Geothermics 77 (2019) 42–61

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Geothermics

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A three-dimensional numerical model to simulate Iranian NW Sabalan geothermal system T ⁎ Mirmahdi Seyedrahimi-Niaraqa, Faramarz Doulati Ardejanib,c, , Younes Noorollahid, Soheil Porkhiale, Ryuichi Itoif, Saeid Jalili Nasrabadif a Faculty of Engineering, University of Mohaghegh Ardabili, Ardabil, Iran b School of Mining, College of Engineering, University of Tehran, Tehran, Iran c Mine Environment and Hydrogeology Research Laboratory, University of Tehran, Tehran, Iran d Department of Renewable Energies Engineering, Faculty of New Sciences & Technologies, University of Tehran, Tehran, Iran e Department of mechanic engineering, Karaj Branch, Islamic Azad University, Karaj, Iran f Department of Earth Resources Engineering, Kyushu University, No. 418 West Building 2, Fukuoka, Japan

ARTICLE INFO ABSTRACT

Keywords: The plan for electricity generation from north-west (NW) Sabalan geothermal field, Iran has been presented since Numerical modelling 1994. Now construction of the first pilot plant is running with a capacity of 5 MWe in this site. A three-di- Natural state model mensional (3D) numerical model of fluid flow and heat transfer of the geothermal reservoir was developed using fl Fluid ow Tough2 simulator code on the basis of the updated conceptual model from the field recent data. The aim of this Heat transfer study was to present an updated 3D model of the geothermal reservoir in this area by taking ten deep wells data NW Sabalan into account and validate an existing model in which the data obtained from only three wells were included. The Geothermal reservoir model covers an area of 92 km2 and extends to an approximate depth of 5.11 km from the land surface. The model includes 17 horizontal layers, (AA through QQ). The thickness of the layers range between 100 m and 1000 m. The model extends from a minimum level of -1000 to a maximum level of 4110 masl. The model has 12,784 grid blocks, in total. Each layer has 752 grid blocks with horizontal dimensions of 250 m × 250 m and 500 m × 500 m. Altogether, 21 rock types were used in the model to assign different horizontal permeability − values from 1.0 × 10 17 to 9.0 × 10-13 m2 based on the conceptual model. The model was first calibrated by varying the permeability, the total magnitude and location of the recharges and well bottom pressure of dis- charge zones. It was further validated by the profiles of temperature and pressure distribution at 10 deep ex- ploration wells data. A close agreement was obtained between the model predictions and the measured downhole temperature and pressure data in the exploration wells. In the best-fitted model, the natural state simulation proved the presence of a high temperature upflow zone in the southern part (Below the ground, between Pads D and E). High temperature fluid at a rate of 108 kg/s and with an enthalpy of 1056.43 kJ/kg (244 °C) recharged over an area of 2.25 km2 from 36 grid blocks from the bottom layer in southern part of the field. The overall natural discharge is about 43 kg/s from hot springs which was simulated using the pressure dependent deliverability method. The results show that there is mostly one inflow of geothermal water at the level of -1000 m at southeast below the land surface between the Pad D and Pad E. The fluid flow moves upward by the fracture zones and faults toward the northwest of area. The faults NW5, NW3 and NNW2 play a major role in this system. Finally, the geothermal fluid discharging by the surface manifestations can be seen as hot springs at the land surface. This model can be used to simulate future production scenarios to evaluate the sustainability of the reservoir.

1. Introduction goal for electricity production. This field is located in the north-west of Iran in Ardabil Province. Its distance from Tehran is 859 Km (Fig. 1). The eighteen geothermal fields have been identified as potential The area has been under geological investigation since 1978. The areas in Iran. Among them, the NW Sabalan geothermal field is the first electricity generation plan in the geothermal field has been presented

⁎ Corresponding author at: School of Mining, College of Engineering, University of Tehran, Tehran, Iran. E-mail addresses: [email protected] (M. Seyedrahimi-Niaraq), [email protected] (F. Doulati Ardejani), [email protected] (Y. Noorollahi), [email protected] (S. Porkhial), [email protected] (R. Itoi), [email protected] (S. Jalili Nasrabadi). https://doi.org/10.1016/j.geothermics.2018.08.009 Received 10 January 2018; Received in revised form 1 August 2018; Accepted 28 August 2018 0375-6505/ © 2018 Elsevier Ltd. All rights reserved. M. Seyedrahimi-Niaraq et al. Geothermics 77 (2019) 42–61

