In geothermal reservoir systems, changes in pore pressure due to production (depletion), injection or temperature changes result in a displacement of the effective stresses acting on the rock matrix of the aquifer. To compensate for these intrinsic stress changes, the rock matrix is subjected to poroelastic deformation through changes in rock and pore volume. This in turn may induce changes in the effective pore network and thus in the hydraulic properties of the aquifer. Therefore, for the conception of precise reservoir models and for long-term simulations, stress sensitivity of porosity and permeability is required for parametrization. Stress sensitivity was measured in hydrostatic compression tests on 14 samples of rock cores stemming from two boreholes of the Upper Jurassic Malm aquifer of the Bavarian Molasse Basin. To account for the heterogeneity of this carbonate sequence, typical rock and facies types representing the productive zones within the thermal reservoir were used. Prior to hydrostatic investigations, the hydraulic (effective porosity, permeability) and geomechanical (rock strength, dynamic, and static moduli) parameters as well as the microstructure (pore and pore throat size) of each rock sample were studied for thorough sample characterization. Subsequently, the samples were tested in a triaxial test setup with effective stresses of up to 28 MPa (hydrostatic) to simulate in-situ stress conditions for depths up to 2000 m. It was shown that stress sensitivity of the porosity was comparably low, resulting in a relative reduction of 0.7–2.1% at maximum effective stress. In contrast, relative permeability losses were observed in the range of 17.3–56.7% compared to the initial permeability at low effective stresses. Stress sensitivity coefficients for porosity and permeability were derived for characterization of each sample and the different rock types. For the stress sensitivity of porosity, a negative correlation with rock strength and a positive correlation with initial porosity was observed. The stress sensitivity of permeability is probably controlled by more complex processes than that of porosity, where the latter is mainly controlled by the compressibility of the pore space. It may depend more on the compaction of precedented flow paths and the geometry of pores and pore throats controlling the connectivity within the rock matrix. In general, limestone samples showed a higher stress sensitivity than dolomitic limestone or dolostones, because dolomitization of the rock matrix may lead to an increasing stiffness of the rock. Furthermore, the stress sensitivity is related to the history of burial diagenesis, during which changes in the pore network (dissolution, precipitation, and replacement of minerals and cements) as well as compaction and microcrack formation may occur. This study, in addition to improving the quality of input parameters for hydraulic–mechanical modeling, shows that hydraulic properties in flow zones largely characterized by less stiff, porous limestones can deteriorate significantly with increasing effective stress.

中文翻译：

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德国南部地热 Malm 储层不同碳酸盐岩类型在不同静水应力条件下孔隙度和渗透率的应力敏感性

在地热储层系统中，由于生产（枯竭）、注入或温度变化引起的孔隙压力变化会导致作用在含水层岩石基质上的有效应力发生位移。为了补偿这些内在应力变化，岩石基质通过岩石和孔隙体积的变化而经受多孔弹性变形。这反过来可能会导致有效孔隙网络的变化，从而导致含水层的水力特性发生变化。因此，对于精确储层模型的概念和长期模拟，参数化需要孔隙度和渗透率的应力敏感性。在对来自巴伐利亚莫拉塞盆地上侏罗纪马尔姆含水层的两个钻孔的 14 个岩芯样本进行静水压缩试验中测量了应力敏感性。为了解释这种碳酸盐岩层序的非均质性，使用了代表热储层内生产区的典型岩石和相类型。在静水研究之前，对每个岩石样品的水力（有效孔隙度、渗透率）和地质力学（岩石强度、动态和静态模量）参数以及微观结构（孔隙和孔喉尺寸）进行了研究，以进行彻底的样品表征。随后，在三轴测试装置中对样品进行测试，有效应力高达 28 MPa（流体静力），以模拟深度达 2000 m 的原位应力条件。结果表明，孔隙度的应力敏感性相对较低，导致在最大有效应力下相对减少 0.7-2.1%。相比之下，相对渗透率损失在 17 的范围内观察到。与低有效应力下的初始渗透率相比，为 3–56.7%。孔隙度和渗透率的应力敏感系数被推导出来用于表征每个样品和不同的岩石类型。对于孔隙度的应力敏感性，观察到与岩石强度呈负相关，与初始孔隙度呈正相关。渗透率的应力敏感性可能受比孔隙度更复杂的过程控制，后者主要受孔隙空间的可压缩性控制。它可能更多地取决于先前流动路径的压实以及控制岩石基质内连通性的孔隙和孔喉的几何形状。一般来说，石灰岩样品表现出比白云质石灰岩或白云岩更高的应力敏感性，因为岩石基质的白云石化可能导致岩石刚度增加。此外，应力敏感性与埋藏成岩作用的历史有关，在此期间可能发生孔隙网络的变化（溶解、沉淀、矿物和胶结物的置换）以及压实和微裂缝的形成。除了提高水力-机械建模的输入参数质量外，这项研究还表明，流动区的水力特性主要以硬度较低的多孔石灰岩为特征，随着有效应力的增加，其水力特性会显着恶化。和替代矿物和水泥）以及压实和微裂纹的形成可能会发生。除了提高水力-机械建模的输入参数质量外，这项研究还表明，流动区的水力特性主要以硬度较低的多孔石灰岩为特征，随着有效应力的增加，其水力特性会显着恶化。和替代矿物和水泥）以及压实和微裂纹的形成可能会发生。除了提高水力-机械建模的输入参数质量外，这项研究还表明，流动区的水力特性主要以硬度较低的多孔石灰岩为特征，随着有效应力的增加，其水力特性会显着恶化。