Rapid evaluation of probable recoverable reserves in tight reservoirs: a case study of Chang 8 reservoir (eighth member of Yanchang Formation) in Honghe oil field
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摘要: 致密油藏储层致密、大面积含油、局部“甜点”富集、油水分布复杂以及后期工程改造带来的储层复杂性,决定了其单井开发动态与可采储量分布具有较大的差异性和较强的不确定性,很难用常规的储量评估方法得出合理的确定性认识。通过引入北美对非常规页岩油气藏储量的评估方法,采用已开发井确定单井储量,再用已开发井储量的概率分布特征来表征区块或者油气藏的储量。对红河油田长8致密特低渗油藏进行了研究,跟踪单井的初期生产动态和产量递减特征,分析可采储量与地质特征、含油性和初期生产动态的相关性,来寻找影响可采储量估算的关键因素。用P10/P90值来判断数据的不确定性,其值在3~8,可获得90%的置信度所需最小的样本量;该值如大于10,说明油藏具有地质、工程改造的复杂性和不同区块的差异性。通过不确定性法建立了关键影响参数和单井可采储量的概率分布,实现了单井可采储量的快速评估和评价。Abstract: Tight oil reservoirs are featured by tight formations, large oil area, locally enriched "sweet spots", complex oil-water distribution, and complicated reservoir conditions brought by later engineering transformation, which determines the development performance of single well. The distribution of recoverable reserves has great variability and uncertainty, so it is difficult to obtain a reasonable and definite understanding using conventional reserve evaluation methods. By introducing the evaluation methods of unconventional shale oil and gas reserves in North America, the developed wells were used to determine the reserves of single wells, and the probability distribution characteristics of reserves of developed wells were used to characterize the reserves of blocks or reservoirs. The Chang 8 (the eight member of Yanchang Formation) tight and ultra-low permeability reservoir in the Honghe oil field was studied. The initial production performance and production decline characteristics of a single well were tracked, and the correlations between recoverable reserves and geological characteristics, oil-bearing properties and initial production performance were analyzed to determine the key factors affecting the estimation of recoverable reserves. The P10/P90 values were used to judge the uncertainty of the estimates. The minimum sample size required for 90% confidence can be obtained when the value is 3-8; if the value is greater than 10, it indicates that the reservoir has the complexity of geological and engineering transformation and the difference of different blocks. The key influencing parameters and the probability distribution of recoverable reserves of single well were established by using the uncertainty method, and the rapid evaluation of recoverable reserves of a single well were realized.
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表 1 红河油田长8油藏不同初产规模的产量递减参数
Table 1. Comparison of production decline parameters of Chang 8 reservoir with different initial production scales in Honghe oil field
单井分类 井数/口 递减类型 第一段递减率/% 第二段递减率/% 平均单井前6个月累产/t 前6个月平均含水率/% 单井可采储量(EUR)/t >10 t/d 52 指数 71.81 48.76 2 584.50 64.16 7 189 5~10 t/d 44 指数 73.96 39.29 1 226.60 73.02 3 646 < 5 t/d 168 指数 47.85 19.64 311.39 87.53 2 021 表 2 SPEE专著三推荐的最小样本量
Table 2. Recommended minimum sample size in monograph 3 by SPEE
P10/P90值 推荐样本量 备注 2 15 不太可能出现的比率 3 35 常见比率 4 60 常见比率 5 75 常见比率 6 100 常见比率 8 130 常见比率 10 170 可能的数据质量/模拟问题 15 290 可能的数据质量/模拟问题 20 420 可能的数据质量/模拟问题 30 670 可能的数据质量/模拟问题 表 3 红河油田长8油藏单井可采储量及关键影响因素概率统计
Table 3. Statistical results of a single well EUR and key influencing factors in Chang 8 reservoir, Honghe oil field
不同概率 单井可采储量/t 初始产量/(t·d-1) 半年累产量/t 半年平均含水率/% P90 558.0 1.5 199.2 99.0 P50 2 642.1 5.7 834.0 51.4 P10 12 509.8 21.1 3 491.4 24.1 分析平均值 5 516.0 9.6 1 556.8 61.1 算数平均值 4 291.8 7.9 1 201.1 55.1 P10/P90 22.4 13.6 17.5 4.5 表 4 红河油田某区块长8油藏单井可采储量(EUR)及关键影响因素概率统计
Table 4. Statistical results of single well EUR and key influencing factors in Chang 8 reservoir, Honghe oil field
不同概率 单井可采储量/t 初始产量/(t·d-1) 半年累产量/t 半年平均含水率/% P90 2 567.6 3.1 287.4 87.6 P50 5 624.9 8.3 1 187.3 39.8 P10 12 322.8 22.0 4 904.7 18.0 分析平均值 6 783.2 11.1 2 190.8 48.1 算数平均值 6 301.2 9.6 1 643.3 43.2 P10/P90 4.8 7.1 17.1 4.9 -
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