| Natural gas. as a kind of clean energy. is being more and more important in the strategic position of energy security in our nation. And there is an upward trend in the degree of foreign dependence on natural gas. The exploration and development of shale gas, which is an alternative energy of natural gas, attract more and more concern and attention. Oil-based drilling fluid and horizontal well technology are mostly used in developing shale gas around the world and the technology is relatively mature that has achieved good results. The development in our nation is still elementary, and the technology of oil-based drilling fluid is not mature enough comparing with abroad where there is a certain gap. Therefore we must develop our own oil-based drilling fluid for shale gas horizontal well, in order to meet the need of shale gas independent scale development by carrying on systematic research. From the existing field drilling experience, we know that the sealing character and borehole stability on shale of oil-based drilling fluid is still bothering well drilling. For this reason, a set of oil-based drilling fluid has been created, based on analyzing the fabric characteristics of shale from Longmaxi formation in Weiyuan and the existing drilling data. The following are the results achieved and awareness:1) The clay mineral in the shale from Longmaxi formation ranges from29.53%to37.43%, quartz content distributes in33.9%～33.9%. brittle mineral distributes in54.69%～54.69%with a small amount of dolomite; The clay mineral mainly consists of illite and containing illite mixed layer, with no montmorillonite, presenting weak dilatability. Bedding, macro cracks, micro cracks, micro pore are developed, the micro crack opening can reach to more than5microns. So the sealing character of oil-based drilling fluid must be intensified.2) The original rock shows high strength and brittleness which ma)’lead to borehole’s easily broken. Therefore, better ability of borehole stability on shale is required.3) By optimizing additive. I have formed the oil-based drilling fluid, which is composed of5#white oil+25%CaCl2(oil/water ratio of85:15)+3%emulsifier ETS-1+1%CaO+2%fluid loss agent CF-1%wettability WEP+1%sealing agent CB+0.6%CaCO3(800mesh)+0.5%shearing potentiator RZL+barite. The API filtration quantity of the system is0. the HTHP filtration quantity is less than2mL, the emulsion-breaking voltage is beyond1200V. The system has better HTHP rheology behavior, stability, inhibitive ability and anti-pollution capacity. The experiment that evaluating sealing property of the system shows good result. The depth of invading sand bed is4cm under the pressure0.69MPa within30min and the HTHP filtration quantity in PPA is0.4) During drilling the horizontal part of WEI-A shale gas well, the drilling fluid showed good stability, inhibitive ability, lubricity, debris carrying capacity and borehole stability on shale, and no obvious collapse occurred in the whole well section. The rheological property of the drilling fluid system was quite stable that can meet the requirement of the horizontal section, but the rheological property can be optimized sequentially. |
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