在犹他州地热能源研究前沿观测站利用通电套管源进行深孔电磁测量的三维建模,以绘制断裂图

IF 1.8 3区 地球科学 Q3 GEOCHEMISTRY & GEOPHYSICS
Evan Schankee Um, David Alumbaugh, Joseph Capriotti, Michael Wilt, Edward Nichols, Yaoguo Li, Seogi Kang, Kazumi Osato
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引用次数: 0

摘要

我们介绍了三维数值建模分析,评估了在犹他州地热能源研究前沿观测站地热站点部署井眼电磁测量工具以探测和成像激发区的情况。由于地热储层的深度达数公里,而激发区的大小仅限于数百米,因此基于地表的可控源电磁测量缺乏灵敏度,无法探测激发引起的电阻率变化。为了克服这一限制,该研究评估了在地热能源研究前沿观测站使用三组件钻孔磁接收器系统的可行性。为了在强化地热储层内部和周围提供足够的电流,我们使用一口注入井作为通电套管源。为了在逼真的三维电阻率模型中有效模拟注水井通电,我们引入了一种新颖的建模工作流程,充分利用了基于三维圆柱网格的电磁建模代码和基于三维四面体网格的电磁建模代码的优势。前者特别适用于对壳体等空心圆柱形物体进行建模,而后者则擅长表现更复杂的三维地质结构。在此工作流程中,我们的第一步是使用三维圆柱形电磁建模代码计算垂直钢套管井沿线的电流密度。随后,我们在复杂的三维电阻率模型中沿井轨迹分布一系列等效电流源。然后,我们使用四面体网格将该模型离散化,并使用三维有限元电磁代码模拟套管源激发的井眼电磁响应。这种多步骤方法使我们能够在复杂的三维电阻率模型中模拟三维套管源电磁响应,而无需使用过多的精细单元对油井进行明确离散。我们将在通电井偏离的电磁建模场景(如地热能源研究前沿观测站)中讨论所建议的工作流程的适用性和局限性。利用该工作流程,我们证明了结合使用通电套管源和井眼电磁接收器系统可提供可测量的磁场振幅和对深层局部受激区的灵敏度。测量结果还能区分平行裂缝各向异性储层和各向同性储层,为了解受刺激区的裂缝系统提供宝贵的信息。除磁场测量外,开放井段的垂直电场测量也对受刺激区高度敏感,可作为探测和成像目标的附加数据。我们还可以在不同位置将地面电极接地,重复进行井眼电磁测量,从而获取额外的多源数据。这种方法可使监测数据的数量增加几倍,为分析深部局部受激区提供更全面的数据集。数值分析表明,利用通电套管和井下电磁测量相结合的方法监测大深度局部激发区是可行的。
本文章由计算机程序翻译,如有差异,请以英文原文为准。
3D modeling of deep borehole electromagnetic measurements with energized casing source for fracture mapping at the Utah Frontier Observatory for Research in Geothermal Energy

We present a 3D numerical modelling analysis evaluating the deployment of a borehole electromagnetic measurement tool to detect and image a stimulated zone at the Utah Frontier Observatory for Research in Geothermal Energy geothermal site. As the depth to the geothermal reservoir is several kilometres and the size of the stimulated zone is limited to several 100 m, surface-based controlled-source electromagnetic measurements lack the sensitivity for detecting changes in electrical resistivity caused by the stimulation. To overcome the limitation, the study evaluates the feasibility of using a three-component borehole magnetic receiver system at the Frontier Observatory for Research in Geothermal Energy site. To provide sufficient currents inside and around the enhanced geothermal reservoir, we use an injection well as an energized casing source. To efficiently simulate energizing the injection well in a realistic 3D resistivity model, we introduce a novel modelling workflow that leverages the strengths of both 3D cylindrical-mesh-based electromagnetic modelling code and 3D tetrahedral-mesh-based electromagnetic modelling code. The former is particularly well-suited for modelling hollow cylindrical objects like casings, whereas the latter excels at representing more complex 3D geological structures. In this workflow, our initial step involves computing current densities along a vertical steel-cased well using a 3D cylindrical electromagnetic modelling code. Subsequently, we distribute a series of equivalent current sources along the well's trajectory within a complex 3D resistivity model. We then discretize this model using a tetrahedral mesh and simulate the borehole electromagnetic responses excited by the casing source using a 3D finite-element electromagnetic code. This multi-step approach enables us to simulate 3D casing source electromagnetic responses within a complex 3D resistivity model, without the need for explicit discretization of the well using an excessive number of fine cells. We discuss the applicability and limitations of this proposed workflow within an electromagnetic modelling scenario where an energized well is deviated, such as at the Frontier Observatory for Research in Geothermal Energy site. Using the workflow, we demonstrate that the combined use of the energized casing source and the borehole electromagnetic receiver system offer measurable magnetic field amplitudes and sensitivity to the deep localized stimulated zone. The measurements can also distinguish between parallel-fracture anisotropic reservoirs and isotropic cases, providing valuable insights into the fracture system of the stimulated zone. Besides the magnetic field measurements, vertical electric field measurements in the open well sections are also highly sensitive to the stimulated zone and can be used as additional data for detecting and imaging the target. We can also acquire additional multiple-source data by grounding the surface electrode at various locations and repeating borehole electromagnetic measurements. This approach can increase the number of monitoring data by several factors, providing a more comprehensive dataset for analysing the deep-localized stimulated zone. The numerical analysis indicates that it is feasible to use the combination of the energized casing and downhole electromagnetic measurements in monitoring localized stimulated zone at large depths.

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来源期刊
Geophysical Prospecting
Geophysical Prospecting 地学-地球化学与地球物理
CiteScore
4.90
自引率
11.50%
发文量
118
审稿时长
4.5 months
期刊介绍: Geophysical Prospecting publishes the best in primary research on the science of geophysics as it applies to the exploration, evaluation and extraction of earth resources. Drawing heavily on contributions from researchers in the oil and mineral exploration industries, the journal has a very practical slant. Although the journal provides a valuable forum for communication among workers in these fields, it is also ideally suited to researchers in academic geophysics.
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