The recovery of natural gas hydrate (NGH) from marine sediments faces challenging due to not only the gas productivity itself but also the possible geohazards, such as seafloor subsidence, submarine ...landslide, wellbore instability and possible sand production. The coupled thermal-hydrodynamic-mechanical (THM) processes during NGH recovery are generally complex, and numerical simulation tools are needed to assess related geomechanical responses. We have extended the Biot consolidation model in our previous simulator TOUGH2Biot, and incorporated into the existing TOUGH + hydrate code, resulting in a THM simulator for the NGH recovery. The THM simulator is used to assess the geomechanical responses to gas recovery from an unconfined hydrate-bearing sediment (HBS) in the Shenhu area, South China Sea. We investigated depressurization using constant bottom-hole pressure through a horizontal well. Results show that methane production quickly reaches a stabilized state and the water rate increases linearly. The drawdown of pore pressure around the well controls the increase in effective stress. Subsidence becomes significant after depressurization due to the quickly propagation of pore pressure. Depressurization in early stage could contribute to more than half of the total subsidence. The decreasing production pressure leads to an increase in the methane production rate but deterioration in subsidence. A decrease in intrinsic permeability of overlying and underlying layer is undesirable due to its decrease in methane production rate and the worse of subsidence. A balance between gas productivity and related geomechanical response must be achieved. The methods and preliminary results presented in this study could help us to understand the geomechanical behaviors during NGH recovery and to design trial production schemes under similar conditions.
•A new simulator for geomechanical behavior for gas production from methane hydrate reservoir is presented.•Geomechanical responses to gas recovery from an unconfined hydrate bearing-sediment are assessed.•Gas productivity and related sediment subsidence should be balanced.•The effects of production pressure and permeability of bonded layers on subsidence are analyzed.
Supercritical geothermal resource is a renewable and carbon-free energy with enormous potential. Quantitative assessment of the geochemical processes involved in the future operation of this resource ...requires a comprehensive understanding of quartz solubility in subcritical and supercritical water. However, quartz solubility behaviors from subcritical to supercritical conditions are unclear, and the quartz solubility model requires refinement by improving the accuracy of experimental measurements. In this study, a series of in-situ sampling and measurements are conducted to investigate quartz solubility at 300–500 °C and 25–50 MPa by exploiting a novel and specifically designed experimental apparatus. The results indicate that the use of in-situ sampling and resistance continuous monitoring procedures can bring an accurate and reliable estimation of quartz solubilities near the supercritical point and in the low-density region. It also emerged that the quartz solubility in water is significantly affected by temperature and pressure both under subcritical and supercritical conditions, showing a positive correlation with pressure at isothermal and a retrograde behavior near the supercritical point. These features are interpreted by the thermodynamic theory, quartz hydrolysis process, and mathematical model. By combining our experimental results with high-quality data from the literature, a five-parameter density model is provided for the calculation of the silica concentrations in aqueous fluids up to supercritical geothermal conditions. To overcome the current limitations in the modeling of supercritical geothermal fluids, future experimental work needs to collect additional data on silica in real geofluids (e.g., saline fluid) to develop an advanced model. The findings of this study can help for a better understanding of geochemical processes in supercritical geothermal systems, as well as ore deposits and vent structures.
•New experimental method enables accurate estimation of quartz solubility.•Density models are evaluated and refined according to the experimental results.•Quartz dissolution behaviors at supercritical geothermal conditions are interpreted.
