| Fractured tight sandstone reservoirs are characterized by its particular engineering geologicy, and it is easily damaged by the invasion of drilling fluid during well drilling, which causes the low efficiency of petroleum reservoir developing. Scholars home and abroad have made a multitude of detailed and thorough researches over reservoir protection, and they have made substantial achievements and developed systematic reservoir protection techniques. However, current reservoir protection theories and techniques mainly focus on water-based drilling fluid, but few researches over oil-based drilling fluid reservoir protection have been conducted. Therefore, in the climate of increasing application of oil-based drilling fluid, it is necessary to study the oil-based drilling fluid reservoir protection.This paper, taking the geological features of fractured tight gas sandstone reservoir and the features of inflowing fluid as the breakthrough point, studies the formation mechanism of the oil-based drilling fluid’s damage, reveals features of single component damage of oil-based drilling fluid, and develops research on the main factors of the damage. Besides, this paper identifies damage mechanism of oil-based drilling fluid and then forms its protection techniques.Firstly, this research, by selecting fractured tight gas sandstone reservoir as the research object and combining the geological characteristics of fractured tight gas sandstone reservoir in Tarim and the use of fluid, studied the formation mechanism of oil-based drilling fluid’s damage to fractured tight gas sandstone reservoir from three aspects:multi-phase fluid, variation of rock strength and rheological property of oil-based drilling fluid, and determined the differences between oil-based drilling fluid and water-based drilling fluid in the aspects such as the causes and types of damage.This research identified the function mechanism and damage characteristics of elements of oil-based drilling fluid, such as the base oil, organic soil, emulsifier, filtrate reducer, wetting agent and weighting agent; theoretical and practical studies were conducted on the main factors of oil-based drilling fluid reservoir damage such as the effect of solid particles, emulsification effect, wetting property, invasion of gas and water, oil trap, features of pressure and temperature and alkalinity changes; and it is proved that particle effect, wetting property variation, oil trap, changes of pressure and temperature features are the main factors of oil-based drilling fluid’s damage to fractured tight gas sandstone.Two methods were adopted to comprehensively evaluate the damage of oil-based fluid to reservoir and develop the damage mechanism of oil-based drilling fluid to the fracture, matrix and slit pore surface of fractured tight gas sandstone reservoir by combining the research on the component damage characteristics and main factors of oil-based drilling fluid; it is proved that wettability assistance, capillary pressure, effects of granule loss and sensitivity damage are the main damage mechanisms of oil-based drilling fluid to the fracture; particulate plugging and liquid damage of slit pore surface are the damage mechanisms of slit pore surface.Owing to the specialty of oil-based drilling fluid reservoir protection, this research revealed the features of fracture and the influence of material characteristics on oil-based drilling fluid reservoir protection, formed its technical ideas, and constructed fractured tight gas sandstone reservoir and oil-based drilling fluid reservoir protection techniques; it studied the relationship between the adding of reservoir protection materials and reservoir protection effect. The result shows that as to the fractures less than 10μm, the adding of a large number of rigid materials can effectively improve reservoir protection effect; as to the fractures between 10 and 200μm, the adding of fibers and rigid materials exert little influence on reservoir protection function, and after acid soluble, fibrous skeleton has little influence on reservoir damage on the condition of acid-insoluble.Based on discrete element method, this research used granular flow numerical simulation technique, and built granular flow model of oil-based drilling fluid in fracture on the granular flow numerical simulation software, and studied the influence of different particle shapes, fracture lengths and grades on the crack plugging efficiency of oil-based drilling fluid. It indicates that granular flow simulation technique based on discrete element method possesses fine adaptability and can effectively simulate the physical heaping and plugging process of oil-based drilling fluid in fracture; the influence of particle shapes on plugging efficiency is caused by the irregularity of fractures and particles; there occur changes of fracture length mainly because external pressures promotes the fracture expansion, thus causing the inapplicability of originally matching particles or further distance between plugging area and wellbore; particle grade is an important factor that affect the plugging efficiency of oil-based drilling fluid, and suitable grade can effectively block and protect reservoir;Prediction on finite element method of dynamic width of the tight gas sandstone reservoir fracture was conducted; current oil-based drilling fluid was optimized from three angles:system design, identification of particles and the adding of them and the effect of particles on drilling fluid property; and field application was adopted. The experiment and field application indicate that after the optimizing, the oil-based drilling fluid has fine property and possesses fine reservoir protection effect.The result of this research systematically demonstrates the damage mechanism of oil-based drilling fluid to fractured tight gas sandstone reservoir, and deepens the realization of oil-based drilling fluid reservoir protection techniques, and provides reference for the reservoir protection in the application of oil-based drilling fluid. |
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