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性质（Petrophysical and rock mechanical properties）是多学科领域中深部地下表征的关键参数，涵盖地热开发（geothermal heat extraction）、油气藏工程（petroleum reservoir engineering）与采矿工程等方向，可用于地球物理数据解释、数值模型参数化，亦是油气藏经济评价的基础。然而，针对各目标层位的此类参数详细信息往往较为匮乏、不一致，或分散于多篇文献之中。因此，地下模型常采用泛化或假设值，导致较高的不确定性。此外，成岩作用、变质作用与热液过程会显著影响岩石的物理化学与力学性质，往往带来极强的物性差异。唯有深入理解其控制因素，方能明确各参数间的统计与因果关系，为可靠的储层评价与建模提供依据。在欧盟地平线2020（EU-H2020，项目编号GA Nr. 727550）的GEMex项目框架内，该项目旨在为超高温非常规地热系统开发可推广的勘探与开发新方法，研究团队采用了全新的工作流程以填补储层物性认知空白。研究选取了墨西哥跨火山带（Trans Mexican Volcanic Belt）东北部的两处破火山口复合体——阿科库尔科（Acoculco）破火山口与洛斯乌梅罗斯（Los Humeros）破火山口作为示范站点。洛斯乌梅罗斯地热系统为蒸汽主导型，自20世纪90年代起开始开发，目前共有65口钻井井筒（其中28口仍在生产）；该系统温度超过380℃，被归类为超高温地热系统。阿科库尔科地热系统目前仅有两口勘探井，在约2km深度处温度可达约300℃，尽管研究区内发育完善的裂缝网络，但几乎不含流体，因此该系统可作为增强型地热系统开发的示范站点。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单元的物性、矿物学与地球化学特征。通过开展不同尺度的地质非均质性识别（露头分析、代表性岩样分析、薄片鉴定与化学分析），可实现可靠的储层物性预测。研究团队从洛斯乌梅罗斯与阿科库尔科破火山口内部及其周边区域的代表性露头，以及拉斯米纳斯（Las Minas）和萨卡特兰（Zacatlán）的抬升“古系统”中采集了超过340件岩石样品；此外，还从洛斯乌梅罗斯地热田16口井中获取了66件岩心样品。对样品开展了颗粒密度与体积密度、孔隙度、渗透率、导热系数、热扩散率、热容，以及超声波波速、磁化率与电阻率的测试分析。随后开展破坏性岩石力学试验（点荷载试验、单轴与三轴试验），以测定抗拉强度、单轴抗压强度、杨氏模量（Young’s modulus）、泊松比、体积模量、剪切模量、断裂韧性、内聚力与摩擦角。此外，还对131件样品开展了X射线衍射（XRD）与X射线荧光光谱（XRF）分析，以获取矿物组合、全岩地球化学与热液蚀变强度相关信息。研究构建了一套完整的岩石物性数据库，包含超过2160件岩心柱塞样测定的34项参数，共汇编了超过31000条数据，涵盖了不同时代（侏罗纪至全新世）的火山岩、沉积岩、变质岩与火成岩，可广泛应用于资源评价、建模与统计分析等多个领域。