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.V1.

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.V1.

From outcrop scanlines to discrete fracture networks, an integrative workflow

Understanding fractures and fracture networks is essential for the investigation and use of subsurface reservoirs. The aim is to predict the fractures and the fracture network when there is no direct access to subsurface images available. This article presents a universal workflow to numerically compute a discrete fracture network by combining the 1D scanline survey method, processed with the newly written SkaPy script, together with the multiple point statistic method (MPS). This workflow is applied to a potential geothermal site in Mexico called Acoculco. We use Las Minas outcrops and quarries as surface analogues for the Acoculco reservoir, as Las Minas and Acoculco are both formed by the influence of a plutonic intrusion into the Jurassic–Cretaceous carbonate sequence of the Sierra Madre Oriental in the Trans-Mexican volcanic belt (TMVB). The intrusion is associated with contact metamorphism and metasomatic phenomena, providing the basis for the mining activities at Las Minas. The results obtained using this workflow demonstrate the feasibility of the approach, which presents a solution combining the efficiency of data processing and an interpretation-driven approach to build realistic discrete fracture networks. This workflow can be used in the process of estimating the permeability of a fracture controlled reservoir, with using only scanline surveys data as input. This is essential in the process of evaluating the feasibility to develop an enhanced geothermal system.publishedVersio

From outcrop scanlines to discrete fracture networks, an integrative workflow

Understanding fractures and fracture networks is essential for the investigation and use of subsurface reservoirs. The aim is to predict the fractures and the fracture network when there is no direct access to subsurface images available. This article presents a universal workflow to numerically compute a discrete fracture network by combining the 1D scanline survey method, processed with the newly written SkaPy script, together with the multiple point statistic method (MPS). This workflow is applied to a potential geothermal site in Mexico called Acoculco. We use Las Minas outcrops and quarries as surface analogues for the Acoculco reservoir, as Las Minas and Acoculco are both formed by the influence of a plutonic intrusion into the Jurassic–Cretaceous carbonate sequence of the Sierra Madre Oriental in the Trans-Mexican volcanic belt (TMVB). The intrusion is associated with contact metamorphism and metasomatic phenomena, providing the basis for the mining activities at Las Minas. The results obtained using this workflow demonstrate the feasibility of the approach, which presents a solution combining the efficiency of data processing and an interpretation-driven approach to build realistic discrete fracture networks. This workflow can be used in the process of estimating the permeability of a fracture controlled reservoir, with using only scanline surveys data as input. This is essential in the process of evaluating the feasibility to develop an enhanced geothermal system

Recurrences of Atrial Fibrillation Despite Durable Pulmonary Vein Isolation: The PARTY-PVI Study

International audienceBACKGROUND: Recurrences of atrial fibrillation (AF) after pulmonary vein isolation (PVI) are mainly due to pulmonary vein reconnection. However, a growing number of patients have AF recurrences despite durable PVI. The optimal ablative strategy for these patients is unknown. We analyzed the impact of current ablation strategies in a large multicenter study. METHODS: Patients undergoing a redo ablation for AF and presenting durable PVI were included. The freedom from atrial arrhythmia after pulmonary vein-based, linear-based, electrogram-based, and trigger-based ablation strategies were compared. RESULTS: Between 2010 and 2020, 367 patients (67% men, 63±10 years, 44% paroxysmal) underwent a redo ablation for AF recurrences despite durable PVI at 39 centers. After durable PVI was confirmed, linear-based ablation was performed in 219 (60%) patients, electrogram-based ablation in 168 (45%) patients, trigger-based ablation in 101 (27%) patients, and pulmonary vein-based ablation in 56 (15%) patients. Seven patients (2%) did not undergo any additional ablation during the redo procedure. After 22±19 months of follow-up, 122 (33%) and 159 (43%) patients had a recurrence of atrial arrhythmia at 12 and 24 months, respectively. No significant difference in arrhythmia-free survival was observed between the different ablation strategies. Left atrial dilatation was the only independent factor associated with arrhythmia-free survival (HR, 1.59 [95% CI, 1.13–2.23]; P =0.006). CONCLUSIONS: In patients with recurrent AF despite durable PVI, no ablation strategy used alone or in combination during the redo procedure appears to be superior in improving arrhythmia-free survival. Left atrial size is a significant predictor of ablation outcome in this population