Study of Tight Oil Flowing Mechanism and Reservoir Producing Conditions for South Edge of Ordos Basin

Xu Ting, Pu Jun, Qin Xuejie, Yi Wei, Song Wenfang
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Abstract

South Ordos sandstone reservoir is mainly featured by tiny pore, which mainstream throat radius is around 50nm, high filtration resistance, resulting in low oil productivity and more obvious non-linear seepage characteristics. As of low formation pressure, well production is poor and declines dramatically, therefore primary recovery is hard to sustain effective development for the reservoir. The core problem of tight oil development focuses on the evaluation of tight matrix flowing capability and reservoir producing condition. In the paper, in Ordos typical tight oil basin, by means of microscopic flowing simulation, numerical simulation as well as lab experiments results, single-phase and oil-water two-phrase flowing mechanisms have been analyzed, revealing tight oil single phase percolating resistance and movable oil saturation, providing key evaluation parameters for effective reservoir division. For oil-water two-phase flowing, Jamin effect is so serious that water flooding is hard to displace the oil in micro-pores, accordingly relative permeability and displacement efficiency are calculated. Tight matrix-fracture coupling model recovery mechanism have been analyzed, effective producing radius and mechanism of matrix are defined in the condition of fracturing horizontal wells developing, according to which productivity percentage of Ordos tight oil between fracture and matrix have been determined. On basis of geology evaluation and reservoir engineering analysis, correlation of geological properties-well dynamic characteristics are set up, then influencing factors have been studied to identify tight oil producing conditions on depletion development at different oil price. As different classified fracture developed in the reservoir, water flooding producing condition has been studied, laying the foundation for study of effective development method and technical strategy. Our research indicates that Ordos tight matrix is of low productivity, with movable water saturation increasing, well productivity sharp decline. During production period, production ratio from fracture is only amounted to 6~14% of accumulation oil. Fully excavating the potential of matrix reserves is predominant to achieve effective development of tight oil. Owing to high start-up pressure gradient, as high as 0.1~0.2MPa/m, for water flooding development, well spacing should be reduced to 50m□ to set up pressure response without fracture developing. While in Ordos basin natural fracture is developed, water channeling is so heavy that accumulative oil is lower than depletion method. CO2 start-up pressure gradient is far smaller than that of water flooding with composite EOR mechanisms, expected to be an effective injection medium for tight oil. It is a critical period how so many shut-in wells could be revitalized under low oil price condition. Relying on research results, Ordos tight oil new development method target has been determined, promoting application research and pilot test on CO2-gelled fracturing fluid and effective injection fluid sustaining matrix displacing pressure in tight oil development.
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鄂尔多斯盆地南缘致密油流动机理及产油条件研究
鄂尔多斯南部砂岩储层以微孔为主,主流喉道半径在50nm左右,过滤阻力大,导致产油能力低,非线性渗流特征较为明显。当地层压力较低时,油井产量较低且下降幅度较大,因此初次采收率难以维持油藏的有效开发。致密油开发的核心问题是致密基质流动能力和储层生产条件的评价。本文以鄂尔多斯典型致密油盆地为研究对象,通过微观渗流模拟、数值模拟和室内实验结果,分析了致密油单相和油水两相渗流机理,揭示了致密油单相渗流阻力和可动油饱和度,为有效划分储层提供了关键评价参数。油水两相流动时,由于贾敏效应严重,水驱难以驱替微孔中的油,因此计算了相对渗透率和驱替效率。分析了致密基质-裂缝耦合模型的采收率机理,确定了压裂水平井开发条件下的有效生产半径和基质的作用机理,据此确定了鄂尔多斯致密油在裂缝-基质间的产能百分比。在地质评价和储层工程分析的基础上,建立了致密油地质物性与井动态特征的相关性,研究了不同油价条件下致密油枯竭开发的影响因素。针对储层中发育的不同类型裂缝,研究了水驱开采条件,为研究有效的开发方法和技术策略奠定了基础。研究表明,鄂尔多斯致密基质产能较低,可动含水饱和度增大,井产能急剧下降。在生产期间,裂缝产油比例仅为聚集油的6~14%。充分挖掘基质储量潜力是实现致密油有效开发的关键。由于启动压力梯度大,可达0.1~0.2MPa/m,水驱开发时应将井距减小至50m□,以建立压力响应而不发生裂缝。鄂尔多斯盆地天然裂缝发育,水窜严重,累计产油量低于枯竭法。CO2启动压力梯度远小于复合EOR机制下的水驱,有望成为致密油的有效注入介质。在低油价条件下,如此多的关停井能否复产是一个关键时期。依托研究成果,确定了鄂尔多斯致密油新开发方法目标,推进了co2胶凝压裂液和有效注入液维持基质驱替压力在致密油开发中的应用研究和中试。
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