Zhengmeng Hou

Simulation of CO2 plume movement in multilayered saline formations through multilayer injection technology in the Ordos Basin, China

Deep saline aquifers still remain a significant option for the disposal of large amounts of CO2 from the atmosphere as a means of mitigating global climate change. The small scale Carbon Capture and Sequestration demonstration project in Ordos Basin, China, operated by the Shenhua Group, is the only one of its kind in Asia, to put the multilayer injection technology into practice. This paper aims at studying the influence of temperature, injection rate and horizontal boundary effects on CO2 plume transport in saline formation layers at different depths and thicknesses, focusing on the variations in CO2 gas saturation and mass fraction of dissolved CO2 in the formation of brine in the plume’s radial three-dimensional field around the injection point, and interlayer communication between the aquifer and its confining beds of relatively lower permeability. The study uses the ECO2N module of TOUGH2 to simulate flow and pressure configurations in response to small-scale CO2 injection into multilayer saline aquifers. The modelling domain involves a complex multilayer reservoir–caprock system, comprising of a sequence of sandstone aquifers and sealing units of mudstone and siltstone layers extending from the Permian Shanxi to the Upper Triassic Liujiagou formation systems in the Ordos Basin. Simulation results indicate that CO2 injected for storage into deep saline aquifers cause a significant pressure perturbation in the geological system that may require a long duration in the post-injection period to establish new pressure equilibrium. The multilayer simultaneous injection scheme exhibits mutual interference with the intervening sealing layers, especially when the injection layers are very close to each other and the corresponding sealing layers are thin. The study further reveals that injection rate and temperature are the most significant factors for determining the lateral and vertical extent that the CO2 plume reaches and which phase and amount will exist at a particular time during and after the injection. In general, a large number of factors may influence the CO2–water fluid flow system considering the complexity in the real geologic sequence and structural configurations. Therefore, optimization of a CO2 injection scheme still requires pursuance of further studies.

Unconventional gas resources in China

Coalbed methane, tight and shale gas are three important unconventional gas resources in China. ‘‘According to data from the Ministry of Land and Resources, at the end of 2013, China had 11, 12 and 25 trillion cubic meters, respectively of remaining technically recoverable resources of coalbed methane, tight gas and shale gas which are still in the early stage of development’’. (Source CNPC and ARA International Limited). China is developing its shale gas exploration and exploitation strategies for economic reasons and social aspects (so-called ‘‘shale gas revolution’’). Huge shale gas reserves provide a reliable source for sustaining China’s economic development and contributing to China’s ‘‘Energiewende’’. Zhao et al. (2015a) analyze the strategic measures and advantages for a safe and vigorous development of shale gas in China and review the developments of hydraulic fracturing technologies in China. Economic exploitation for shale gas is highly dependent on the complexity of the fracture network caused by hydraulic fracturing technology, so it is necessary to accurately assess the effect of the fracture network on gas flow behavior and productivity (Li et al. 2015c). This thematic issue is particularly dedicated to recent researches in unconventional gas resources in China related to the reservoirs indicated in Fig. 1 in order to substitute coal by ‘‘clean’’ gas energy resources as a transitional technology towards renewable energy resources. The thematic issue compiles theoretical, experimental as well as field studies in China. Different reservoir types are under investigation such as tight and shale gas reservoirs as well as coal mines with coalbed methane resources. Fundamental aspects on process understanding in unconventional gas reservoirs as well as the international context of research and technology deployment are discussed in this volume. Tight gas reservoirs have become an important resource for the world’s gas supply. Such reservoirs have very low permeability (usually below 0.1 mD) and show a strong stress sensitivity to fluid transport properties and a considerable productivity decline during the production process due to decreasing reservoir pressure as well as increasing effective stress. In an experimental study by Albrecht and Reitenbach (2015) several measurement series were performed on plugs from the North-German Rotliegend tight gas reservoirs to determine the effects of changing stress and pore pressure conditions on reservoir & Olaf Kolditz olaf.kolditz@ufz.de

