海洋天然气水合物地层钻井的钻井液研究
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摘要
天然气水合物是由水分子和天然气分子在一定温度和压力下形成的似冰雪状结晶化合物,又称笼形水合物或“可燃冰”。由于形成天然气水合物的气体主要为甲烷,因而也被称为甲烷水合物。自然界中的天然气水合物主要分布在大陆边缘的海洋深水区和陆上永冻地区,前者占了已发现数量的绝大多数。天然气水合物研究已成为当代地球科学研究和能源工业发展的一大热点,该研究涉及新能源的勘探开发、温室效应、全球碳循环和气候变化、古海洋、海洋地质灾害、天然气储运、油气管道流动安全等,并有可能对地质学、环境科学和能源工业的发展产生深刻的影响。因而世界上许多国家都从各自的关注点对天然气水合物展开了广泛的调查研究。特别是近年来能源短缺日益加剧,油气价格居高不下,使得各国更加关注具有能量密度高、储量大和分布广等特点的天然气水合物,纷纷加大对其勘探和开发研究的力度。对于我国来说,本身油气资源就不足,加之经济的快速发展,导致能源缺口越来越大。因此,从我国能源战略安全和经济可持续发展的角度来说,也应加大天然气水合物勘探开发的力度。
要对赋藏在地下一定深处的天然气水合物进行勘探和开发,钻井是必不可少的重要手段。天然气水合物的相平衡性质决定了此类地层钻井与一般油气地层钻井有很大不同,也导致这类地层钻井面临更加复杂的井内问题。首先在钻井时,储层井壁和井底附近地层应力会释放,地层压力会降低；同时,钻头切削岩石、井底钻具与井壁及岩心的摩擦都会产生大量的热能,此外循环泥浆温度控制不当,这些都可能使孔内温度升高。在钻井过程中井壁地层压力和温度的变化将不可避免地导致天然气水合物发生分解。当固态水合物起胶结或骨架支撑作用时,分解本身就会使井壁坍塌。而分解产生的水增加了井壁地层的含水量,使颗粒间的联系减弱,导致井壁不稳；逸出的气体又影响了钻井液的比重和流变性,对井壁稳定愈发不利,甚至还可能引发井涌和井喷等钻井事故。其次,钻井是一个非绝热过程,钻井液与地层间的热交换和水合物分解时吸热会导致循环钻井液和井内的温度发生变化,使钻井液的关键参数发生变化,如粘度、密度和化学稳定性等,井内的应力和孔隙水压力也会发生改变。最后,水合物分解释放的气体进入井内,与钻井液一起上返到地表。在此过程中,如果温度和压力条件适当,在钻杆或阀门,特别是防喷器等部位还会生成水合物栓塞。而且,含水合物地层一般为未固结或半固结砂岩或泥质砂岩,这使井内稳定的问题更加严峻。井壁的不稳定会导致井壁坍塌、卡钻、压裂、钻井液漏失或井控失败。在某些极端的情况下,还会造成钻井报废,甚至人员伤亡和钻井设备损坏。因此,保证井眼温度和流动安全是水合物地层钻井的关键。
本文的研究来自863计划专项课题——“海底天然气水合物地层钻井钻井液工艺技术研究”(课题编号：2006AA09Z316),属于探索性研究课题。针对海洋天然气水合物钻井方面所面临的难题,本文认为制定适合天然气水合物钻井的钻井工艺,设计满足要求的钻井液工艺方案是解决问题的关键。在海洋含天然气水合物地层中钻进时,一方面要考虑钻井液性能和温度,保证其能在抑制天然气水合物分解的前提下进行钻进,另一方面还必须考虑在井内地层环境温度较低情况下循环钻井液的选择,该类地层钻进方法主要为分解抑制法,即通过采用适当的钻井液密度,维持井内压力,冷却钻井液以及调整相关钻进参数,将天然气水合物维持在稳定状态的钻进方法。低温钻井液一是抑制井内出露天然气水合物的分解,二是抑制钻井液中天然气水合物的形成。
全文共分六章,主要内容如下：
第一章：介绍了天然气水合物的结构、性质和特点,对国内外天然气水合物研究的历程进行了回顾,对水合物勘探钻井情况进行了总结,然后分析了水合物钻井所面临的主要问题,据此提出了本论文的研究内容及技术路线；
第二章：首先对天然气水合物沉积地层的性质进行了总结,包括天然气水合物的分布形式、天然气水合物稳定区以及天然气水合物微观模型,然后对海洋水合物地层的特点进行了分析,包括地质特征、孔隙度和水合物饱和度、渗透性,有效导热系数、力学性质以及相平衡特征等。最后指出了天然气水合物藏的分类方法及开发利用水合物资源所应采取的战略。
第三章：对天然气水合物侵入地层的特征进行了分析,包括钻井液侵入水合物地层的过程及主要特性,水合物在多孔介质中的分解特性,并建立了钻井液侵入水合物地层的基本模型,进行了数值模拟研究,分析了模拟结果和保持井壁稳定对钻井液性能的要求。
