friction of the bit cutting rocks and the friction of the drilling tools, the borehole and the core, and the improper control of the circulating mud will result in rising of temperature near wellbore, and cause the gas hydrate dissociating. If gas hydrate plays the role of cementing sediments, the dissociation of gas hydrate will lead to borehole collapse. The water generated by dissociation increases water content of the wellbore wall, weakens the interaction between granules and makes the borehole unstable. And the released gas influences the specific gravity and the rheology of the drilling fluid, which does harm to the stability of the borehole and even initiates the well accidents, such as surge or blowout. Secondly, the process of drilling is a non-adiabatic process. Heat exchange between drilling fluid and the formation and the heat absorbed by dissociation of gas hydrate lead to a change of temperature of the fluid circulating in the well and the borehole, which alters critic parameters of the drilling fluid, such as viscosity, density and chemical stability of the drilling fluids, etc., and the stress around the well and the pore water pressure also changed. Finally, the gas released by the dissociation of gas hydrate flows into the wellbore, returns to the ground with the drilling fluids. During this course, if the temperature and pressure conditions in the well are suitable, gas hydrate plug will form in drilling pipes or valves, especially in blowout preventer. Furthermore, the gas hydrate formations are unconsolidated or semi-consolidated sandstone or argillaceous sandstone, which makes the problem of the stability of the wellbore in these sediments more severe. The instability of the wellbore wall leads to collapse of the borehole, sticking of drilling tool, cracking of the borehole, leakage of the drilling fluid or losing control of the well surge. Under some extreme circumstances, it will cause drilling well abandon, even loss of human life and drilling facilities. Subsequently, ensuring stability of the borehole wall and flow assurance within circulating system is pivotal during drilling in gas hydrate bearing formations.The study of the works comes from a major project of 863 Plan—the research of drilling fluid technique for drilling in marine natural gas hydrate containing formations (No. 2006AA09Z316), which is an exploratory issue. For the challenges faced with drilling operations in marine gas hydrate bearing sediments, the works indicates that it is essential to develop a propriate drilling technique, and design a drilling fluid system that meets the requirements the demand of safety drilling. When drilling through sediments containing natural gas hydrates, on the one hand, the performance and temperature of drilling fluid should constrain the dissociation of gas hydrate, and on the other hand, the selection of circulating drilling fluid should also be taken into account for the low temperature encountered. The main approach of drilling in these formations is decomposition inhibiting method, namely choosing drilling fluid with appropriate density, maintaining the pressure in the wellbore, cooling the mud and adjusting related drilling parameters to keep the gas hydrate in a stable state. The low-temperature drilling fluid can limit the dissociation of gas hydrate outcropped within borehole, and also inhibit the formation of gas hydrate plug in the circulating mud.The thesis consists of six chapters and the contents of each section are as following:Chapter 1:the structures, properties, and characteristics of natural gas hydrate are introduced, and the history of natural gas hydrate research overviewed. Based on the analysis of the main problems faced with drilling through gas hydrate bearing formations, the detailed content of study and technical roadmap are presented.Chapter 2:firstly detailed properties of natural gas hydrate bearing formations are introduced, including distributing mode of natural gas hydrate, the gas hydrate stable zone, and micro-structure of natural gas hydrate. Then the characteristics of natural gas hydrate bearing formations are analyzed, such as its geologic property, porosity, hydrate saturation, effective heat transfer coefficient, mechanics properties and phase equilibrium of gas hydrate formation. Finally, a classification of gas hydrate reservoir was introduced and a strategy of gas hydrate resource exploitation was suggested.Chapter 3:The characteristics of drilling fluid invasion in gas hydrate formation were analyzed, including the process and main properties of drilling fluid invasion, and gas hydrate decomposing in porous media. Then a model of drilling fluid invasion was built, and a numerical study was conducted, the result of which implied the requirements of wellbore stability on drilling fluid performance.Chapter 4:The silicate drilling fluid for drilling in gas hyrate bearing formations was studied. At first the current situation of drilling fluid that can inhibit gas hydrate formation, the technique of drilling fluid enabling inhibit gas hydrate decomposing was introduced. Then the characteristics of drilling fluid for deepwater drilling and the requirements of drilling in gas hydrate formation on the performance of drilling fluid were discussed. Based on the discussion, the performance of silicate drilling fluid was studied in detail by an analysis of the current state of silicate drilling fluid study, the mechanism of wellbore wall stability, the optimization of treating agent for silicate drilling fluid, etc.Chapter 5:The effects of the silicate drilling fluid on gas hydrate formation and dissociation inhibition were studied systematically. Firstly, the