The flow state of gas in coals is very complicated. We should pay attention to whether the permeability calculated by Darcy's law is in accordance with the actual situation. We conducted an experiment on coal permeability and deformation under fixing confining pressure and increasing axial stress conditions. The objective is to investigate the variation of Reynolds number Re. In this study, the dynamic evolution of the Reynolds number is calculated under the relevant assumptions. The Reynolds number increases with an increase in the axial stress. In addition, the larger the value of initial Reynolds numbers, the greater the value of Re in the postpeak, and the possibility of nonlinear flow state is higher. Further, if the mass density (rho) and fluid viscosity (mu) are constant, the decrease in the amplitudes of the flow rate is less than the increase in the equivalent diameter of the seepage path. Moreover, the tensile stress generated around the pores and fractures parallel or nearly parallel to the axial stress direction with increase in the axial stress results in an increase in the Reynolds numbers and equivalent diameter of the seepage path increase due to the development, expansion, and penetration of the cracks.
The 2016 Mw 7.8 Kaikōura earthquake induced groundwater level changes throughout New Zealand. Water level changes were recorded at 433 sites in compositionally diverse, young, shallow aquifers, at distances of between 4 and 850 km from the earthquake epicentre. Water level changes are inconsistent with static stress changes but do correlate with peak ground acceleration (PGA). At PGAs exceeding ~2 m/s2, water level changes were predominantly persistent increases. At lower PGAs, there were approximately equal numbers of persistent water level increases and decreases. Shear-induced consolidation is interpreted to be the predominant mechanism causing groundwater changes at accelerations exceeding ~2 m/s2, whereas permeability enhancement is interpreted to predominate at lower levels of ground acceleration. Water level changes occur more frequently north of the epicentre, as a result of the fault’s northward rupture and resulting directivity effects. Local hydrogeological conditions also contributed to the observed responses, with larger water level changes occurring in deeper wells and in well-consolidated rocks at equivalent PGA levels.
The formation of the fracture network in shale hydraulic fracturing is the key to the successful development of shale gas. In order to analyze the mechanism of hydraulic fracturing fracture propagation in cemented fractured formations, a numerical simulation about fracture behavior in cemented joints was conducted based firstly on the block discrete element. And the critical pressure of three fracture propagation modes under the intersection of hydraulic fracturing fracture and closed natural fracture is derived, and the parameter analysis is carried out by univariate analysis and the response surface method (RSM). The results show that at a low intersecting angle, hydraulic fractures will turn and move forward at the same time, forming intersecting fractures. At medium angles, the cracks only turn. At high angles, the crack will expand directly forward without turning. In conclusion, low-angle intersecting fractures are more likely to form complex fracture networks, followed by medium-angle intersecting fractures, and high-angle intersecting fractures have more difficulty in forming fracture networks. The research results have important theoretical guiding significance for the hydraulic fracturing design.
Oil-based drilling fluids (OBDFs) have a strong wellbore stabilization effect, but little attention has been paid to the formation damage caused by oil-based drilling fluids based on traditional knowledge, which is a problem that must be solved prior to the application of oil-based drilling fluid. For ultradeep fractured tight sandstone gas reservoirs, the reservoir damage caused by oil-based drilling fluids is worthy of additional research. In this paper, the potential damage factors of oil-based drilling fluids and fractured tight sandstone formations are analyzed theoretically and experimentally. The damage mechanism of oil-based drilling fluids for fractured tight sandstone gas reservoirs is analyzed based on the characteristics of multiphase fluids in seepage channels, the physical and chemical changes of rocks, and the rheological stability of oil-based drilling fluids. Based on the damage mechanism of oil-based drilling fluids, the key problems that must be solved during the damage control of oil-based drilling fluids are analyzed, a detailed description of formation damage characteristics is made, and how to accurately and rapidly form plugging zones is addressed. This research on damage control can provide a reference for solving the damage problems caused by oil-based drilling fluids in fractured tight sandstone gas reservoirs.
