垂直串珠串结构碳酸盐岩断喀斯特储层井压力动态分析——以塔里木盆地顺北油田为例

Cao Wei, Shiqing Cheng, Yang Wang, Ruyuan Shang, Langtao Zhu, Haiyang Yu
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摘要

塔里木盆地新发现的断喀斯特碳酸盐岩储层是由大规模构造断裂活动和多期岩溶作用形成的。地面露头、地震反射和测井资料表明,储层中存在大型溶洞、孔洞、构造裂缝和基质共存。裂缝与溶洞串联,形成垂直串珠结构是最常见的模式。结果表明,常规模型难以与实测压力数据相匹配。为了填补这一空白,本文总结了该结构的三种模式,并提出了三种新的模型来估计地层性质。通过简化垂向串珠结构,建立了多缝区-多洞区串联连接的物理模型。建立了相应的数学模型,其中裂缝区流动服从达西定律,溶洞区流动服从自由流动。重要的是,重力被认为是由于沿垂直方向流动。然后对典型流型进行了分析,并进行了敏感性分析。研究表明,压力导数曲线在溶洞储层和孔洞/基质间渗流层中表现出相似的“凹”特征。不同之处在于,溶洞储层的压力导数为单位斜率,而孔洞/基质渗流的压力导数不等于单位斜率。因此,通过压力导数斜率可以区分大溶洞介质和孔洞/基质介质。不仅如此,在型曲线上垂直串珠结构的典型特征是洞型储集型和线性流动型交替出现。这一特性有助于工程师识别垂直串珠结构。一个新的发现是,重力效应会导致后期的单位斜率压力和压力导数,传统上认为这是一个封闭边界的存在,但事实并非如此。最后,将该方法应用于顺北油田两处传统方法难以获得较好拟合质量和解释结果的实例。除了常规属性,该方法还允许我们估计其他属性(如洞穴高度、洞穴半径),这些属性是传统方法难以获得的。
本文章由计算机程序翻译,如有差异,请以英文原文为准。
Pressure Transient Analysis of Wells in the Fault-Karst Carbonate Reservoirs with Vertical Beads-on-String Structure: Case Studies in Shunbei Oilfield, Tarim Basin of Northwestern China
The newly discovered the fault-karst carbonate reservoirs in Tarim Basin are formed by large-scale tectonic fault activities and multiple-stage karstification. The ground outcrop, seismic reflection and well logging show that the large caves, vugs, tectonic fractures and matrix coexist in the reservoirs. The fractures interconnecting with caves in series to form vertical beads-on-string structure is the most common pattern. It is found that conventional models are difficult to match the recorded pressure data. To fill this gap, this work summarizes three kinds of patterns for this structure and presents three novel models to estimate formation properties. The physical models of multi-fracture-region multi-cave-region series connection are established by simplifying vertical beads-on-string structure. The corresponding mathematical models are developed, in which the flow in fracture regions obey Darcy's law, while the flow in cave regions obeys free flow. Importantly, the gravity is considered due to the flow along vertical direction. Then typical flow regimes are analyzed and sensitivity analysis is conducted. Our work shows that pressure-derivative curves show similar ‘concave’ characteristic for the cave storage regime and vug/matrix interporosity flow regime. The difference is that the pressure derivative for cave storage regime is unit slope, while this slope is not equal to unity for vug/matrix interporosity flow regime. Therefore, large cave and vug/matrix medium can be distinguished by slope of pressure derivative. More than that, the typical characteristic of vertical beads-on-string structure on type curve is that the cave storage regimes and linear flow regimes alternately appear. This characteristic helps the engineers identify vertical beads-on-string structure. A novel finding is that gravity effect could lead to unit-slope pressure and pressure derivative at late times, which is traditional recognized as the presence of a closed boundary when it may not be the case. Lastly, this methodology is applied to two cases from Shunbei Oilfield in which it is difficult to obtain good fitting quality and interpretation results using traditional methods. Besides conventional properties, the proposed methodology allows us to estimate other properties (e.g., cave height, cave radius), which are not readily obtained from conventional methods.
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