Greywacke basement rocks in New Zealand host conventional geothermal reservoirs and may supply important hotter and deeper geothermal energy resources in the future. This work combines petrological analyses and physical property measurements of Waipapa greywacke, a basement unit hosting New Zealand geothermal reservoirs, in order to understand better how structurally controlled flow networks develop and channel geothermal fluids within it. Results show intact Waipapa greywacke has high tensile and triaxial compressive strengths, and low intrinsic permeability (~ 10–21 m2). Permeability of intact Waipapa greywacke does not increase significantly during triaxial loading to failure and is accompanied by minimal changes ultrasonic wave velocities. These data taken together suggest that microcrack development during brittle deformation is very limited. Upon failure, the permeability increases by two orders of magnitude and shows similar permeability to tests performed on synthetic, single, mode I fractures in intact Waipapa greywacke. Permeability persists in Waipapa greywacke fractures under confining pressures of at least 150 MPa. It is concluded that Waipapa greywacke rocks will not allow fluid flow through the matrix of the rock and that substantial geothermal fluid flow will only occur through macrofracture networks.

中文翻译：

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Waipapa graywacke的物理性质表征：新西兰重要的地热储层基底岩

新西兰的 Greywacke 基底岩石拥有传统的地热储层，未来可能提供重要的更热和更深的地热能源。这项工作结合了怀帕帕灰瓦克的岩石学分析和物理特性测量，这是一个拥有新西兰地热储层的地下室单元，以便更好地了解结构控制的流动网络如何发展和引导地热流体在其中。结果表明，完整的 Waipapa 灰瓦具有较高的拉伸强度和三轴抗压强度，以及较低的固有渗透率（~ 10-21 m2）。在三轴加载至失效期间，完整的 Waipapa 灰瓦克的渗透性不会显着增加，并且伴随着超声波速度的最小变化。综合这些数据表明，脆性变形过程中的微裂纹发展非常有限。失效后，渗透率增加了两个数量级，并显示出与在完整的 Waipapa 灰瓦克合成、单一、I 型裂缝上进行的测试相似的渗透率。在至少 150 MPa 的围压下，Waipapa 灰瓦克裂缝的渗透性仍然存在。得出的结论是，Waipapa 灰瓦岩不允许流体流过岩石基质，大量地热流体只能通过大裂缝网络发生。