Tianfu Xu

A study on the interaction of mud acid with rock for chemical stimulation in an enhanced geothermal system

In order to explore the thermal energy from the low-permeability hot dry rocks, both thermal and shear stimulations were employed to create connective fracture networks that allow fluid flow in the reservoir, which is the concept of enhanced geothermal system (EGS). However, strong hydraulic stimulation may lead to preferential short-circuiting flow paths where the fluid transports too fast to take in sufficient heat from the reservoir. To solve this problem, the chemical stimulation was proposed recently, which injects aqueous chemical agents such as acids into reservoir to enlarge the fractures by dissolving the minerals in the rocks. This study examines in the laboratory the effects of using mud acid to enhance the permeability of the rock cores that were recovered from Yingcheng Formation in Songliao Basin, China. The results suggest that the permeability in the core increases with the injection rate of the mud acid. The core scanning electron microscope analysis shows that the increase in permeability is mainly induced by the dissolution of K-feldspar and albite. Secondary amorphous silica was formed due to the acid–rock interaction. The chemical stimulation in the laboratory can be reproduced by reactive transport simulation. The experimental and numerical studies confirmed that the injection of mud acid allows the permeability enhancement during field chemical stimulation for an EGS reservoir.

Multiphase Flow Behavior of Layered Methane Hydrate Reservoir Induced by Gas Production

Gas hydrates are expected to be a potential energy resource with extensive distribution in the permafrost and in deep ocean sediments. The marine gas hydrate drilling explorations at the Eastern Nankai Trough of Japan revealed the variable distribution of hydrate deposits. Gas hydrate reservoirs are composed of alternating beds of sand and clay, with various conditions of permeability, porosity, and hydrate saturation. This study looks into the multiphase flow behaviors of layered methane hydrate reservoirs induced by gas production. Firstly, a history matching model by incorporating the available geological data at the test site of the Eastern Nankai Trough, which considers the layered heterogeneous structure of hydrate saturation, permeability, and porosity simultaneously, was constructed to investigate the production characteristics from layered hydrate reservoirs. Based on the validated model, the effects of the placement of production interval on production performance were investigated. The modeling results indicate that the dissociation zone is strongly affected by the vertical reservoir’s heterogeneous structure and shows a unique dissociation front. The beneficial production interval scheme should consider the reservoir conditions with high permeability and high hydrate saturation. Consequently, the identification of the favorable hydrate deposits is significantly important to realize commercial production in the future.

Classical and integrated multicomponent geothermometry at the Tengchong geothermal field, Southwestern China

To reconstruct deep fluid chemical composition and increase the confidence in estimated reservoir temperatures, a more integral geothermometry method was compared to other classical geothermometers. Here, we apply the integrated multicomponent geothermometry (IMG) method using the GeoT code to estimate reservoir temperatures at the Tengchong geothermal field in Southwestern China. Results show reservoir temperatures calculated using the quartz geothermometer are closest to those estimated with the IMG method. The concentrations of Al and Mg, as well as selected minerals for geothermometry computations, are key factors for successfully using the IMG. Using the IMG method together with classical geothermometers can significantly increase confidence in reservoir temperature estimations. The methods presented and simulation program used here may be useful for analysis of other geothermal fields under similar conditions.

Effects of Formation Dip on Gas Production from Unconfined Marine Hydrate-Bearing Sediments through Depressurization

The effects of geologic conditions and production methods on gas production from hydrate-bearing sediments (HBS) have been widely investigated. The reservoir was usually treated as horizontal distribution, whereas the sloping reservoir was not considered. In fact, most strata have gradients because of the effects of geological structure and diagenesis. In this study, based on currently available geological data from field measurements in Shenhu area of the South China Sea, the effects of formation dip on gas production were investigated through depressurization using a horizontal well. The modeling results indicate that the strategy of horizontal well is an effective production method from the unconfined Class 2 HBS. The predicted cumulative volume of methane produced at the 1000 m horizontal well was 4.51 × 107 ST m3 over 5-year period. The hydrate dissociation behavior of sloping formation is sensitive to changes in the reservoir pressure. As in unconfined marine hydrate reservoir, the sloping formation is not conducive to free methane gas recovery, which results in more dissolved methane produced at the horizontal well. The obvious issue for this challenging target is relatively low exploitation efficiency of methane because of the recovery of very large volumes of water. Consequently, the development of the favorable well completion method to prevent water production is significantly important for realizing large scale hydrate exploitation in the future.

