综述+基于测井岩石物理性质聚类分析的“双甜点”识别方法：以致密砂岩储层为例

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【岩石脆性综述-经典回顾】岩石脆性指数及其在岩石工程不同领域中的应用：综述（关键词：岩石脆性；适用性；水力压裂；裂缝扩展；破碎效率）

文献信息：Meng F, Wong L N Y, Zhou H. Rock brittleness indices and their applications to different fields of rock engineering: A review[J]. Journal of Rock Mechanics and Geotechnical Engineering. 2021, 13(1): 27.4

摘要译文：

脆性是控制岩石在加载和卸载条件下力学行为和破坏特征的一个重要参数，例如可压性、可切削性、可钻性和岩爆倾向等都与岩石脆性密切相关。因此，正确评估岩石脆性非常重要。然而，岩石脆性的定义和测量方法繁多，且尚未标准化。本文首先对岩石脆性的定义进行了回顾，简要讨论了一些具有代表性的岩石脆性定义。研究了岩石脆性在岩石工程不同领域中的发展和作用。归纳了岩石力学相关文献中的80个脆性指数，并讨论了一些指数的测量方法、适用性和局限性。结果表明：（1）大量脆性指数和脆性定义都是基于对岩石破裂行为的不同关注角度获得的；（2）脆性指数通常不具有广泛适用性；（3）“脆性”这一术语常被滥用，许多脆性指数缺乏理论基础，不能真正反映岩石脆性。在本次调研的基础上，确定了三种测量方法：（1）断裂前的弹性变形；（2）峰后应力-应变曲线特征；（3）基于断裂力学理论的方法，这些方法还可以进一步完善和统一，成为标准的岩石脆性测量方法。对于岩石力学领域而言，建立具有代表性且实用性强的岩石脆性定义意义重大。本研究将对实际应用中选择合适的脆性指数提供全面的指导，并对岩石脆性标准的制定，岩石脆性测量方法的优选打下坚实的基础。

摘要原文：

Brittleness is an important parameter controlling the mechanical behavior and failure characteristics of rocks under loading and unloading conditions, such as fracability, cutability, drillability and rockburst proneness. As such, it is of high practical value to correctly evaluate rock brittleness. However, the definition and measurement method of rock brittleness have been very diverse and not yet been standardized. In this paper, the definitions of rock brittleness are firstly reviewed, and several representative definitions of rock brittleness are identified and briefly discussed. The development and role of rock brittleness in different fields of rock engineering are also studied Eighty brittleness indices publicly available in rock mechanics literature are compiled, and the measurement method, applicability and limitations of some indices are discussed. The results show that (1) the large number of brittleness indices and brittleness definitions is attributed to the different foci on the rock behavior when it breaks; (2) indices developed in one field usually are not directly applicable to other fields; and (3) the term "brittleness" is sometimes misused, and many empirically-obtained brittleness indices, which lack theoretical basis, fail to truly reflect rock brittleness. On the basis of this review, three measurement methods are identified, i.e. (1) elastic deformation before fracture, (2) shape of post-peak stress-strain curves, and (3) methods based on fracture mechanics theory, which have the potential to be further refined and unified to become the standard measurement methods of rock brittleness. It is highly beneficial for the rock mechanics community to develop a robust definition of rock brittleness. This study will undoubtedly provide a comprehensive timely reference for selecting an appropriate brittleness index for their applications, and will also pave the way for the development of a standard definition and measurement method of rock brittleness in the long term.

图1 材料在拉伸（左半部分）和压缩（右半部分）加载条件下的脆性和韧性行为。图右侧显示了试验样品在不同围压下的脆性和延展性特征。

图2 井筒附近波速随着远离井筒而增加的声波路径示意图

（研究认为，脆性高的岩石在钻井过程中容易在井壁产生大量微裂纹，导致径向弹性波速变化较大，以此可以通过测井数据判定储层脆性）

(a) (b)

图3 (a)贯入试验示意图; (b)三种不同脆性岩石的贯入试验曲线

（贯入试验常用来评价滚刀贯入过程中不同强度岩石的破坏行为，也可以间接表征岩石脆性）

(a) (b)

图4 (a)脆性试验的原理概述; (b)钻井速率指数（DRI）图版

（脆性高的材料易碎且破碎后颗粒小，冲击试验后脆性材料细颗粒占比较高，因此，可以通过冲击后的细粒含量作为岩石脆性的指标）

图4 点载荷试验（PLT）示意图

（将抗拉强度（S_t）和脆性（K_b）作为材料常数，分别通过不同h和P值的PLT的最佳拟合回归线的截距和斜率来确定材料脆性特征）

【致密砂岩油藏双甜点识别-前沿追踪】基于测井岩石物理性质聚类分析的“双甜点”识别方法：以致密砂岩储层为例（关键词：甜点识别、聚类分析、测井岩石物理性质、致密砂岩油藏）

文献信息：Xiaofeng Zhou; Jianguang Wei; Fahimeh Hadavimoghaddam; Abdumalik Gayubov; Mehdi Ostadhassan; Yinghe Chen; Yuan Wang; Bo Yang; Ibragimov Jovdat Jamshid ogli; Baoping Pu; Xiao Zhang; Songze Liu.“Double sweet spot" identification method via cluster analysis of petrophysical properties from well logg data: A case study of a tight sandstone reservoir[J]. Geoenergy Science and Engineering, 2023, Vol.227: 211821

