Electricity generation from enhanced geothermal systems by oilfield produced water circulating through reservoir stimulated by staged fracturing technology for horizontal wells: A case study in Xujiaweizi area in Daqing Oilfield, China
Introduction
In recent years, the issue of energy has received considerable attention because of rapid economic development. The oil crises in the 20th century have significantly affected many countries. Therefore, energy security has become an issue of national security, with various countries developing energy policies to ensure the security of energy supply. However, conventional fossil energy is non-renewable and environmentally harmful. To deal with the greenhouse effect and protect the living environment, alternative energy sources that are renewable and environmentally benign need to be found and utilized. Geothermal energy is one of the few renewable energy resources that can provide 24-h-a-day power because of its continual stability and wide spatial distribution [1].
Geothermal resources can be divided into conventional HDR (hydro-thermal resources and hot dry rock) resources [2]. The existence of hydro-thermal resources is usually related to eruptive volcanic activities and high terrestrial heat flow values. The hydro-thermal energy can be exploited by extracting the fluid contained in the geothermal reservoir (water, steam, or various gases) [2]. HDR resources refer to high temperature rocks that have a very low porosity and permeability. Energy contained in HDR resources can be extracted by artificially stimulating the reservoir to form an artificially altered geothermal system called EGS (enhanced geothermal system) [1].
HDR has attracted increasing attention in China because of its cleanliness and enormous potential. Total HDR resource reserve in the continental area of China within 3 ∼ 10 km depth is 20.9 M EJ, which is equivalent to energy contained in 714.9 × 1012 t standard coal. If the recoverable fraction is 2%, this value is 168 times the quantity of conventional hydro-thermal resource and is equivalent to 4400 times total energy consumption in 2010 in China [3]. China Geological Survey Bureau released a report of the preliminary assessment of the HDR resource potential of China in 2011. The assessment results show that the favorable regions of developing HDR resources in Chinese mainland are Southern Tibet, western Yunnan Province (Tengchong County), southeastern coastal areas (Zhejiang, Fujian, and Guangdong Provinces), North China (Bohai Bay Basin), Fenwei graben, and northeast China (Songliao Basin) [3], [4]. Among these regions, southeast coast of China, North China, and northeast China are economically developed regions that are densely populated and highly industrialized. Demands of electricity power in these regions are very high, and excessive dependence on fossil fuels has caused serious environmental problems. Therefore, developing HDR resources in these regions is of great importance.
EGS has been studied for more than 30 years. However, previous studies were mainly conducted by research institutions in Europe and the United States. EGS research started relatively late in China. Only a few Chinese research institutions previously conducted some theoretical work or participated in some international cooperation projects [5]. However, this situation has been changing in recent years. In 2012, Ministry of Science and Technology of China launched an 863 program (National High Technology Research and Development Program) named “Key technology research on HDR resource development and utilization”. In 2013, China Geological Survey launched a program named “HDR resource investigation and related technology research” [6]. HDR resource also attracts increasing attention of local government and energy companies. In August 2013, an HDR exploration project is launched in Lijin County, Shandong Province. Total investment of this project is $2.05 million. It is the first HDR resource survey project in Shandong Province [7]. In 2013, an HDR exploration project is launched in Gonghe Basin of Qinghai Province, supported by Qinghai Provincial Government. It is reported that the bottom-hole temperature is 168 °C at a depth of 2735 m, which indicates a huge potential for HDR development in this area [8]. State-owned key enterprises including CNPC (China National Petroleum Corporation) and Sinopec Group have entered the geothermal market and invested in the development of HDR resource [9].
HDR resource development is a complicated engineering; many science and technology issues need to be solved during the process. These issues mainly include locating the target area, reservoir stimulation technology, micro-earthquake monitoring technology [10], establishment of geothermal geological models, multi-field coupling effects of deep underground environment, heat transfer characteristics of the medium, and energy conversion efficiency et al. [11]. Capital investments of an EGS project are very large, which is why reserves of the target region need to be evaluated and heat production potential from the stimulated reservoir must be predicted [12], [13], [14], [15].
Garnish and Shock [11] set several economic standards to assess the performance of an EGS project in the 1980s. Among these standards, two of the most important are sufficiently high production temperature (190 °C) and flow rate (100 kg/s). Recently, with the development of power generation technology using medium-low temperature geothermal resource, the high temperature demand of EGS has gradually declined [3]. However, the low level of production flow rate is still a main bottleneck constraining the economic operation of an EGS project. An earlier study showed that the production flow rate from a single fracture is far lower than the commercial standard [16]. Therefore, staged fracturing technology for horizontal wells is proposed to be used to enhance the production flow rate in this work.
Drilling costs account for a large proportion of the total capital costs of EGS development [3], while oilfields have many abandoned deep wells (CNPC has more than 200,000 deep wells in China) [17]. Some abandoned wells located in thermal anomaly regions can be used for geothermal development after some renovations. In China, water content of the produced oil has reached 80% ∼ 90% in some old oilfields. It is reported that more than 5 × 108 t oilfield produced water with an average temperature of 40 ∼ 50 °C are withdrawn and need to be disposed yearly [17]. There is no doubt that significant economic and environmental benefits can be gained if this kind of resource can be properly utilized.
