Petrophysical and mechanical rock property database of the Los Humeros and Acoculco geothermal fields (Mexico)

Petrophysical and rock mechanical properties are key parameters for the characterization of the deep subsurface in different disciplines such as geothermal heat extraction, petroleum reservoir engineering or mining, are commonly used for the interpretation of geophysical data and the parameterization of numerical models and as thus are the basis for economic reservoir assessment. However, detailed information regarding these properties for each target horizon are often scarce, inconsistent or spread over multiple publications. Thus, subsurface models are often populated with generalized or assumed values resulting in high uncertainty. Furthermore, diagenetic, metamorphic and hydrothermal processes significantly affect the physiochemical and mechanical properties often leading to high variability. A sound understanding of the controlling factors is needed to identify statistical and causal relationships between the properties as basis for a profound reservoir assessment and modelling. Within the scope of the GEMex project (EU-H2020, GA Nr. 727550), which aims to develop new transferable exploration and exploitation approaches for super-hot unconventional geothermal systems, a new workflow was applied to overcome the gap of knowledge of the reservoir properties. Two caldera complexes located in the northeastern Trans Mexican Volcanic Belt - the Acoculco and Los Humeros caldera - were selected as demonstration sites. The Los Humeros geothermal system is steam dominated and has been exploited since the 1990’s with 65 wellbores (28 still producing). With temperatures above 380 °C, the system is characterized as a super-hot geothermal system. The geothermal system in Acoculco (presently consisting of two exploration wells) is characterized by temperatures of approximately 300 °C at a depth of about 2 km. It contains almost no fluids, even though a well-developed fracture network exists in the study area. Therefore, the system serves as a demonstration site for the development of an enhanced geothermal system. The workflow starts with outcrop analogue and reservoir core sample studies in order to define and characterize the properties of all key units from the basement to the cap rock as well as their mineralogy and geochemistry. This allows the identification of geological heterogeneities on different scales (outcrop analysis, representative rock samples, thin sections and chemical analysis) enabling a profound reservoir property prediction. More than 340 rock samples were taken from representative outcrops inside of the Los Humeros and Acoculco calderas, the surrounding areas and from exhumed ‘fossil systems’ in Las Minas and Zacatlán. Additionally, 66 core samples from 16 wells of the Los Humeros geothermal field were obtained. Samples were analyzed for particle and bulk density, porosity, permeability, thermal conductivity, thermal diffusivity, heat capacity, as well as ultra-sonic wave velocities, magnetic susceptibility and electric resistivity. Afterwards destructive rock mechanical tests (point load tests, uniaxial and triaxial tests) were conducted to determine tensile strength, uniaxial compressive strength, Young’s modulus, poisson ratio, bulk modulus, shear modulus, fracture toughness, cohesion and friction angle. In addition, XRD and XRF analyses were performed on 131 samples to provide information about the mineral assemblage, bulk geochemistry and the intensity of hydrothermal alteration. An extensive rock property database was created comprising 34 parameters determined on more than 2160 plugs. More than 31,000 data points were compiled covering volcanic, sedimentary, metamorphic and igneous rocks from different ages (Jurassic to Holocene), thus facilitating a wide field of applications regarding resource assessment, modeling and statistical analyses.

岩石物理与岩石力学特性（Petrophysical and rock mechanical properties）是多学科中表征深部地下的关键参数，涵盖地热开采、油气藏工程与采矿工程等领域，常被用于地球物理数据解译与数值模型参数化，亦是油气藏经济评价的核心基础。
然而，针对各目标层位的此类特性的详细信息往往稀缺、不一致，或分散于多篇学术文献中。因此，地下模型通常采用泛化或预设值填充，进而引入较高的不确定性。此外，成岩、变质与热液作用会显著改变岩石的物理化学与力学特性，往往导致物性参数存在较大变异性。
要明确这些特性间的统计与因果关系，为深入的储层评价与建模奠定基础，需先深入理解其控制因素。
在GEMex项目（欧盟“地平线2020”计划，项目编号GA No.727550）的框架下，该项目旨在为超高温地热系统（super-hot geothermal system）开发全新可迁移的勘探与开发方法，研究团队采用全新工作流程以填补储层物性的认知空白。研究选取墨西哥跨火山带（Trans Mexican Volcanic Belt）东北部的两处破火山口复合体（caldera complexes）——阿库库尔科（Acoculco）破火山口与洛斯乌梅罗斯（Los Humeros）破火山口——作为示范场址。
洛斯乌梅罗斯地热系统以蒸汽为主，自20世纪90年代起投入开发，共完成65口钻井（28口仍在生产），其地层温度超过380℃，被归类为超高温地热系统。
阿库库尔科地热系统目前仅包含2口勘探井，在约2km深度处的温度约为300℃；尽管研究区内发育有完整的裂缝网络，但该系统几乎不含流体，因此可作为增强型地热系统（Enhanced Geothermal System）开发的示范场地。
该工作流程首先开展露头类比与储层岩心样品研究，以明确并表征从基底到盖层的所有关键地层单元的物性特征，及其矿物学与地球化学属性。通过露头分析、代表性岩样测试、薄片观察与化学分析等手段，可识别不同尺度下的地质非均质性，从而实现高精度的储层物性预测。
研究团队从洛斯乌梅罗斯与阿库库尔科破火山口内的代表性露头、周边区域，以及拉斯米纳斯（Las Minas）与萨卡特兰（Zacatlán）的剥露“化石系统”中采集了340余块岩样。此外，还从洛斯乌梅罗斯地热田的16口钻井中获取了66份岩心样品。
对所有样品开展了多项物性测试，包括颗粒密度与体密度、孔隙度、渗透率、热导率、热扩散率、热容量，以及超声波波速、磁化率与电阻率。
随后开展破坏性岩石力学试验，涵盖点荷载试验、单轴与三轴压缩试验，以测定抗拉强度、单轴抗压强度（uniaxial compressive strength）、杨氏模量（Young’s modulus）、泊松比（poisson ratio）、体积模量（bulk modulus）、剪切模量（shear modulus）、断裂韧性（fracture toughness）、内聚力（cohesion）与摩擦角（friction angle）。
此外，还对131份样品开展了X射线衍射（XRD）与X射线荧光（XRF）分析，以获取矿物组合、全岩地球化学特征与热液蚀变强度相关信息。
本研究构建了一套大规模岩心物性数据库，涵盖超过2160份岩芯试样的34项物性参数测定结果，共汇编31000余条数据点，覆盖侏罗纪至全新世不同地质时代的火山岩、沉积岩、变质岩与火成岩，可广泛应用于资源评价、数值建模与统计分析等诸多领域。