Estimation of Biot’s effective stress coefficient from well logs

Effective stress governs the mechanical response of rock formations to variations in stress and pore pressure, which affect wellbore stability and reservoir integrity during drilling and production. Biot’s coefficient is employed to calculate effective stresses from total stress and pore pressure. Therefore, the measurement of Biot’s coefficient becomes crucial. However, the laboratory measurement of Biot’s coefficient is expensive and laborious. This paper presents three methods for computing Biot’s coefficient using logging data. The first method calculates Biot’s coefficient using the existing empirical correlations between porosity and Biot’s coefficient. The second and third methods calculate Biot’s coefficient using dynamic rock and solid bulk modulus, computed using rock and solid wave velocities, respectively. However, the second and third methods calculate the necessary solid wave velocities in different ways. The second method calculates solid shear and compressive velocities (Vs and Vp) using a newly developed correlation between the differential pressure, porosity and wave velocity of sandstone. The third method calculates solid wave velocities based on the significant finding that the Vp/Vs ratio with respect to the S-wave velocity is constant for sediments including highly compacted sand. Case studies were undertaken using logging data from the Gulf Coast Gas Wells. It was found that Biot’s coefficient calculated using the first method was highly dependent on the chosen relation, while the coefficients calculated using the second and third methods were related to well logs. Results from the third method show that Biot’s coefficient deflects to higher values in situations where gamma ray surveys read low API values. This is in agreement with the phenomenon that rocks with a smaller API should have lower a clay content and bigger value of Biot’s coefficient. Therefore, the third method is more reliable and also requires fewer input parameters.

Study of the relationship between surface subsidence and internal pressure in salt caverns

AbstractBecause of the ultra-low permeability and favorable rheological properties of rock salt, salt caverns are globally considered to be an ideal medium for energy storage. The prediction and control of surface subsidence above storage caverns are the key problems that require steady attention to ensure their long-term safety and stable operation. For salt caverns already in existence factors such as depth, shape and geological conditions cannot be changed. Therefore, the operating condition (i.e., internal pressure) becomes a key factor in influencing surface subsidence. However, cyclic pressure change during the operation phase has not been seriously considered by analytical investigation yet. In this paper, theoretical models for the volume convergence rates in spherical and cylindrical caverns have been derived, thereby developing a new concept “equivalent internal pressure” that of rock salt is consideredalso takes cyclic internal pressure into consideration. The analytic solution for surface subsidence was then derived from a combination of transfer and distribution functions. Analytical and numerical solutions for different conditions were compared and verified, while the FDM code, FLAC3D, was used for numerical simulations. This comparison reveals that the use of “equivalent internal pressure” is suitable for predicting surface subsidence in the long-term cyclic operation conditions.

Effect of water on carbonation of mineral aerosol surface models of kaolinite: a density functional theory study

Gas–solid interfacial phenomena always play a significant role in the multiphase process in atmospheric chemistry. The mineral aerosols have desirable interfacial reactivity on the carbonation of kaolinite. In addition, carbonation of kaolinite may play a role in carbon dioxide capture. It is not well-known about the mechanisms of this reaction. In this paper, the carbonation of kaolinite cluster models with or without water on the atomic scale is studied. We simulate the corresponding reaction paths and accurately calculate the transition states with the homologous enthalpies via using the density functional theory (DFT) method. The study shows that the reaction barriers are lowered down seriously with the existence of water. In addition, water can help stabilize the reaction regions thereby firming the structure of carbonated product.