Forty-five gas samples have been collected from natural gas manifestations at the island of Kos-the majority of which are found underwater along the southern coast of the island. On land, two anomalous degassing areas have been recognized. These areas are mainly characterized by the lack of vegetation and after long dry periods by the presence of sulfate salt efflorescence. Carbon dioxide is the prevailing gas species (ranging from 88 to 99%), while minor amounts of N-2 (up to 7.5%) and CH4 (up to 2.1%) are also present. Significant contents of H-2 (up to 0.2%) and H2S (up to 0.3%) are found in the on-land manifestations. Only one of the underwater manifestations is generally rich in N-2 (up to 98.9%) with CH4 concentrations of up to 11.7% and occasionally extremely low CO2 amounts (down to 0.09%). Isotope composition of He ranges from 0.85 to 6.71 R/RA, indicating a sometimes-strong mantle contribution; the highest values measured are found in the two highly degassing areas of Paradise beach and Volcania. C-isotope composition of CO2 ranges from -20.1 to 0.64 parts per thousand vs. V-PDB, with the majority of the values being concentrated at around -1 parts per thousand and therefore proposing a mixed mantle-limestone origin. Isotope composition of CH4 ranges from -21.5 to +2.8 parts per thousand vs. V-PDB for C and from -143 to +36 parts per thousand vs. V-SMOW for H, pointing to a geothermal origin with sometimes-evident secondary oxidation processes. The dataset presented in this work consists of sites that were repeatedly sampled in the last few years, with some of which being also sampled just before and immediately after the magnitude 6.6 earthquake that occurred on the 20(th) of July 2017 about 15km ENE of the island of Kos. Changes in the degassing areas along with significant variations in the geochemical parameters of the released gases were observed both before and after the seismic event; however, no coherent model explaining those changes was obtained. CO2 flux measurements showed values of up to about 10(4) gxm(-2)x d(-1) in both the areas of Volcania and Kokkino Nero, 5 x 10(4) g x m(-2) x d(-1) at Paradise beach, and 8 x 10(5)gxm(-2) x d(-1) at Therma spring. CO2 output estimations gave values of 24.6, 16.8, 12.7, and 20.6 t x d(-1), respectively, for the above four areas. The total output of the island is 74.7t x d(-1) and is comparable to those of the other active volcanic/geothermal systems of Greece (Nisyros, Nea Kameni, Milos, Methana, and Sousaki).
Permeability is an important physical property of rock. In rock and rock-like materials, permeability after yielding is closely related to plastic flow behavior. Theoretical analysis and experimental investigations are effective and reliable ways of studying the evolution of permeability. In this paper, fluid flow tests of sandstone samples under 11 stress states were conducted using the MTS 816.02 rock mechanics testing system and a self-designed permeation system. A new plastic flow rule was proposed based on the Mohr-Coulomb yield criterion and a nonassociated flow rule. The applicability of the flow rule was verified by comparing the estimated and experimental values of the equivalent shear strain of the Mohr-Coulomb criterion. Two new coefficients were defined to reflect the influence of the volume deformation and shear deformation on the permeability. A permeability model for plastic flow was established, which can be used to calculate both single-step and multistep permeability estimates of the sandstone samples during plastic flow. The errors of both estimation methods were analyzed. The experimental results showed that the plastic multiplier for the sandstone samples was positive during unloading conditions and negative during loading conditions. The experimental value error of the single-step permeability estimation was less than 16.5%. The experimental value errors of the multistep permeability estimations varied between 15.6% and 16.7%, indicating that the iterative format of the multistep permeability estimation method was generally stable and highly precise. A comparison of the permeability influence coefficients indicated that the influence of the equivalent shear strain on the permeability was smaller than that of the volumetric strain.
In this paper, we investigated the gas permeation properties of fractured rock during the process of stage load axial stress and cyclic load and unload confining pressure and come to some conclusions as follows: (1) the relationship between radial strain and deviatoric stress satisfied the quadratic polynomial function, and the relationship between permeability and deviatoric stress satisfied the exponential function under different axial stress levels; (2) in the staged load axial stress process, the axial strain-deviatoric curves of specimens conform to the quadratic polynomial function. The axial strain-deviatoric curves of specimens conform to linear function when confining pressure is 8.6MPa or 2.0MPa at different axial stress levels; (3) we used the permeability damage rate and maximum permeability damage rate to evaluate the recovery degree and reduction extent of permeability; (4) we used the volume expansion ratio and the maximum volume expansion ratio to evaluate the expansion degree and increase extent of rock samples' volume.