current state of development of kinetic gas hydrate inhibitor was introduced, and the performance of the silicate drilling fluid on gas hydrate formation and decomposition inhibition was evaluated and the results were analyzed.Chapter 6:The main conclusions of the dissertation were put forward and the defects were pointed out, and suggestions for future research and main innovative points of the paper were also illustrated.By theoretical analysis and experimental study mentioned above, preliminary results that meaningful and have some significant practical value were obtained. Some understandings and concluions attained during the works are as follows:(1) In the drilling of marine gas hydrate formations, because the stress near the borehole wall and bottom is released, the pressure of formation decreased. The heat generated from cutting of the rock, friction between drilling tools and the core or borehole wall will break temperature and pressure conditions at which gas hydrate is stable, and triggering the decomposition of hydrates, resulting in borehole instability, thereby causing the drilling operation adversely affected. To ensure the safety of natural gas hydrate formation drilling carried out smoothly, great efforts shold be taken to control the temperature and pressure within the well, applying drilling fluid with good performance at low temperature and minimize the gas hydrate decomposition, in order to maintain borehole stability and the safety of drilling operation.(2) The decomposition of the gas hydrate and drilling fluid invasion will increase the pore pressure, and bring a negative impact on the safety of drilling operation. If the permeability of the formation is low and the gas hydrate is heated very quickly, gas and water generated from the decomposition can not flow away in time, pore pressure of the formation which depends on the permeability of sediments and warming rate will increase rapidly. The faster the pore pressure increases, the smaller the range of safe drilling of the drilling fluid density window, and the more difficult to carry on drilling operation. In addition, the pore pressure increase require higher drilling fluid density to maintain wellbore stability, which not only seriously affects the penetration rate, but also increases wall permeability and pore pressure near the wellbore, further reducing the stability of borehole wall, and bring a vicious circle, greatly increasing the complexity of the borehole.(3) The invasion of drilling fluid and dissociation of gas hydrate will also weaken the rock matrix, so the strength of the formation decreased as well. Especially for the zone near wellbore wall, disturbed by heat, the intensity of gas hydrate dissociation is very high, because of high hydraulic gradient, the pore pressure increased sharply. As a result, this zone is the most fragile and indanger of collapsing. So the effect of hydrate dissociation on stress field is relized by increasing pore pressure, and altering effective stress of formation.(4) During the process of drilling, the filtered liquor entering gas hydrate formation, and water released by hydrate dissociation, casuing the water content of the formation increased. This increase will intensify the hydration of rock matrix, and causing the borehole wall unstable. Moreover, the dissociation also results in the increase of formation permeability. So the drilling fluid exerts permeate pressure to the formation, leading to the collapse pressure increase and fracturing gradient decease, making the wellbore wall unstable.(5) From the perspective of drilling fluid, reducing the temperature of the drilling fluid, making it close to the original formation temperature can minimize thermal disturbance and decomposition rate in hydrate formation. Therefore, the speed and extent of pore pressure and moisture content increase in the hydrate formation reduced. Meanwhile, the selection of drilling fluid with appropriate density assits in the stability of gas hydrate formation. In addition, with the good filtration and walling performance, the drilling fluid can form dense impermeable mud layer at the surface of the borehole wall, controlling hydration scattering in the formation also promote stability of wall. Therefore, designing of a suitable drilling fluid system plays an important role when drilling through gas hydrate containing formations, and the drilling fluid treatment agent and hydrate decomposition inhibitor which has synergy effect on sloughing prevention is the emphasis of experimental study in the future.(6) The decreasing of drilling fluid temperature has significant impact on its performance, particularly its rheology and filtration. The addition of thermodynamic inhibitors and kinetic inhibitors will not only have an important effect on the general performance of drilling fluid, including rheology and filtration properties, but also greatly affects gas hydrate formation and dissociation inhibition. Therefore, in the process of drilling fluid for natural gas hydrate drilling designing, the combianation of these two types of inhibitor should be taken into consideration to make sure that the drilling fluid has a good low-temperature performance and gas hydrate inhibition performance.(7) Based on the rhelogical testing of drilling fluid at low temperatures, combined with the results of gas hydrate formation inhibition and decomposing inhibition simulating experiment in the laboratory, the recommended formulations of silicate drilling fluid were put forward:①2% bentonite+0.3-0.5% HV-PAC+2% SMP-2+3% Na2Si03+3% KCl+0.5-1% PVPK90+ 0.2-0.4% XC+10% NaCl,②2%bentonite+1-2% LV-CMC+2% SMP-2+3% Na2SiO3+3% KCl+0.5-1% PVPK90+0.2-0.4% XC+10% NaCl.