第四章：研究了水合物地层钻井中使用的硅酸盐钻井液体系。首先分析了抑制水合物生成的钻井液应用现状、抑制水合物分解的钻井液工艺和水合物抑制剂的国内外研究现状；接着讨论了天然气水合物钻井对钻井液的性能要求；最后分别从海洋钻井液特点及性能要求、硅酸盐钻井液研究现状及其井壁稳定机理、硅酸盐钻井液体系主要处理剂的优选、不同处理剂对硅酸盐钻井液流变性的影响及钻井液处理剂对水合物的影响等方面详细研究了硅酸盐钻井液体系的性能。
第五章：对所得硅酸盐钻井液对水合物生成和分解抑制的影响进行了相关实验研究。首先分析了新型水合物动力学抑制剂的研究现状,然后分别进行了钻井液对水合物生成和分解的抑制实验,并对实验结果进行了分析和研究。
第六章：给出了论文研究的主要结论与认识,指出了论文存在的不足,提出了对今后研究的发展方向,并说明了论文的主要创新点。
通过上述的理论分析与实验研究,初步取得了一些有意义和具有实用价值的成果,得到了以下几点结论和认识：
(1)在海洋天然气水合物地层中钻进时,由于储层井壁和井底附近地层应力会释放,地层压力降低。钻头切削岩石、井底钻具与井壁及岩心的摩擦都会产生大量的热能,使水合物稳定存在的温度压力条件被破坏,引发水合物的分解,造成井壁失稳,从而对钻井作业造成不利影响。要确保天然气水合物地层钻井安全顺利地进行,就必须采取措施对井内温度和压力进行严格控制,利用具有良好低温性能的钻井液尽可能控制并降低水合物的分解程度,以保持井壁稳定和井内安全；
(2)钻井液侵入以及水合物的分解会不同程度地导致地层孔隙压力增大,进而对安全钻井产生不利影响。如果地层渗透性较差而水合物受热很快,分解产生的气体和水不能及时流走,会导致孔隙压力急剧增大,其增加程度取决于沉积层的渗透系数和增温速度。孔隙压力增加得越大,安全钻进的钻井液密度范围越小,越不利于正常钻进。此外,孔隙压力增加使得保持井壁稳定所需钻井液密度也要增大,这不仅严重影响钻速,而且增加井壁渗透,使井周的孔隙压力增大,从而进一步降低井壁的稳定性,形成恶性循环,大大增加井眼的复杂程度；
(3)钻井液侵入以及水合物的分解引起的孔隙压力增大也会使骨架应力降低,地层抵抗破坏的能力下降。尤其是在近井壁区域,由于受井内温度扰动大,水合物分解比较剧烈,加之水力梯度大,导致此范围内的孔隙压力增加程度高,成为井壁最脆弱易失稳区域。其中,水合物分解对井壁应力场的影响是通过改变孔隙压力进而改变地层有效应力的方式来表现的；
(4)钻井过程中地层含水量由于进入地层的钻井液滤液与地层中水合物分解产生的水而逐渐变大。当地层含水量过高时,会使水合物地层骨架水化加剧,导致井壁更加不稳。而且,水合物分解还会使地层的渗透性增大,钻井液向井壁渗透产生渗透压力,使地层的坍塌压力提高,破裂压力降低,使井壁更易坍塌与压裂,降低了井壁稳定性；
(5)从钻井液的角度来看,通过降低自身的温度,使其接近原始地层的温度,就能减少对地层中水合物的热扰动,降低分解速度,从而降低地层中的孔隙压力和含水量升高的速度和幅度。同时,选择适当的钻井液密度,也有利于稳定地层中的水合物。此外,具有良好滤失护壁性能的钻井液能够在井壁上形成致密不渗透泥皮,控制地层的水化分散,也能促进井壁的稳定。因此,设计合适的钻井液体系是水合物地层钻井工作中的重要环节,其中具有协同防塌效果的钻井液处理剂和水合物分解抑制剂应是今后实验研究的重点；
(6)钻井液温度的降低会对其性能(尤其是其流变性与滤失性)产生较大影响。其中,热力学抑制剂与动力学抑制剂的添加不仅对钻井液流变、滤失等常规性能产生影响,而且更为重要的是,其对水合物的生成与分解抑制也起到至关重要的作用。因此,水合物地层钻井液体系研究中需要考虑两者的配合使用,从而保证钻井液具有良好的低温性能与水合物抑制性能；
(7)通过钻井液低温流变性测试,结合水合物的生成抑制与分解抑制实验,得到了含热力学与动力学抑制剂的硅酸盐钻井液推荐配方,①膨润土浆+0.3-0.5%HV-PAC+2%SMP-2+3%Na2SiO3+3%KCl+0.5-1%PVPK90+0.2-0.4%XC+10%NaCl,②膨润土浆+1-2%LV-CMC+2% SMP-2+3%Na2SiO3+3%KCl+0.5-1%PVPK90+0.2-0.4%XC+10%NaCl.
