QUANTITATIVE ESTIMATES OF OIL LOSSES DURING MIGRATION, PART II: MEASUREMENT OF THE RESIDUAL OIL SATURATION IN MIGRATION PATHWAYS

Recent experimental observations have demonstrated that losses of oil in a secondary migration pathway depend largely on the characteristics of the pathway itself. Accurate assessments of the residual saturation are required to quantify losses during migration. In the present paper, the authors report on experimental procedures designed to evaluate the oil saturation at various stages of the migration/invasion process. The experiments made use of both apparatuses filled with an artificial medium composed of glass beads, and cores composed of reservoir sandstones from oilfields in NE China. The saturation of residual oil in the pathways was measured by nuclear magnetic resonance imaging.
Experiment results show that mobile oil continues to migrate in a porous medium when the supply of oil has been stopped, but that the migration pathway shrinks and may become disconnected into isolated segments. Once active migration ceases, the residual oil saturation varies significantly from location to location within a pathway, and the average residual oil saturation falls to 30∼60%, depending on grain size and composition. A porous medium composed of large grains commonly contains large pores and pore throats and thus may correlate with low residual oil saturations.     

QUANTITATIVE ESTIMATES OF OIL LOSSES DURING MIGRATION, PART I: THE SATURATION OF PATHWAYS IN CARRIER BEDS

Estimates of hydrocarbon losses during migration are critical to petroleum resource assessments based on mass balance calculations. Using knowledge acquired from physical experiments, we conducted numerical experiments to qualitatively simulate migration processes on a basin scale, and we have estimated the proportions of oil lost along different parts of the migration pathway. Between the point where oil is expelled from a source rock and its arrival in a trap, migration pathways were divided into three sections, namely vertical and lateral pathways within the area of the effective source rock (W₁), and lateral pathways outside this area (W₂).
With reference to the concept of an independent migration unit (IMU), a prism-shaped domain with square top and bottom faces, the proportion of vertical pathways (Q₁) and lateral pathways (Q₂) in carrier beds in W₁ may be estimated, using parameters obtained in migration experiments. The proportion of oil lost in W₂ was obtained by statistical analysis of modelled results. It is found that the loss of oil within W₂ decreases rapidly with increasing distance from the contact between W₁ and W₂.
Relative oil losses in migration pathways were estimated by considering two typical migration models: a sloping rectangular plate model, and a circular synclinal model. Simulation results show that, for sandstone carrier beds at a basin scale, Q₂ is one order of magnitude less than Q₁, and losses (Q₃) in W₂ are two orders of magnitude less than those (Q₁+ Q₂) in W₁.     

核磁共振成像技术分析油运移过程中含油饱和度

利用核磁共振成像技术，观察玻璃珠填装管状模型与河砂填装管状模型内油的运移过程，测量并分析油运移路径形态特征和内部含油饱和度。原始油柱内油饱和度可达83％；路径形成油沿路径运移时，油在路径内的饱和度为40％～50％；运移完成后，在没有油继续供给的条件下，原有路径收缩，宽度减小，路径内的油呈不连续状态，残余油饱和度降为20％～35％。实验装置中孔隙介质的颗粒成分对路径内的油饱和度有一定影响。图2表1参16     

石油二次运移不同模式条件下含油饱和度测量

石油二次运移具有活塞式、指进式、优势式3种不同模式。利用湿填模型,在仅有浮力作为运移动力、以泵压和浮力综合作用作为运移动力的实验条件下,用不同粒度、不同充填介质填装的玻璃管模型系统,观测在3种运移模式条件下路径含油饱和度、残余油饱和度及残余油在通道中所占比例等变化规律。结果表明,路径含油饱和度和残余油量占通道的比例因运移模式不同而明显不同,残余油饱和度则基本不受运移模式的影响。     

核磁共振成像技术在油气运移路径观察与分析中的应用

利用核磁共振成像技术观察圆管填玻璃珠物理模型内油的运移过程,对成像处理方法进行改进后,可以定量地观察和分析运移路径的形态特征及其内部的油饱和度.实验结果表明,被油占据的运移路径在整个通道内的比例取决于路径的形态,而路径内的油饱和度并非100%.在实验模型的尺度上,运移路径上的油饱和度在运移过程中可达80%以上;在运移结束后,因运移路径收缩、分离,油饱和度只占20%～0%,甚至更低.     