A preliminary site selection system for a CO2-AGES project and its application in China

Because of the high costs involved in CO2 sequestration in deep saline formations, it has been difficult for its widespread application in the short term. Recently, however, a new technology called “combined geothermal production–CO2 sequestration technology” has made the technique more attractive not only by increasing the CO2 storage capacity, but also by decreasing the operational costs, through the utilization of geothermal energy, either directly or for the purpose of producing electricity. A CO2-aided (or CO2-involved) geothermal extraction system (CO2-AGES) is presented, based on existing technologies and the new ideas of “combined geothermal production-CO2 sequestration” introduced in this paper. This system can be used to extract geothermal energy from sedimentary aquifer formations (low–medium temperature) at different stages. The purpose of this paper is to set up an evaluation system for selecting a suitable site for this CO2-AGES system. There are both similarities and differences in the procedure needed for selecting a site for a conventional pure CO2 sequestration and for this CO2-AGES system. Both are carried out at different scales, including basin, region and target formation scales. The biggest difference is the temperature gradient, which plays an opposite role in the two systems. By using the preliminary ranking and screening method presented in this paper, it can be shown that the Bohaiwan, Songliao and Qiangtang basins have the highest potential for the application of this CO2-AGES technology. However, if CO2 sequestration is considered alone, the Ordos, Tarim and Bohaiwan basins should be selected as the best three operational sites in China. While preliminary evaluation methods can provide some useful information on the selection of the best sites for the “combined geothermal production–CO2 sequestration technology”, more detailed work is still required because of the strong uncertainties that exist in the determination of the upper and lower boundaries for each indicator.

Subsurface energy systems in China: production, storage and conversion

The Thematic Issue is dedicated to different aspects of using the potential of the geologic subsurface as a resource for energy production (e.g., hydrothermal and petrothermal resources), energy storage as well as for safe deposition of energy waste, and energy conversion (e.g., as biochemical reactors to convert hydrogen and carbon dioxide into methane). Figure 1 shows an overview map of hydrothermal systems in China including a classification to high-, midand low-temperature reservoirs and basins (Kong et al. 2014). Current research efforts concerning hydrothermal resources focus on the sustainable development of large-scale geothermal fields. Pang et al. (2012) designed a roadmap of geothermal energy development in China and reported the recent progress in geothermal research in China (Pang et al. 2014). Recently, Pang et al. (2015) presented a new classification of geothermal resources based on the type of heat source and followed by the mechanisms of heat transfer. The present Thematic Issue is focusing on petrothermal resources and particularly enhanced geothermal systems. Some of the basins indicated in Fig. 1 are also of interest for unconventional gas resources which is discussed in a separate Thematic Issue in Environmental Earth Sciences (Hou et al. 2015a). Geothermal energy resources are considered to provide a significant contribution to renewable energy supply from both shallow and deep systems (Arola et al. 2014; Huenges et al. 2013; Kolditz et al. 2013; Scheck-Wenderoth et al. 2013). Understanding the thermally induced coupled processes is important for reservoir operations (McDermott et al. 2006; Kolditz and Diersch 1993). Breede et al. (2014) provide a systematic review of enhanced or engineered geothermal systems (EGS). Ramos et al. (2015) provide a systematic review of projects implemented worldwide and a methodology for screening geothermal projects. Geothermal shallow surface systems based on ground source heat pumps have already proved to be a feasible option for the heating and cooling of buildings. The present work studies the concept of using abandoned mines for geothermal heat recovery in various countries such as Germany, the Netherlands, Norway, Russia, Spain, UK and United States. De Filippis et al. (2015) are discussing the geothermal potential of the Salento peninsula in southern Italy. They propose a simple methodology for the assessment of low-enthalpy geothermal energy systems based on stochastic approaches. They found that coastal areas might lower potential due to saltwater intrusion problems. Hou et al. (2015b) introduce the first & Hongwei Zhou zhw@cumtb.edu.cn

Worldwide Status of CCUS Technologies and Their Development and Challenges in China

Carbon capture, utilization, and storage (CCUS) is a gas injection technology that enables the storage of CO2 underground. The aims are twofold, on one hand to reduce the emissions of CO2 into the atmosphere and on the other hand to increase oil/gas/heat recovery. Different types of CCUS technologies and related engineering projects have a long history of research and operation in the USA. However, in China they have a short development period ca. 10 years. Unlike CO2 capture and CO2-EOR technologies that are already operating on a commercial scale in China, research into other CCUS technologies is still in its infancy or at the pilot-scale. This paper first reviews the status and development of the different types of CCUS technologies and related engineering projects worldwide. Then it focuses on their developments in China in the last decade. The main research projects, international cooperation, and pilot-scale engineering projects in China are summarized and compared. Finally, the paper examines the challenges and prospects to be experienced through the industrialization of CCUS engineering projects in China. It can be concluded that the CCUS technologies have still large potential in China. It can only be unlocked by overcoming the technical and social challenges.