The development of sandstone-type geothermal energy is an important part of the development of geothermal resources and has great significance in promoting environmental protection and energy ...structural transformation. In sandstone geothermal energy development, recharging is the main method to ensure bottom hole pressure. However, the pressure and temperature changes of sandstone reservoirs under recharge conditions have not been extensively studied. It is easy to ignore the hydraulic relationship between the production and the injection wells, which leads to an increased risk of thermal breakthrough. Therefore, a three-dimensional hydrothermal coupling model is established, and simulation studies of different flow rates, well lengths, and well spacings are completed in this paper. Here, we show the numerical simulation results. The low temperature expansion zone and hydrostatic pressure near the injection well increase with increasing flow rate, and the maximum expansion of the low temperature zone is about 350 m. The low temperature expansion area near the injection well has a small relationship with the well spacing, and the increase in hydrostatic pressure is proportional to the well spacing. As the length of the well increases, the increase in hydrostatic pressure near the injection well decreases, indicating that the injected water under the long well section easily enters the reservoir. When no thermal breakthrough occurs and the hydrostatic pressure drops significantly near the production well, it is recommended that the flow rate be controlled at approximately 20-25 L/s, the well spacing should be 600-800 m, and the well length should be greater than 100 m.
•An improved phase field model is proposed for pore-scale reactive transport.•The reliability of the model is verified by the benchmark problems.•The applicable conditions and scope of Kozeny-Carman ...law are clarified.
Based on the phase-field method, a fluid–solid coupling modeling method was proposed to simulate the reaction transport in porous media at the pore scale and realize the dynamic tracking of the fluid–solid interface. We skillfully combined the phase-field module in COMSOL with the Heaviside function to realize the numerical solution of the model. The feasibility and reliability of the model are verified by comparing it with the results of previous studies on the related benchmark problems (the dissolution process of a single calcite crystal). To further explore the practicability of the model, we simulate the dissolution process in two-dimensional porous media generated by random circles under different transport conditions and reaction rates. The results show that the solute can be transported downstream of the model region under advection-dominated transport conditions, which are common in engineering applications, and the dissolution reaction in the medium takes place synchronously. When diffusion is the main transport mode which often occurs in natural conditions, and the reaction is relatively fast, the solute is difficult to transport downstream of the model region, mineral dissolution mainly occurs near the solute inlet, and mineral disappearance is carried out in a horizontal push mode. The permeability-porosity relationship is predicted by the Kozeny-Carman law for different transport conditions and reaction rates. The results show that the Kozeny-Carman law is more accurate in predicting permeability over a porosity range of 15% for advection-dominated reactive transport, considering specific surface area variation.
The natural gas hydrates detected in Shenhu Area, South China Sea are considered to have great potential for exploitation and to become a research hotspot. However, details of the dynamic ...accumulation and evolution processes of gas hydrate over a long time in this area remain unclear. In this study, we developed a numerical model to predict the accumulation of gas hydrates in the marine sediments by introducing the biogenic methanogenesis module to the framework of the existing simulator TOUGH+HYDRATE. A one-dimensional dynamic model was applied to reproduce the formation of gas hydrate at site SH2 in Shenhu Area. The burial of sediments and associated phenomena (e.g., evolution of temperature, sediment compaction and consequent reduction in sediment porosity and permeability, fluid expulsion) are taken into account. Modeling results indicate that with the conventional conversion coefficient (CC) of organic carbon to methane of 0.15, the amount of in-situ biogenic methane is not sufficient to form the highly saturated hydrate detected at this location and the remaining methane (91.4%) is supplied by the upward flow of fluids. The tectonic movement that took place about 1.5 Ma created pathways for the fluids to migrate vertically. The pore water and methane gas fluxes in the upward fluids were determined as 2 × 10−10 kg/s·m2 and 1 × 10−11 kg/s·m2, respectively, using the proved hydrate saturation profile and measured pore water chlorinity from recovered core samples. The distribution of hydrate in the sediments is significantly influenced by pore water salinity. The inhibitory effect of salt on hydrate formation, which shifts the pressure of three-phase (i.e., solid hydrate, methane gas and pore water) equilibrium to actual pore water pressure, could be the major reason for the appearance of a three-phase co-existence zone in the hydrate concentrated zone. Additionally, the water flux and diffusivity of salt have a profound effect on the formation of gas hydrate by affecting the salinity of pore water in the gas hydrate concentrated zone. The results of this study are of great significance for guiding marine gas hydrate exploration and resource evaluation.