Land uplift induced by injection: a feasible method to evaluate the security of CO 2 capture and sequestration projects

Technology of CO2 capture and sequestration (CCS) is one of the many solutions to reduce greenhouse gases and alleviate the current global warming, but its security is important and needs to be evaluated. A simulator which links TOUGHREACT and FLAC3D was used to simulate the process of coupled temperature-hydrologic-mechanics (THM) in CCS. A test on laboratory scale was set up and water was injected into compacted sand covered by low permeability clay to study the land uplift displacement. The results were used to verify the accuracy of the simulator for calculating the THM coupling. The effects of injection quantity, injection time, and injection mode on land uplift were also studied on the constructed model. At last, a land uplift evaluation system was built to quantify the CO2 escape if any. The evaluation process can be divided into five steps: model generalization, acquisition of model parameters, numerical modeling, simulation and analysis, monitor comparison, and evaluation of model results. The major output of this study will provide a feasible method for quantitative analysis of CO2 leakage in CCS projects.

Effect of hydrological spatial variability on the heat production performance of a naturally fractured geothermal reservoir

In this work, the effect of hydrological spatial variability on the heat production performance of a naturally fractured geothermal reservoir was investigated. Using the geological information of an abandoned natural gas exploration well in northern Songliao Basin, northeast China, the hydraulic properties of the fractured reservoir were studied based on DFN models. The heterogeneous distribution of reservoir hydrological properties was simulated using random field method. Evolution characteristics of production temperature, heat extraction rate, injection pressure, reservoir flow impedance, and the spatio-temporal distribution of temperature and pressure were studied. A sensitivity analysis was performed on correlation distance and coefficient of variation for the random field model. The results indicate that spatial variability of the hydrological parameters has significant influence on the heat production performance. Besides, correlation distance and coefficient of variation are two key parameters that influence the distributions of reservoir parameters and cause the variation of heat production performance in random field models.

Experimental Study on the Effects of Stress Variations on the Permeability of Feldspar-Quartz Sandstone

The multistage and discontinuous nature of the injection process used in the geological storage of CO2 causes reservoirs to experience repeated loading and unloading. The reservoir permeability changes caused by this phenomenon directly impact the CO2 injection process and the process of CO2 migration in the reservoirs. Through laboratory experiments, variations in the permeability of sandstone in the Liujiagou formation of the Ordos CO2 capture and storage (CCS) demonstration project were analyzed using cyclic variations in injection pressure and confining pressure and multistage loading and unloading. The variation in the micropore structure and its influence on the permeability were analyzed based on micropore structure tests. In addition, the effects of multiple stress changes on the permeability of the same type of rock with different clay minerals content were also analyzed. More attention should be devoted to the influence of pressure variations on permeability in evaluations of storage potential and studies of CO2 migration in reservoirs in CCS engineering.

On Fluid and Thermal Dynamics in a Heterogeneous CO2 Plume Geothermal Reservoir

CO2 is now considered as a novel heat transmission fluid to extract geothermal energy. It can achieve both the energy exploitation and CO2 geological sequestration. The migration pathway and the process of fluid flow within the reservoirs affect significantly a CO2 plume geothermal (CPG) system. In this study, we built three-dimensional wellbore-reservoir coupled models using geological and geothermal conditions of Qingshankou Formation in Songliao Basin, China. The performance of the CPG system is evaluated in terms of the temperature, CO2 plume distribution, flow rate of production fluid, heat extraction rate, and storage of CO2. For obtaining a deeper understanding of CO2-geothermal system under realistic conditions, heterogeneity of reservoir’s hydrological properties (in terms of permeability and porosity) is taken into account. Due to the fortissimo mobility of CO2, as long as a highly permeable zone exists between the two wells, it is more likely to flow through the highly permeable zone to reach the production well, even though the flow path is longer. The preferential flow shortens circulation time and reduces heat-exchange area, probably leading to early thermal breakthrough, which makes the production fluid temperature decrease rapidly. The analyses of flow dynamics of CO2-water fluid and heat may be useful for future design of a CO2-based geothermal development system.

Use of a CO2 Geological Storage System to Develop Geothermal Resources: A Case Study of a Sandstone Reservoir in the Songliao Basin of Northeast China

The concept of a carbon dioxide(CO2) plume geothermal (CPG) system, which uses the CO2 geological storage system to develop geothermal resources has been proposed recently. On the basis of the geological structures and geothermal conditions in the Songliao Basin in North East China, a three-dimensional model of a sandstone layer with a temperature of 120°C was developed using TOUGHREACT. Numerical simulations for operating both the geothermal system using CO2 only and that using water only were carried out, and the results compared. A number of comprehensive processes including flow (under gravity, buoyancy and injection pressure), and heat transfer have been considered. A cold fluid (20°C) injection well and a hot fluid production well were specified in the geothermal reservoir. The heat extraction rates for CO2 and water as heat transfer medium were analysed and compared. Modeling results indicate that compared to the CPG system, the variation range of temperature of the water system reduces by about 50% during the same simulation period. This phenomenon is caused by the favorable properties of CO2 such as low density and viscosity. The heat extraction rate for CO2 significantly increased by nearly 200% more than water in this sandstone reservoir under the present simulation conditions. Thus, the fundamental understanding and the scientific base for the future CPG development could be provided by this research.