| Since the 1990's, the exploration and development of oil & gas world wide had turn their majorities from packed oil & gas field, high/normal pressured,permeability homogeneous sandstone reservoir, to the exploration and development of complex small and medium-sized oil and gas fields, fault block oil and gas reservoir, thin reservoir, low pressure, low permeability and low-yield oil and gas fields, heavy and extra-heavy oil reservoir. Under these situations, underbalanced drilling technology using low-density circulating fluid has been rapidly developed.At present, the newly explored oil and gas reserves are mainly in low or ultra-low permeability reservoir. Application of the underbalanced drilling technology can effectively protect the reservoir, increase the discovery and producing rate of reserves, and can solve engineering problems. In recent years, the UBD technology has been regarded as one of the major technologies of the CNPC to researching, testing, and popularizing, which played a key role in the founding of some important oil and gas fields in Sichuan, Daqing, Liaohe and some other areas.During the development of Puguang Gas Field, contractors encountered a series of engineering difficulties such as hard rock; heavily well loss and difficulty to control the well path, while using traditional mud drilling means more drilling time, more accidents and higher costs. Gas drilling which made the full use of the advantages of UBD used there, had overcame the problems of low ROP, seriously loss happened in traditional drilling, and greatly accelerated the development pace of the gas field, however, complex down hole situations such as water invasion and blasting restrict the use of these technology. Foam drilling technology is getting more popular in oil companies because of it's unique advantages, and be used more and more in Puguang gas field.Foam drilling as a branch of UBD has the following main features:1. High ROP and longer bit life;2. Little formation damage, low polluting to cores and cuttings, and be conducive to the timely discovery of the reservoir;3. In comparing with the pure gas drilling, foam drilling are more suitable in watered situations, and lower air volume is needed;4. Foam with certain flexibility can provide well protection to drill strings, moreover, it can reducing the possibilities of down hole explosion;5. During the foam drilling, the annular upward velocity is low, which is conducive to the protection of the borehole; 6. Strong suspension ability, which is about 10 times that of conventional drilling fluid;7. Low consumption of water, just about 1 / 10 to 1 / 500 of liquid drilling fluid; which is suitable for drilling in drought and water shortage areas, high mountains, cold or freezing areas.The foam used in foam drilling is nonreusable. Stable characteristics of foam are necessary for a successful drilling, of which foam stability is a very important factor. As to the theoretical study on foam stability, experts and scholars had given many different views, but some of which are controversial. For example, as reasons to cause the bursting of bubbles, most scholars thought that the unload liquid in film of bubbles caused the rupture. While Excpova thought that foam stability has nothing to do with the unload liquid in film. Domestic researches were mainly focus on the practical applications; further study on theory part is needed.In this article, the author studied the foam stability mechanism in connection with the formula, temperature, pressure, solid and formation water by means of experiments, in the sake of some regularities and suggestions for practical uses.In the first chapter of the article, the author gave a brief introduction of the background and purpose of the article, and also, introduced the basic characteristics of foam drilling; and then gave some introductions on the developments of foam drilling in applications and theoretical fields at home and abroad.Chapter II introduced the basic knowledge of foam fluid. In the chapter, relevant knowledge on foam, performance, stability and rheology character is recounted respectively and summed up.Chapter III mainly discussed the influence on foam properties by temperature. At first, it discussed the basic principles of foam compound design, principles in selection and contentment determination of foaming agents and stabilizer; in the second section, it gives a detailed introduction on micelle forming mechanism and its influential factors of surfactants.It draws the following conclusions through experiments and analyses in the third section:1. Formability of foaming liquor in closely related to the type of surfactants, stabilizers it uses and their concentration, foaming temperature.2. When rising the foaming temperature, coefficient of foaming will increasing by a certain range; temperature operate on the stability of foam is presented as an inverted parabola; the coefficient increases as temperature rises up, when the temperature exceeds a certain value, it decreases.Studies of foam stability impacted by pressure, solid contentment and formation water are carried out in chapter IV. The main results are:1. A pressuring and observation equipment is designed and manufactured, which can be used to conducting the pressure experiments, and real-time observation; by certain modification, it also can be used to carry the permeability experiments of loose soil and sand-like samples;2. With the pressure rises up, and the degree of compression increases, foam stability declines;3. Bubble sizes and film thicknesses in foam system are in largely diversity, small bubbles in the foam system quickened the dying out process of foam system, existence of small bubbles have no conducive to the stability system;4. Solid on its own has little effect to the foam stability, while when the formation water encountered, its stability will shapely deterioration;5. Foam's suspension ability will drop with the declining of foam quality, but if the foam quality is too high, is will also not conducive to suspense cuttings because of thin liquid film, and low film strength.Conclusions and suggestions are drawn in Chapter V.
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