| Low permeability reservoirs are widely distributed in large oil and gas fields or main basins in China. Due to the extremely poor permeability of the rock matrix, the fracture is important for the obvious improvement of the rock physical property. However because of complexity of fracture development and distribution, it is difficult to develop low permeability oilfields. At present, as to the kind of oilfield, the producing degree of reserves is low, the development effect is unsatisfied, and the economic results are poor. Therefore, it is of great practical significance to research how to develop this kind of oilfield economically and effectively for stable and sustained development of Chinese petroleum industry.Changling is a north-south strike sag, which generates hydrocarbon in the south of Songliao Basin. The northern part of the sag is deeper than the south. Yaoyingtai Oilfield is located in the south of the sag, its oil layers are mainly distributed in Qingshankou Formation, and it is a structural-lithologic hydrocarbon reservoir. Its oil-water distribution is complicated; there is no unified oil-water contact in the whole region.It is revealed that the lithology character of the oilfield reservoir is mainly fine sandstone-siltstone, with thin sand body and good horizontal communication. It belongs to low-to-moderate porosity (12%-15%) and extremely low permeability (1×10-3μm2-10×10-3μm2), with developed fracture, artificial hydraulic fracture generally according with natural fracture, mainly going from east to west. The reservoir is of poor oil and water differentiation; in the partial horizons, the initial oil saturation is low, and oil and water are yielded at the same time in the initial period of development. Its natural recovery phase is not enough and the oil layers submitting most reserves at present have no natural productivity, so hydraulic fracturing is main completion method in the region. Except that, the stress sensitivity of the rock is severely.Geologic characteristics and fracture development of low permeability fracture reservoir have been recognized, by the way of studying on tectonic stress field and description of natural fracture in reservoir. Natural micro-fractures in Qingl segment are developed, most of which are shear fractures in groups. The direction of natural fractures is NE85.9°-NE153.6°, basically going from east to west. The direction of artificial fractures is in concord with that of natural fractures, which also goes from east to west basically. Under original formation conditions, most of the natural fractures are closed, that is to say, there are potential fractures existing, but the induced fractures are easy to be formed during drilling or oil testing. Through artificial fracturing, isolated dissolved pores and micro-pores can be communicated, so as to improve seepage flow of the low permeability reservoirs.By means of laboratory and field experiment investigation and optimization of fracturing technical design, the combined slanted line/multi-slug sand fracturing technique has been researched for fractured reservoir with high filtration, adopting multistage sand filling technique and combining with new type of proppant (precoated sand), and the obvious stimulation effect has been obtained. The average oil produced per day of the single well has been improved from 3.2t in 2005 to 3.2t in 2006 and 4.9t in 2007, which increases more than 50%. The water content has been reduced from 84% in 2005 to 77% in 2006 and 74% in 2007.The matched fracturing techniques have been put forward, such as the diagnostic technology for filtration of natural fractures, the net pressure fitting technology for fracturing, the fluid loss control technology by using of sands in dual media, the optimization of fracture parameters, the optimization of fracturing fluid and proppant concentration, the fracture height control technology and the post-fracture fluid discharging technology.
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