摘要译文：

近年来，由于水平井和水力压裂技术的发展，非常规资源的勘探力度大幅增加。但是，高昂的钻井成本、使得水力压裂中射孔位置的选择至关重要。因此，识别储层“甜点”在石油和天然气领域有着重要意义。本研究提出了一种基于聚类分析测井岩石物理性质的“双甜点”识别新方法。首先，介绍了一种基于地球物理响应（脆性指数和渗透率）的工程甜点识别方法，用于对目标区域进行分类。进行三轴压缩实验以获得岩石样品的静态杨氏模量和泊松比。同时，为了计算动态杨氏模量和泊松比，提出了一种基于多元线性回归理论来确定横波时差的新模型。通过聚类分析，建立渗透率与归一化Rick man指数的交会图，然后根据储层岩石物理性质对目标区域中的工程甜点进行表征。通过绘制基于聚类分析理论的孔隙度和含油饱和度交汇图，对目标区域的地质甜点进行了表征，并将储层岩相划分为四类：砂岩 A（So＞48%，8＜φ＜19%）；砂岩 B（31＜So＜48%，6＜φ＜18%）；砂岩 C（18＜So＜31%，8＜φ＜20%）和砂岩 D（0＜So＜18%，8＜φ＜17%）。然后，运用提出的基于储层物理参数（含油饱和度和孔隙度）的地质甜点识别方法对目标地层进行分类。通过阿奇公式确定岩心电阻率与含水饱和度之间的关系，建立目标储层中的含油饱和度预测模型。根据 47 个岩心样品的实验孔隙度和渗透率测量值以及测井数据，提出了孔隙度解释模型。通过绘制基于聚类分析理论的孔隙度和含油饱和度的交汇图，对目标地层的地质甜点进行表征，将储层岩相划分为四类：砂岩 A（K＞0.1mD，BI＞38）；砂岩 B（K＞0.1mD，33＜BI＜38）；砂岩 C（K＞0.1mD，BI＜33）和砂岩 D（0.01＜K＜0.1mD，10＜BI＜60）。分析了大庆致密砂岩油藏井1的综合“双甜点”识别结果。岩相AA是最有利的储层岩相类型，岩相DD是最不利的储层岩相类型。最后，将本研究提出的综合“双甜点”识别方法应用于国内某致密砂岩油藏重复压裂井段的筛选。重复压裂后，发现井组平均产油量由1.10 吨/天逐渐增加至5.63 吨/天。该“双甜点”识别新方法对致密油储层具有较强的实用性。

摘要原文：

In recent years, interest in unconventional resource plays has grown substantially due to horizontal drilling and hydraulic fracturing techniques. However, the high cost of drilling, choosing the right locations for perforation intervals during hydraulic fracturing is a crucial issue. Therefore, identifying so called "sweet spot" with high potential for hydrocarbon production plays an important role in oil and gas industry. In this study a new "double sweet spot" identification method using cluster analysis of well-logged petrophysical properties of facies is presented. Firstly, an engineering sweet spot identification method based on geophysical responses parameters (brittleness index and permeability) is introduced to classify the target zone. Triaxial compression tests are performed to obtain the static Young' s modulus and Poisson' s ratio of the rock samples. To calculate dynamic Young' s modulus and Poisson' s ratio, a novel model for determining interval transit time of S-wave is proposed based on the multiple linear regression theory according to well logging data. By performing the cluster analysis, an intersection diagram of permeability and calculated normalized Rickman`s brittleness is built and then engineering sweet spots in target zone are characterized based on well-logged petrophysical properties of facies. By plotting the intersection diagram of porosity and oil saturation based on the cluster analysis theory, geological sweet spots in target zone were characterized and four types of rock petrophysical facies are divided: Sandstone A (So＞48%, 8＜φ＜19%); Sandstone B (31＜So＜48%, 6＜φ＜18%); Sandstone C (18＜So＜31%, 8＜φ＜20%) and Sandstone D (0＜So＜18%, 8＜φ＜17%). Secondly, target formation classification is carried out using a proposed geological sweet spot identification method based on reservoir physical parameters (oil saturation and porosity). The relationship between core electrical resistivity and water saturation is determined by traditional Archie' s formula and then an oil saturation prediction model in target reservoir is established. Based on the experimental porosity and permeability measurements of 47 core samples and well logging data, a porosity interpretation model is proposed. By plotting the intersection diagram of porosity and oil saturation based on the cluster analysis theory, geological sweet spots in target formation are characterized and four types of rock petrophysical facies are given: Sandstone A (K＞0.1mD, BI＞38); Sandstone B (K＞0.1mD, 33＜BI＜38); Sandstone C (K＞0.1mD, BI＜33) and Sandstone D (0.01＜K＜0.1mD, 10＜BI＜60). The integrated "double sweet spot" identification results of Well 1 in Daqing tight sandstone oil reservoir are given. Rock AA is the most favorable type of petrophysical rock facies and rock DD is the worst favorable type of petrophysical rock facies. Finally, an integrated "double sweet spot" identification method proposed in this study was applied to screen proper interval candidates during refracturing operation in a tight sandstone oil reservoir in China. After refracturing operation a gradually increase of average oil production in well group was found from 1.10 t/d to 5.63 t/d. This novel "double sweet spot" identification method has applicability in other tight oil reservoirs around the world.