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.
要对赋藏在地下一定深处的天然气水合物进行勘探和开发,钻井是必不可少的重要手段。天然气水合物的相平衡性质决定了此类地层钻井与一般油气地层钻井有很大不同,也导致这类地层钻井面临更加复杂的井内问题。首先在钻井时,储层井壁和井底附近地层应力会释放,地层压力会降低；同时,钻头切削岩石、井底钻具与井壁及岩心的摩擦都会产生大量的热能,此外循环泥浆温度控制不当,这些都可能使孔内温度升高。在钻井过程中井壁地层压力和温度的变化将不可避免地导致天然气水合物发生分解。当固态水合物起胶结或骨架支撑作用时,分解本身就会使井壁坍塌。而分解产生的水增加了井壁地层的含水量,使颗粒间的联系减弱,导致井壁不稳；逸出的气体又影响了钻井液的比重和流变性,对井壁稳定愈发不利,甚至还可能引发井涌和井喷等钻井事故。其次,钻井是一个非绝热过程,钻井液与地层间的热交换和水合物分解时吸热会导致循环钻井液和井内的温度发生变化,使钻井液的关键参数发生变化,如粘度、密度和化学稳定性等,井内的应力和孔隙水压力也会发生改变。最后,水合物分解释放的气体进入井内,与钻井液一起上返到地表。在此过程中,如果温度和压力条件适当,在钻杆或阀门,特别是防喷器等部位还会生成水合物栓塞。而且,含水合物地层一般为未固结或半固结砂岩或泥质砂岩,这使井内稳定的问题更加严峻。井壁的不稳定会导致井壁坍塌、卡钻、压裂、钻井液漏失或井控失败。在某些极端的情况下,还会造成钻井报废,甚至人员伤亡和钻井设备损坏。因此,保证井眼温度和流动安全是水合物地层钻井的关键。
本文的研究来自863计划专项课题——“海底天然气水合物地层钻井钻井液工艺技术研究”(课题编号：2006AA09Z316),属于探索性研究课题。针对海洋天然气水合物钻井方面所面临的难题,本文认为制定适合天然气水合物钻井的钻井工艺,设计满足要求的钻井液工艺方案是解决问题的关键。在海洋含天然气水合物地层中钻进时,一方面要考虑钻井液性能和温度,保证其能在抑制天然气水合物分解的前提下进行钻进,另一方面还必须考虑在井内地层环境温度较低情况下循环钻井液的选择,该类地层钻进方法主要为分解抑制法,即通过采用适当的钻井液密度,维持井内压力,冷却钻井液以及调整相关钻进参数,将天然气水合物维持在稳定状态的钻进方法。低温钻井液一是抑制井内出露天然气水合物的分解,二是抑制钻井液中天然气水合物的形成。
全文共分六章,主要内容如下：
第一章：介绍了天然气水合物的结构、性质和特点,对国内外天然气水合物研究的历程进行了回顾,对水合物勘探钻井情况进行了总结,然后分析了水合物钻井所面临的主要问题,据此提出了本论文的研究内容及技术路线；
第二章：首先对天然气水合物沉积地层的性质进行了总结,包括天然气水合物的分布形式、天然气水合物稳定区以及天然气水合物微观模型,然后对海洋水合物地层的特点进行了分析,包括地质特征、孔隙度和水合物饱和度、渗透性,有效导热系数、力学性质以及相平衡特征等。最后指出了天然气水合物藏的分类方法及开发利用水合物资源所应采取的战略。
第三章：对天然气水合物侵入地层的特征进行了分析,包括钻井液侵入水合物地层的过程及主要特性,水合物在多孔介质中的分解特性,并建立了钻井液侵入水合物地层的基本模型,进行了数值模拟研究,分析了模拟结果和保持井壁稳定对钻井液性能的要求。
第四章：研究了水合物地层钻井中使用的硅酸盐钻井液体系。首先分析了抑制水合物生成的钻井液应用现状、抑制水合物分解的钻井液工艺和水合物抑制剂的国内外研究现状；接着讨论了天然气水合物钻井对钻井液的性能要求；最后分别从海洋钻井液特点及性能要求、硅酸盐钻井液研究现状及其井壁稳定机理、硅酸盐钻井液体系主要处理剂的优选、不同处理剂对硅酸盐钻井液流变性的影响及钻井液处理剂对水合物的影响等方面详细研究了硅酸盐钻井液体系的性能。