Accurate recognition of the types of alteration fluid and the development mechanisms are important concerns in studying deep marine carbonate reservoirs. Major fluid types, such as seawater, meteoric water, deep burial formation water, hydrothermal fluid, and thermochemical sulfate reduction- (TSR-) derived fluid, were identified based on carbon, oxygen, and strontium isotope compositions of many samples from the Tarim, Sichuan, and Ordos basins in China. Compared with normal marine limestones, seawater calcite cement has similar isotopic compositions. Calcite cement precipitated from meteoric water has extremely light oxygen isotope compositions, and its delta O-18(V-PDB) reaches -18.8 parts per thousand. Due to the fractionation of oxygen isotopes at high temperatures (101.2 similar to 145.6 degrees C), calcite precipitated from deep burial formation water and deep hydrothermal fluid has moderately light oxygen isotope compositions. The TSR process consumes organic matter to produce CO2/CO32-, and the calcite from TSR-derived fluid has very light carbon isotopes (delta O-18(V-PDB), -18.9 parts per thousand) due to the incorporation of organic CO2/CO32-. Formation water and TSR-derived fluid generally originate and are confined within the carbonates and are consequently termed endogenous fluids. The Sr-87/Sr-86 ratios of calcite cements from endogenous fluids are basically the same as those of surrounding carbonates. Meteoric water and hydrothermal fluid originate outside the carbonate strata and are exogenous fluids. The Sr-87/Sr-86 ratios of calcite cements from exogenous fluids are higher than those of surrounding carbonates, up to 0.710558. For karst carbonate reservoirs developed in tectonic uplift-meteoric water environments, the reservoir spaces of karst caves and fractures occur principally under and near unconformity surfaces and megacrystalline calcite cements occur below the karst zone. In deep fault-hydrothermal fluid environments, high-quality carbonate reservoirs develop downward into ultradeep strata. In deep burial-TSR-derived fluid environments, dissolution porosity can be well preserved for a long geological time due to high CO2 and H2S concentrations.
A series of flow experiments were performed on matched fractures to study the problem of non-Darcy flow in fractured media. Five rock fractures of different roughness were generated using indirect tensile tests, and their surface topographies were measured using a stereo topometric scanning system. The fracture was assumed to be a self-affine surface, and its roughness and anisotropy were quantified by the fractal dimension. According to the flow tortuosity effect, the nonlinear flow was characterized by hydraulic tortuosity and surface tortuosity power law relationships based on Forchheimer's law. Fracture seepage experiments conducted with two injection directions (0 degrees and 90 degrees) showed that Forchheimer's law described the nonlinear flow well. Both the proposed model and Chen's double-parameter model gave similar results to the experiment, but the match was closer with the proposed model. On this basis, a new formula for the critical Reynolds number is proposed, which serves to distinguish linear flow and Forchheimer flow.
Shale damage investigation is important in shale gas development. This paper is concerned with the experimental identification of ultrasonic wave velocities and damage mechanic parameters of Longmaxi shale under water-based mud soaking and confining pressure loading. The wave velocities increased with increasing confining pressure, while wave velocities decreased with increasing soaking time. The anisotropy of Young's modulus decreases when confining pressure increases. As soaking time increases, the anisotropy coefficient increases. As soaking time and confining pressure rise, the damage parameters also show complex changes. The results are beneficial for shale gas development.
During July 2016, the first integrated heat flow, CO2, and He-3 emission survey was conducted across 0.5km(2) of the summit cone and crater of Teide volcano, Tenerife, Canary Islands, Spain. The thermal energy released from Teide summit cone by diffuse degassing was 2.2 MW, and the heat flux calculated through Dawson's method was 8.1MW, difference due to the comparison of purely convective areas as the crater with diffusive areas as the flanks of the volcano. Diffuse CO2 output was 211 +/- 20t d(-1), and He-3 emission was estimated to be within a range between 0.35 and 0.89 mol y(-1). The obtained values of diffuse degassing and heat fluxes are close to others obtained for similar volcanic areas. The calculation of He-3/heat ratio for the first time in this volcanic system supports the presence of an important mantle source for the degassing of Teide volcano.