•Biogenic methane generation module was embedded in the TOUGH+Hydrate code.•The fluxes of pore water and methane in the upward fluids flow were determined using the measured data.•The formation mechanism for the three-phase (water, methane gas and hydrate) co-existence zone was ascertained.•The influence of pore water salinity on hydrate formation and distribution was highlighted.
Pre-Darcy flow is widely found in many naturally porous media, such as low-permeability clay. Nonflowing liquid boundary layers strongly influence fluid flow in low-permeability porous media. There ...is evidence in the literature that the porosity and permeability of low-permeability porous media change with variations in the effective stress, which is called stress sensitivity. In this paper, a new mathematical model for pre-Darcy flow in low-permeability porous media is developed and validated with experimental data from the literature. The nonflowing boundary layer thickness can be obtained from the proposed model instead of the empirical equation. First, an equation for calculating the pore radius with stress sensitivity is derived. Two parameters εp and εr are defined to calculate the nonflowing boundary layer thickness. Second, based on the capillary bundle model, the apparent flow velocity and apparent liquid permeability of the porous media are determined. Finally, the stress sensitivity and boundary-layer effects on the apparent flow velocity and apparent liquid permeability are analyzed. The research results show that the effect of stress sensitivity on apparent flow velocity increases with increasing pore compressibility. When the pore compressibility Cp>0 MPa−1, the apparent liquid permeability increases continuously with increasing pressure gradient. When the pore compressibility Cp=0 MPa−1, the apparent liquid permeability continues to increase with increasing pressure gradient. The apparent liquid permeability eventually tends to a constant value. The apparent flow velocity decreases with εp, and εr decreases at a given pressure gradient. The larger εp and εr are, the faster the rate of increase in the apparent liquid permeability with increasing pressure gradient. The proposed model can estimate the apparent flow velocity at different pressure gradients with a high accuracy. This work is important for understanding flow phenomena in low permeability porous media, such as landslides caused by seepage of water in clay.
•A low-velocity pre-Darcy flow model in low permeability porous media is developed.•The nonflowing boundary layer thickness was obtained from a new model.•The relationship between the boundary layer and the pressure gradient is established.•The effect of stress sensitivity on velocity and permeability is analyzed.•The effect of nonflowing boundary layer on velocity and permeability is analyzed.
Investigation of water-rock reactions effects on the shear strength of gypsum-bearing mudstone is necessary for underground engineering design and assessment of engineering safety. A comprehensive ...study of the multi-scale deterioration effects and mechanisms of water-rock reactions on gypsum-bearing mudstone can contribute to developing better engineering applications. The 25 sets of laboratory dissolution tests and 30 sets of conventional rock mechanical compression tests conducted in this study demonstrate the progressive modification of rock properties on multiple scales, including changes in mineralogy, porosity, grain structure, microstructures, and complexity of micropores, with sodium sulfate and gypsum being the most active minerals. Water-rock reactions cause irreversible morphological changes in microstructures. For instance, the number of intracrystalline pores increased, the contact between grains changed from face-to-face contact to line-to-line contact, and the volume of micropores, mesopores and macropores inside the rock increased by a maximum of 80.0%, 59.9% and 71.4% respectively. The maximum increase in the porosity of the samples was 5.33%. The increasing number of microphysical changes leads to defects that develop into weak areas, significantly decreasing the shear strength of gypsum-bearing mudstone. The maximum decrease in the cohesion of the samples was 79.06% throughout the dissolution process. It was found that the decrease in the cohesion of the samples c at the early stage of dissolution is mainly affected by the increase in the micropore volume. In the middle and late stages, the cohesion c of the samples decreased in a straight line, which is mainly caused by the increase in the macropore volume and porosity. The fluctuation in the cumulative pore volume during the whole dissolution process leads to fluctuation in the internal friction angle φ. The multi-scale degradation model of rock samples induced by water-rock reactions should be formulated to accurately evaluate the effect of water-rock reactions on rock shear strength.