Migration and occurrence of high wax oils in the Damintun Depression, Northeast, China: Implication for primary controls of petroleum migration pathways in heterogeneous carrier beds

This paper discusses migration styles and primary controls of petroleum migration pathways in heterogeneous carrier beds in the Damintun Depression, Bohai Bay Basin. There are two types of crude oils in the Damintun Depression: a high wax oil (wax hydrocarbon content between 8 and 60%) and a normal oil (wax hydrocarbon content lower than 8%). The high wax oil and normal oil are distinctly different both in biomarker compositions and in distribution patterns of carbon isotopic ratios for individual alkanes, and were confirmed to have been generated from different source rocks. Modeling of secondary migration pathways of both the high wax oil and the normal oil was conducted using a simple three-dimensional model, which assumes that the positions of petroleum migration pathways are controlled by the morphology of the sealing surfaces without taking into consideration the influence of permeability heterogeneity (the morphology-controlled migration model). The modeling results have reasonably well predicted the occurrences of both the high wax oil and the normal oil. All commercial petroleum accumulations are along the predicted preferential petroleum migration pathways (PPMP) formed by focusing of numerous “small petroleum streams” close to the kitchens. The focusing of oils originating from a large area of the generative kitchens is essential for the formation of large oilfields. The strong porosity and permeability heterogeneities of the carrier beds, and the relatively high prediction accuracy of the modeling, suggest that the preferential petroleum migration pathways, although influenced by permeability contrast at local scales, can be effectively predicted at a basin or depression scale under certain circumstances using a model that does not take into consideration the effect of heterogeneity. Most dry holes are not on the predicted petroleum migration pathways, suggesting that the three-dimensional migration pathway modeling may provide a useful tool to reduce exploration risk.     

An interative procedure to determine depth and time of primary oil migration and expulsion efficiency of source rocks

This study seeks to propose an iterative procedure for estimating time and depth of primary oil migration and expulsion efficiency of a source rock. The procedure is based on two assumptions: (a) oil moves out of the source rock as a separate phase; and (b) oil migration does not start until residual oil saturation is reached. Oil saturation increases with oil generation and porosity reduction during burial. Changes in total volume, pore and oil volumes by oil generation and compaction are calculated at discrete time steps. Oil expulsion begins when oil saturation reaches residual oil saturation and relative permeability of oil rises above zero. The volume of oil expulsion can also be calculated iteratively after oil migration starts. The difference between oil generation and residual oil gives the volume of oil expulsion at each time step. The mass ratio of oil expelled to total oil that is generated provides the expulsion efficiency of the source rock.     

单井化学示踪剂法测残余油饱和度——回顾与展望

经过第一次和第二次采油后 ,还有大量的石油滞留地下深处不能被有效采出。而每年新发现的石油地质储量在不断减少 ,勘探开发的难度越来越大 ,这就要求必须采用先进的方法和技术将残余在地下的原油采出 ,也就有了确定SOR的需要。在进行三次采油的潜力评价中 ,SOR是一个至关重要的参数。测残余油饱和度的方法较多 ,但各有优缺点。单井化学示踪剂是其中一种较为有潜力的方法 ,它始于2 0世纪 60年代后期 ,由Cooke首先提出 ,它利用地层的色谱效应 ,用示踪剂法测残余油饱和度。Deans于 1 967年用此法在室内成功地测定了模型内的残余油饱和度 ,1 968年应用于现场获得成功 ,1 971年取得了美国专利权。该法首次现场测试获得成功以来已广泛应用。单井示踪法成本低 ,测定简便 ,测定体积大并且可调 ,应用范围广 ,灵敏准确 ,是测定残余油饱和度的理想方法。作者调研了大量国内外资料 ,对单井化学示踪剂测残余油饱和度的产生背景、应用及其发展作了回顾 ,并在与其它测残余油饱和度进行对比的基础上 ,对其应用发展前景作了展望。     

碎屑岩输导层非均质性与油气运聚成藏

碎屑岩地层普遍具有非均质性。然而,在勘探尺度的油气运聚和成藏研究中,输导层内部成分、结构和物性等方面的非均质性被忽略,致使输导层的非均质性特征及其对地层流体和油气运聚的作用没有得到应有的重视。碎屑岩输导层的非均质性受沉积构造控制,在浅层埋藏过程中即已发生明显的差异性成岩作用,其中渗透性岩石被低渗透性隔夹层所分隔,具有一定的空间结构特征。在结构非均质性输导层中,油气运移路径的分布极不均匀,与传统上物性表现为宏观均质性的输导层模型相差甚远。结构非均质性输导层中的油气总体向上倾方向运移,在原先认为应发生垂直运移的层段运移的油气受隔夹层阻碍,运移路径变得十分复杂,侧向运移则可发生在输导层的上、中、下任意部位,但处于上倾方向的圈闭仍然是油气运移的最终指向和有利目标。在运移途中,油气可在任何地方聚集,其单个油气量较小,但数量众多,分布范围很广,总量可能远大于处于高点圈闭中的油气藏。结构非均质性输导层中,油气的运移路径和聚集方式与传统认识不同,为油气运聚和成藏过程的认识带来许多启示,勘探应该具有更为广泛的目标选择,洼陷区和斜坡区都可能成为有利勘探区域。     

原油二次运移过程中的逾渗主脊实验研究

利用Hele-Shaw模型开展二维空间原油持续二次运移过程实验研究。在初始运移路径形成后改变注入煤油的颜色,验证二次运移过程中的逾渗主脊现象,初步从机理上指出初始运移路径形成后毛管力的变化是逾渗主脊形成的主要原因。逾渗主脊的尺寸和分形维数都要小于对应的初始运移路径的尺寸和分形维数。油气以这样的方式发生二次运移,加速了运移速率,二次运移过程中的烃损失量只限于初始路径。