Geogenic noble gases are contained in crustal rocks at inter- and intracrystalline sites. In this study, bedded rock salt from southern New Mexico was deformed in a variety of triaxial compression states while measuring the release of naturally contained helium and argon utilizing mass spectrometry. Noble gas release is empirically correlated to volumetric strain and acoustic emissions. At low confining pressures, rock salt deforms primarily by microfracturing, rupturing crystal grains, and releasing helium and argon with a large amount of acoustic emissions, both measured real-time. At higher confining pressure, microfracturing is reduced and the rock salt is presumed to deform more by intracrystalline flow, releasing less amounts of noble gases with fewer acoustic emissions. Our work implies that geogenic gas release during deformation may provide an additional signal which contains information on the type and amount of deformation occurring in a variety of earth systems.
Although there are many existing analytical studies of tidal groundwater level fluctuations in coastal aquifer systems, few of them focus on an offshore submarine aquifer. Here, we consider tidal groundwater head fluctuations in a submarine leaky confined aquifer overlain by a semipermeable seabed. Both the seabed and the confined aquifer are assumed to extend horizontally infinitely. A one-dimensional mathematical model is established to describe the problem, and the analytical solution is derived. The impacts of the tidal loading efficiency, hydraulic conductivity and elastic storage of the semipermeable layer and aquifer on the groundwater head fluctuations in the aquifer system are analyzed and discussed. Solution analyses indicated that tidal loading effects tend to enhance the amplitude of the tidal groundwater fluctuation in the confined aquifer system and to reduce the phase shift between the groundwater head and the sea tide fluctuations.
The widely used application of horizontal well makes it significant to effectively evaluate rate performance of horizontal well in oil and gas reservoir. However, most models in previous work only focus on rate decline analysis (RDA) of horizontal well with single section (HWSS); they hardly address the problem that production rate distributes nonuniformly along horizontal wellbore in analyzing rate transient behaviors. However, only some horizontal segments contribute to the total production rates, and the production of each section along horizontal wellbore is not the same in fact, which may be caused by reservoir heterogeneity, selective completion, and nonuniform formation damage along horizontal wellbore. Therefore, the effect of these phenomena on rate decline characteristics cannot be ignored. The aim of this paper is to propose an analytical model to investigate transient rate response of a horizontal well with multiple sections (HWMS). The compound type curves, including the normalized production curve, the normalized production integral curve, and the production integral derivative curve, are developed to distinguish the different cases. The influences of some sensitive parameters on decline curves are further discussed. Results show obvious differences on the decline curves between the HWMS and HWSS. The parameters are sensitive on decline curves, which explore the feasible application on production performance evaluation and parameters interpretation through history matching the production data with the compound type curves in this paper.
Water-based hydraulic fracturing for the exploitation of shale gas reservoirs may be limited by two main factors: (1) water pollution and chemical pollution after the injection process and (2) permeability decrease due to clay mineral swelling upon contact with the injection water. Besides, shale rock nearly always contains fractures and fissures due to geological processes such as deposition and folding. Based on the above, a damage-based coupled model of rock deformation and gas flow is used to simulate the fracturing process in jointed shale wells with CO2 fracturing. We validate our model by comparing the simulation results with theoretical solutions. The research results show that the continuous main fractures are formed along the direction of the maximum principal stress, whilst hydraulic fractures tend to propagate along the preexisting joints due to the lower strength of the joints. The main failure type is tensile damage destruction among these specimens. The preexisting joints can aggravate the damage of the numerical specimens; the seepage areas of the layered jointed sample, vertical jointed sample, and orthogonal jointed sample are increased by 32.5%, 29.16%, and 35.05%, respectively, at time t=39s compared with the intact sample. The preexisting horizontal joints or vertical joints promote the propagation of hydraulic fractures in the horizontal direction or vertical direction but restrain the expansion of hydraulic fractures in the vertical or horizontal direction.