Fig. 1. Geographical situation of NW Sabalan geothermal field. since 1994 (Fotouhi, 1995; Noorollahi et al., 2009; Porkhial et al., of previous three wells (2002–2004), creating a new structure from 2015; Kosari and Sattari, 2015). model with considering geological units and delineated faulted zones in From 1998–2005, the detailed geo-based study was directed by the the wells with dense network and finally developing a more reliable joint cooperation of renewable energy organization of Iran (SUNA) and model. This developed porous-medium and natural-state (pre-ex- Sinclair Knight Merz Ltd (SKM) of New Zealand. The NW Sabalan ploitation) model was calibrated by comparing with the measured geothermal field was eventually distinguished as an important potential temperature and pressure profiles of 10 wells. The Tough2 simulator for power generation purpose (Noorollahi et al., 2009). code was used applying the equation of state for water and steam Accordingly, during the time period of 2002–2004, three deep ex- (EOS1) (Pruess et al., 1999). The simulated results are quantitatively ploration and delineation wells (NWS-1, NWS-3 and NWS-4) have been evaluated using Root Mean Square Error (RMSE). drilled. Besides, a shallow injection well named NWS-2 (Fig. 1b, PADs A, B and C) was also drilled, in order to evaluate the subsurface geology 2. Conceptual model and provide data for assessment and modelling the reservoir (Najafi and Ghobadian, 2011; Porkhial et al., 2015). The location of these wells was In order to develop an appropriate numerical model for the study determined based on the results obtained from a magnetotellurics (MT) geothermal field, some interpolation and interpretation of the geolo- survey conducted in 1998 (SKM, 2005a). The wells NWS-1 and NWS-4 gical structures were required. To do this, a new updated conceptual were the first deep wells drilled in Sabalan geothermal site (Porkhial model was developed by adding new information to the existing con- et al., 2015; Kosari and Sattari, 2015). A maximum temperature of ceptual models. This important data includes results of geological sur- 242 °C was measured in well NWS-1 at a depth of 832 m. Subsequently, veys, temperature and pressure distribution in recent deep wells along from 2008 to 2011, SUNA performed new geophysical exploration with those obtained from the previous wells, geophysical surveys and studies. Based on the new results, the wells NWS-5 and NWS-11 on Pads new geochemical data associated with the hot springs. A and C, respectively, NWS-6, NWS-7 and NWS-10 on Pad D and NWS-8 A geological map at a scale of 1:20,000 of Sabalan region is shown and NWS-9 on Pad E were drilled (Fig. 2a). Specification of ten ex- in Fig. 2a. The reservoir conceptual model was developed along AB line ploration wells is summarised in Table 1. with 12 km length, from north to south of the study area. Ten wells In 2011, Noorollahi and Itoi have developed a numerical model for were projected on the AB line. The deviated wells at depths were NW Sabalan geothermal field and validated the model predictions using plotted using Datamine Studio3 Version 3.21.7164 software and used measured data from three exploration wells. The model predicted that a for developing a conceptual model. In addition, the elevation points power plant can be designed with a capacity of 50 MWe and ages of along the cross section were extracted from a digital topography map in more than 30 years. However, this work employed the information Geographical Information System (GIS) environment. obtained from only three exploration wells for development of the conceptual and numerical models. Also, it ignored to include some 2.1. Geology and tectonic setting important geological features and structures such as the subsurface distribution of geological units and delineated faults and lineaments. The surface geological map (Fig. 2a) of the area was modified from These three wells situated out of the main upflow zone. KML (1999). Mt. Sabalan is a large stratovolcano comprising a wide- The aim of this study was to update an existing 3D numerical model ranging central structure which is constructed on a likely tectonic horst of the geothermal reservoir in the study area by incorporating the data of underlying intrusive and effusive volcanic rocks. Subsurface geology obtained from seven new deep wells (2008–2011) to the existing data is described along the cross section AB, using geological data and

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