Correction of linear fracture density and error analysis using underground borehole data

IF 2.6 2区 地球科学 Q2 GEOSCIENCES, MULTIDISCIPLINARY
Dongsheng Cao , Lianbo Zeng , Enrique Gomez-Rivas , Lei Gong , Guoping Liu , Guoqing Lu , Paul D. Bons
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引用次数: 0

Abstract

Fracture data acquired from drill core and borehole image logs require corrections for the bias due to fracture orientation, that are usually achieved by the Terzaghi correction technique. Previous studies often approximate the wellbore as a one-dimensional scanline, assuming that the length of the core axis within the sampling range is equal to the scanline length. This study refers to the commonly used workflow as the original Terzaghi correction method which is known to perform poorly when the angle (θ) between the core axis and the fracture is small. To address this issue, we propose an extension of the Terzaghi correction method that also considers the core diameter and is a function of both the fractures and the host layer dipping angle. The new method resolves the fracture density problem by selecting a new direction for the scanline perpendicular to the fracture and calculating the projection length of the sampling space in this direction. The fracture spatial arrangements and observed number of sampled fractures mainly affect the performance of this new approach. However, possible errors in the new correction method pertaining to the equidistant fracture density should decrease with increasing number of sampled fractures. It is found that an acceptable correction range exists for equidistant fracture density, though, when the corrected density is not within the acceptable correction range, the observed sampled fractures will not be equal to the true observed value. This means the corrected fracture density would be in an unacceptable range of errors. Moreover, the new method can provide results within the acceptable range when the angle between the fracture and the core axis is less than 20°. At the same time, this new method is free from other disadvantages in the original Terzaghi correction method, particularly, when the ratio of layers’ thickness to the core diameter is low. Therefore, the improved approach presented here is especially applicable to thin layers (no more than two times the core diameter) and conditions where the angle between the fracture and the core axis is less than 20°, which can contribute to fracture density characterization in the subsurface.

利用地下钻孔数据进行线性断裂密度校正和误差分析
从钻孔岩心和井眼图像测井记录中获取的断裂数据需要对断裂走向造成的偏差进行校正,通常采用特扎吉校正技术。以往的研究通常将井筒近似为一维扫描线,假设取样范围内的岩心轴线长度等于扫描线长度。本研究将常用的工作流程称为原始 Terzaghi 校正方法,众所周知,当岩心轴线与裂缝之间的夹角 (θ) 较小时,该方法的性能较差。为解决这一问题,我们提出了特尔扎吉校正法的扩展方法,该方法还考虑了岩心直径,并且是裂缝和寄主层倾角的函数。新方法通过为垂直于裂缝的扫描线选择一个新方向并计算该方向上采样空间的投影长度来解决裂缝密度问题。裂缝空间排列和观测到的采样裂缝数量主要影响这一新方法的性能。不过,随着取样断裂数量的增加,新校正方法在等距断裂密度方面可能出现的误差也会减小。研究发现,等距断裂密度存在一个可接受的校正范围,不过,当校正密度不在可接受的校正范围内时,观测到的取样断裂将不等于真实的观测值。这意味着校正后的裂缝密度将处于不可接受的误差范围内。此外,当裂缝与岩心轴线的夹角小于 20°时,新方法可提供可接受范围内的结果。同时,这种新方法也不存在原始特尔扎吉校正方法的其他缺点,尤其是在岩层厚度与岩心直径之比很小的情况下。因此,本文介绍的改进方法尤其适用于薄层(不超过岩心直径的两倍)和断裂与岩心轴线夹角小于 20° 的情况,有助于地下断裂密度的表征。
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来源期刊
Journal of Structural Geology
Journal of Structural Geology 地学-地球科学综合
CiteScore
6.00
自引率
19.40%
发文量
192
审稿时长
15.7 weeks
期刊介绍: The Journal of Structural Geology publishes process-oriented investigations about structural geology using appropriate combinations of analog and digital field data, seismic reflection data, satellite-derived data, geometric analysis, kinematic analysis, laboratory experiments, computer visualizations, and analogue or numerical modelling on all scales. Contributions are encouraged to draw perspectives from rheology, rock mechanics, geophysics,metamorphism, sedimentology, petroleum geology, economic geology, geodynamics, planetary geology, tectonics and neotectonics to provide a more powerful understanding of deformation processes and systems. Given the visual nature of the discipline, supplementary materials that portray the data and analysis in 3-D or quasi 3-D manners, including the use of videos, and/or graphical abstracts can significantly strengthen the impact of contributions.
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