| The average permeability of Ⅲ and Ⅳ oil formations of Nan Yishan shallow reservoir is2.98and it is an ultra-low permeability reservoir with low natural productivity, fast production decline, high water injection pressure, and great difficulty in stable production after water breakthrough. Since the water flooding development taken in October,2003the development effects of the oilfield was improved to some extent. However, the problems in oilfield development still existed in the following main aspects:the adaptability of flooding pattern was weak, that is to say, the present inverted nine-spot flood pattern was unable to adapt to the pore-fracture reservoirs; the percentage of invalid and inefficient water injection was high, and stratum energy had an tendency to descend on the whole; properties inside the layer system varied a lot, reserves producing was low and the water-drive thickness was less than one third; the range of flood conformance in the plane was limited, the directions along which fractures grew were advantageous for water injection movement, and in the direction perpendicular to fractures the effect of water displacing oil was bad; most production wells relied on natural energy with low production and inefficiency.Aimed at the main problem in the development of Ⅲ and Ⅳ oil formations of Nan Yishan shallow reservoir, with a comprehensive application of multi-discipline theories, methods, technologies, and the combination of macro analysis and micro analysis, qualitative and quantitative analysis, dynamic analysis and static analysis, this paper studied the main influence factors on reservoir development, demonstrated the economic and technical policies for reasonable reservoir development, and acquired the following achievements and understandings.(1)The total amount of oil produced by oil wells in Nan Yishan shallow reservoir was connected with the period when oil wells had been put into production, but the structure location and reservoir characteristics was the main influence factor. The high-productivity wells (the total amount of oil is more than5000tons) of Ⅲ and Ⅳ oil formations gathered near Well Nanqian2-1at the northwest of Fault Eight. The amount of total oil production from oil wells directly reflected the characteristic of hydrocarbon accumulation. According to the distribution of relatively-high production wells, the structure shaft at the northwest of Fault Eight was the major part of oil-gas abundant zone in the block. Based on the exploitation status of the oil formations and the fitting of actual water content and degree of recovery, it was known that the effect of oilfield development was changing for the better, and this paper further analyzed the change patterns of reservoir pressure and dynamic fluid level. The strata energy of the layer system had a tendency to descend on the whole, reflecting that the injection-production system needed to be improved and the water injection work needed to be enhanced.(2)The flattened effective thickness of Ⅲ and Ⅳ oil formations was30.6m. For sand bodies the flattened effective thickness possible to form water drive of was22.7m,2.27times as much as the economical oil formation thickness10m at present. It theoretically met the requirements of subdivision strata series, but in practice the effective thickness is usually maintained at about twice of the economical thickness to fulfill the demand of development adjustment. Therefore, Ⅲ and Ⅳ oil formations didn’t have the effective thickness for subdividing strata series and based on integrated analysis the writer thought it was much reasonable to adopt a set of strata system to develop.(3)When the injector producer distance was around300m, sudden water flooding of oil wells would not take place according to the characteristics of water flood development in ultra-low permeability reservoirs. Sudden water flooding of some wells demonstrated that there were micro-fractures in the layer. Fractures of Ⅲ and Ⅳ oil formations mainly grew in the northwest and southeast, and only in part of the area between Fault Four and Fault Eight grew fractures in the northeast and southwest. In general, fractures didn’t develop much.(4) For Qaidam Basin which is seriously short of water resources, gas injection production is the tendency in the future. This paper analyzed the gas injection experiment and its effect on Nan Yishan reservoir. The pros and cons of the two development methods were comprehensively analyzed in this paper, and according to the characteristics of Nan Yishan shallow reservoir, the development method of water injection recharging energy was suitable for it.(5)At present for Ⅲ and Ⅳ oil formations "the inverted nine-spot of the numbers of the reasonable injection-production wells in Ⅲ and Ⅳ oil formations calculated by mobility ratio square root method was1to2.(10)When the price of crude oil was50to75$/bbl, the threshold controlled reserves of a single well in Ⅲ and Ⅳ oil formations was2.47—5.45×104t and the threshold oil production per day was0.74—1.64ton.(11)The production ability of a single well in Ⅲ and Ⅳ oil formations calculated by the producing oil index method was5.95ton per day and at the beginning of production the stable oil production per day is3.0ton.This paper made the most of the existing geological achievements and development dynamic data, analyzed the development status of the reservoir, studied the main problem in reservoir developments, used dynamic development to study the distribution patterns of surplus oil, demonstrated the suitable well pattern and spacing for the development of Nan Yishan Oilfield, deployed the scheme to improve the reservoir adjustment, and decreased the declining production rate of old wells in old blocks to realize highly effective exploitation and reasonable development. After the application of the achievements on the oilfield, it worked preliminarily, the strata energy began to recover and the declining production rate of oil wells slowed down. The practice proves that the demonstration of the techniques policies in this paper is reliable with good practicability and maneuverability and of significance in the development of the similar oilfields. |
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