第五章：对所得硅酸盐钻井液对水合物生成和分解抑制的影响进行了相关实验研究。首先分析了新型水合物动力学抑制剂的研究现状,然后分别进行了钻井液对水合物生成和分解的抑制实验,并对实验结果进行了分析和研究。
第六章：给出了论文研究的主要结论与认识,指出了论文存在的不足,提出了对今后研究的发展方向,并说明了论文的主要创新点。
通过上述的理论分析与实验研究,初步取得了一些有意义和具有实用价值的成果,得到了以下几点结论和认识：
(1)在海洋天然气水合物地层中钻进时,由于储层井壁和井底附近地层应力会释放,地层压力降低。钻头切削岩石、井底钻具与井壁及岩心的摩擦都会产生大量的热能,使水合物稳定存在的温度压力条件被破坏,引发水合物的分解,造成井壁失稳,从而对钻井作业造成不利影响。要确保天然气水合物地层钻井安全顺利地进行,就必须采取措施对井内温度和压力进行严格控制,利用具有良好低温性能的钻井液尽可能控制并降低水合物的分解程度,以保持井壁稳定和井内安全；
(2)钻井液侵入以及水合物的分解会不同程度地导致地层孔隙压力增大,进而对安全钻井产生不利影响。如果地层渗透性较差而水合物受热很快,分解产生的气体和水不能及时流走,会导致孔隙压力急剧增大,其增加程度取决于沉积层的渗透系数和增温速度。孔隙压力增加得越大,安全钻进的钻井液密度范围越小,越不利于正常钻进。此外,孔隙压力增加使得保持井壁稳定所需钻井液密度也要增大,这不仅严重影响钻速,而且增加井壁渗透,使井周的孔隙压力增大,从而进一步降低井壁的稳定性,形成恶性循环,大大增加井眼的复杂程度；
(3)钻井液侵入以及水合物的分解引起的孔隙压力增大也会使骨架应力降低,地层抵抗破坏的能力下降。尤其是在近井壁区域,由于受井内温度扰动大,水合物分解比较剧烈,加之水力梯度大,导致此范围内的孔隙压力增加程度高,成为井壁最脆弱易失稳区域。其中,水合物分解对井壁应力场的影响是通过改变孔隙压力进而改变地层有效应力的方式来表现的；
(4)钻井过程中地层含水量由于进入地层的钻井液滤液与地层中水合物分解产生的水而逐渐变大。当地层含水量过高时,会使水合物地层骨架水化加剧,导致井壁更加不稳。而且,水合物分解还会使地层的渗透性增大,钻井液向井壁渗透产生渗透压力,使地层的坍塌压力提高,破裂压力降低,使井壁更易坍塌与压裂,降低了井壁稳定性；
(5)从钻井液的角度来看,通过降低自身的温度,使其接近原始地层的温度,就能减少对地层中水合物的热扰动,降低分解速度,从而降低地层中的孔隙压力和含水量升高的速度和幅度。同时,选择适当的钻井液密度,也有利于稳定地层中的水合物。此外,具有良好滤失护壁性能的钻井液能够在井壁上形成致密不渗透泥皮,控制地层的水化分散,也能促进井壁的稳定。因此,设计合适的钻井液体系是水合物地层钻井工作中的重要环节,其中具有协同防塌效果的钻井液处理剂和水合物分解抑制剂应是今后实验研究的重点；
(6)钻井液温度的降低会对其性能(尤其是其流变性与滤失性)产生较大影响。其中,热力学抑制剂与动力学抑制剂的添加不仅对钻井液流变、滤失等常规性能产生影响,而且更为重要的是,其对水合物的生成与分解抑制也起到至关重要的作用。因此,水合物地层钻井液体系研究中需要考虑两者的配合使用,从而保证钻井液具有良好的低温性能与水合物抑制性能；
(7)通过钻井液低温流变性测试,结合水合物的生成抑制与分解抑制实验,得到了含热力学与动力学抑制剂的硅酸盐钻井液推荐配方,①膨润土浆+0.3-0.5%HV-PAC+2%SMP-2+3%Na2SiO3+3%KCl+0.5-1%PVPK90+0.2-0.4%XC+10%NaCl,②膨润土浆+1-2%LV-CMC+2% SMP-2+3%Na2SiO3+3%KCl+0.5-1%PVPK90+0.2-0.4%XC+10%NaCl.
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.
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