•Mechanical tests are conducted after water-rock reactions at different times.•The multi-scale degradation mechanism is analyzed in detail.•The cohesion c decreases (in a straight line) with the time of water-rock action.•The angle of internal friction φ fluctuates slightly.•Defects form weak areas, resulting in a significant decrease in rock shear strength.
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.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
Sustainable and profitable energy production from Enhanced Geothermal System (EGS) requires a comprehensive understanding of the coupled effects of elastic and plastic deformation on the hydraulic ...evolution of geothermal fractures. Four flow-through tests were conducted on granite samples with a rough single fracture at 25–180 °C. Each test was performed on three cycles of loading-unloading processes within a confining pressure of 5–30 MPa. Experimental results indicated that the hydraulic properties are negatively correlated with confining pressure in a logarithmic manner. The coupled effects of elastic and plastic deformation induced by stress loading are the main factor affecting fracture hydraulic properties. Plastic deformation is associated with the mineral grains crush occurring in fracture contacting asperities, which is a permanent and irreversible process. Compared to the results at 25 °C, a larger reduction in permeability is observed at 180 °C, as revealed by a maximum reduction in hydraulic aperture up to >40% at this temperature. This means that larger plastic deformations in the fractures are associated with higher temperatures. Therefore, the addition of proppant is very important for sustainable geothermal development for fractured geothermal reservoirs under high-stress conditions. The ion concentration detection of the effluent solution confirms the existence of free-face dissolution in high-temperature scenarios. Generally, free-face dissolution is beneficial to low-permeability geothermal reservoirs due to its positive effects on fracture hydraulic properties. As the number of loading-unloading cycles increases, the hysteresis effect induced by plastic deformation becomes less and less. Compared to the first loading-unloading stage, the maximum decline of permeability decreases from 73% to 6% and 1% during the second and third stages, respectively. However, pressure dissolution under long-term stress loading should be further investigated, because it may disrupt the self-propping balance of geothermal fracture asperities.
•Flow-through experiments are conducted with cycle-loading confining pressure.•Mechanical deformation and chemical reaction coupled effects on fracture are analyzed.•The hydraulic properties of fracture show logarithmic variation during loading.•Free-face dissolution is confirmed by ion concentration detection.•A clear hysteretic effect at high temperatures is caused by plastic deformation.
Nature gas hydrate is a new kind of clean and potential resources. Depressurization is regarded as the most effective and promising hydrate production technology. One of the key points in improving ...the gas production effectiveness of depressurization is whether pressure gradient could transmit in strata effectively. Single well method is widely used in hydrate exploit which is circumscribed in expanding the range of hydrate decomposition. Consequently, the well structure and production strategy needs to be optimized for improving the gas recovery efficiency. The multilateral well technology is proposed for increasing the gas productivity of the reservoir greatly by increasing the multilateral branches. In this paper, we established a numerical simulation model based on the geological data NGHP-02-16 site in the KG basin to evaluate the gas production performance of the reservoir by depressurization. It mainly focuses on investigating the gas production performance of multilateral wells with different combinations of geometric parameters of multilateral branches, such as different dip angle, numbers, and spacing of lateral branches. The result shows that the multilateral well method can effectively increase the gas production rate with the water production rate increase slightly. The cumulative gas production volume of a single vertical well is about 2.85×106 m3, while it is of the multilateral well can reach 4.18×106 m3 during a one-year production. The well interference, the effective influence radius of each multilateral branch, and the vertical depth of the lateral branch are the main factors which affect the gas production efficiency of the multilateral well. The optimization of the geometric parameters of lateral should consider not only the gas production efficiency but also the well interference between the lateral branches.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
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