This paper investigates the feasibility of generating electricity from a proposed EGS project in Xujiaweizi area, Daqing Oilfield, China. Reservoir is stimulated by using staged fracturing technology through abandoned wells. Oilfield produced water is used to extract geothermal energy by circulating through the fractured reservoir. The comprehensive utilization of HDR resource and medium-low temperature geothermal resource in oilfield produced water can be achieved.
Section snippets
Methodology
As one of the key exploration regions of Daqing Oilfield, the Xujiaweizi area has more than 50 years of exploration history. The geological and geothermal data that were accumulated throughout its exploration history were used in this study.
The site was introduced from several aspects including geographical position, geological structure, genesis of geothermal resource, and distribution features of current subsurface temperature field. The HDR resource potential assessment was carried out by
Geographical position
Songliao Basin is located in the central area of northeast China, within 119 40′ E to 128 24′ E longitudes and 42°25′ N to 49 23′ N latitudes. The basin extends in NNE direction, across Heilongjiang, Jilin, Liaoning, and Inner Mongolia provinces. The basin is divided into the southern and the northern parts by the boundaries between Jilin Province and Heilongjiang Province. The basin has an area of 26 × 104 km2, with extents of 700 km from south to north and 370 km from east to west (Fig. 2).
General considerations
Reservoir stimulation is the key technology of HDR development. Recently, the development of unconventional oil and gas resources in North America and other regions has greatly promoted the progress of hydraulic fracturing technology. Reservoir stimulation technology for unconventional oil and gas can be divided into four approaches: (1) Small-scale single-layer fracturing, (2) large-scale single-layer fracturing, (3) multi-layer fracturing in vertical wells, and (4) staged fracturing for
General considerations
In general terms, existing geothermal power plants can be divided into three types according to the power generation technologies: (1) Dry steam power plant, (2) flash steam power plant, and (3) binary cycle power plant. The heat source temperatures of dry steam and flash steam plants are required to be higher than 230 °C and 150 °C, respectively [29]. The heat source temperature of the target formation in Xujiaweizi area is 160 °C ∼ 180 °C. Taking into account the inevitable temperature
EGS design
To meet the commercial standard, nine to ten fractures need to be created according to the simulation results. Thus, the horizontal section of the well can be determined as 3000 m based on the fracture spacing given in Section 5.4.5. According to the experience of oil and gas industry, this length can be achieved technically [43].
According to the simulation results, We maintains at an average level of 3.61 MW in the first five years of production and then decreases to an average level of
Economic analysis
Costs for a geothermal power plant include investment cost and O&M (operation and maintenance) costs [44]. Investment cost can be divided into three categories: (1) drilling costs, (2) stimulation costs, and (3) costs for surface installation. O&M costs involve costs for power self-consumption and other O&M costs like personnel and insurances. The LCOE (levelized cost of electricity) is the most common approach used for evaluating the economic feasibility of a power generation system [1]. A
Conclusions
The feasibility of electricity generation from an enhanced geothermal system by oilfield produced water circulating through the reservoir in Xujiaweizi area was investigated. First, the HDR resource reserves were assessed by using the volumetric method. Fracturing and hydro-thermal simulations were conducted to evaluate the heat production power and electricity generation power from the reservoir. Based on the simulation results, an EGS design scheme was proposed. Economic and environment
Acknowledgments
This study was supported by the National High Technology Research and Development Program of China (863 Program) (No.2012AA052801), the Natural Science Foundation of China (Grant No. 41372239), Specialized Research Fund for the Doctoral Program of Higher Education of China (NO. 20110061110055).
References (51)
Geothermal energy technology and current status: an overview
Renew Sustain Energy Rev
(2002)- et al.
HDR/HWR reservoir: concepts, understanding and creation
Geothermics
(1999) - et al.
Numerical simulation of heat production potential from hot dry rock by water circulating through two horizontal wells at desert peak geothermal field
Energy
(2013) - et al.
Fluid circulation and heat extraction from engineered geothermal reservoirs
Geothermics
(1999) - et al.
Numerical investigation of fluid flow and heat transfer in a doublet enhanced geothermal system with CO2 as the working fluid (CO2-EGS)
Energy
(2014) - et al.
Microhole arrays for improved heat mining from enhanced geothermal systems
Geothermics
(2013) - et al.
Numerical simulation of heat production potential from hot dry rock by water circulating through a novel single vertical fracture at desert peak geothermal field
Energy
(2013) - et al.
Hot water generation for oil sands processing from enhanced geothermal systems: process simulation for different hydraulic fracturing scenarios
Appl Energy
(2014) - et al.
Design and testing of the organic Rankine cycle
Energy
(2001) Geothermal power generation in the world 2005–2010 update report
Geothermics
(2012)