Development of a semi-analytical method to calculate depth-dependent temperature and stress changes: investigation of micro-seismicity at Unterhaching Uha GT-2 geothermal well

Since October 2007 when the hydrothermal production of geothermal energy from the deep underground started at Unterhaching, near Munich (Germany), several micro-seismic events have been observed. Three of the five strongest events, with local magnitudes (ML) ranging from 2.0 to 2.4, occurred in 2008 and were partially felt by the residents. The reasons for these events are not clear. These micro-seismic events could have been induced by a variety of factors. The strongest micro-seismic event was observed soon after the injection of cold water, pointing to an intrinsic relationship between micro-seismic event and cold water injection. One of the possible reasons for the observed micro-seismic events could be the reinjection of cooled thermal water into the hotter underground inducing stress redistribution and shear failures in the reservoir formations around the injection wellbore Uha GT-2. To justify this reason, further investigation will be carried out in another simulation study to verify whether the shear failures induced by cold water injection were the major cause for the occurrence of these micro-seismic events. Nevertheless, before the simulation of shear failures, temperature profiles in both the wellbore and surrounding rocks as well as their corresponding thermal stresses during cold water injection should be established in this study. In this paper, a new semi-analytical simulation method has been developed to calculate simultaneously the temperature profiles in the wellbore and the surrounding rock formations and also determine the corresponding thermal stresses in the surroundings near wellbore field and subsequently verified by analytical solution. Results show that great tensile thermal stresses in the reservoir were initiated by the injection of the cooled thermal water in the injection wellbore Uha GT-2. The results of the simulated thermal stresses indicate a great potential for the ensuing shear failures to affect an extensive area and could be used as input data for the planned shear failure simulations in the future.

Numerical investigation of the hydromechanical response of a natural fracture during fluid injection using an efficient sequential coupling model

Hydraulic fracturing is a complicated hydromechanical coupled process, especially in shale gas and deep geothermal reservoirs, in which natural fractures exist. Due to the geological complexity caused by invisibility, and the challenge and high cost in field investigations, numerical modeling becomes an alternative. In this paper, an integrated numerical model is developed to investigate the hydromechanical behavior of a natural fracture during the fluid injection. In the developed model, the mechanical behavior of the fracture including fracture opening, closure, shear dilation, and shear failure is described by proposed constitutive equations; meanwhile, the hydraulic process is simplified as the fluid flows through two parallel planes. The coupled mechanical and hydraulic equations are sequentially formulated in an implicit schema by combining the finite different method and the finite volume method. The advantage of this numerical schema is that the two coupled processes are solved separately and only one sub-iteration is needed. Thus, the solution is efficient and stable than that formulated in a monolithic coupling. Besides, the implicit formulation of the flow equation makes it possible to set a relative large time step. The developed model is verified through three numerical examples. Then, it is used to investigate the hydromechanical behavior of a natural fracture during the fluid injection with a fictive reservoir. Sensitivity studies with variations in the stress state, the fluid injection rate, the fluid viscosity, and the injection form are conducted. The simulation results show that the mechanism in the far field is mainly dominated by shear dilation in contact condition, whereas the mechanism near the injection could be mixed shear–tension in either the contact or the separation conditions. With the increase in the shear stress and the injection length, decrease in the injection rate and the fluid viscosity, the fracture state near the injection will change from separation to contact, the injection pressure will decline below the primary normal stress, and the dominated mechanism is shear dilation. The findings in this study give a better understanding of the mechanical mechanism and the pressure response of a natural fracture during the fluid injection.