The permeability structure resulting from high fluid pressure stimulation of a geothermal resource is the most important parameter controlling the feasibility and the viability of enhanced geothermal systems ( EGS ), yet is the most elusive to constrain. Linear diffusion models do a reasonably good job of constraining the front of the stimulated region because of the t 1/2 dependence of the perturbation length, but triggering pressures resulting from such models, and the permeability inferred using the diffusivity parameter, drastically underestimate both permeability and pressure changes. This leads to incorrect interpretations about the nature of the system, including the degree of fluid pressures needed to induce seismicity required to enhance the system. Here, I use a minimalist approach to modeling and show that all of the observations from Basel (Switzerland) fluid injection experiment are well matched by a simple model where the dominant control on the system is a large‐scale change in permeability at the onset of slip. The excellent agreement between observations and these simplest of models indicates that these systems may be less complicated than envisaged, thus offering strategies for more sophisticated future modeling to help constrain and exploit these systems. The evolution of the permeability field in the Basel enhanced geothermal system was modelled using a simple non‐linear diffusion model with a step‐wise increase in permeability when the failure condition is reached. This simple model reproduces all the observations obtained during that experiment.
B. Walter et al. integrated structural interpretation of remote sensing images, field work, and geochemistry to determine the role of the different regional structural features that may control different fluid outflow zones, as well as the nature and the source of the different fluids along a rift zone in western Uganda. Remaining in the Upper Rhine Graben study area, J. Freymark et al. used a data-based 3D structural model of the central Upper Rhine Graben for 3D coupled simulations of fluid and heat transport. To assess the influence of the main faults bordering the graben on the hydraulic and the deep thermal field, the authors carried out a sensitivity analysis on fault width and permeability, evaluating the implications for the deep temperature distribution.  P. Calcagno, C. Baujard, L. Guillou-Frottier, A. Dagallier, A. Genter, "Estimation of the deep geothermal potential within the Tertiary Limagne basin (French Massif Central): an integrated 3D geological and thermal approach," Geothermics, vol. 51, pp. 496-508, DOI: 10.1016/j.geothermics.2014.02.002, 2014.
Surface deformation due to fluid extraction can be detected by satellite-based geodetic sensors, providing important insights on subsurface geomechanical properties. In this study, we use Differential Interferometric Synthetic Aperture Radar (DInSAR) observations to measure ground deformation due to fluid extraction at the Los Humeros Geothermal Field (Puebla, Mexico). Our main goal is to reveal the pressure distribution in the reservoir and to identify reservoir compartmentalization, which can be important aspects for optimizing the production of the field. The result of the PS-InSAR (Persistent Scatterer by Synthetic Aperture Radar Interferometry) analysis shows that the subsidence at the LHGF was up to 8 mm/year between April 2003 and March 2007, which is small relative to the produced volume of 5×106 m3/year. The subsidence pattern indicates that the geothermal field is controlled by sealing faults separating the reservoir into several blocks. To assess if this is the case, we relate surface movements with volume changes in the reservoir through analytical solutions for different types of nuclei of strain. We constrain our models with the movements of the PS points as target observations. Our models imply small volume changes in the reservoir, and the different nuclei of strain solutions differ only slightly. These findings suggest that the pressure within the reservoir is well supported and that reservoir recharge is taking place.
Subsurface temperature data is usually only accessible as point information with a very limited number of observations. To spatialize these isolated insights underground, we usually rely on interpolation methods. Unfortunately, these conventional tools are in many cases not suitable to be applied to areas with high local variability, like densely populated areas, and in addition are very vulnerable to uneven distributions of wells. Since thermal conditions of the surface and shallow subsurface are coupled, we can utilize this relationship to estimate shallow groundwater temperatures from satellite-derived land surface temperatures. Here, we propose an estimation approach that provides spatial groundwater temperature data and can be applied to natural, urban, and mixed environments. To achieve this, we combine land surface temperatures with anthropogenic and natural processes, such as downward heat transfer from buildings, insulation through snow coverage, and latent heat flux in the form of evapotranspiration. This is demonstrated for the city of Paris, where measurements from as early as 1977 reveal the existence of a substantial subsurface urban heat island (SUHI) with a maximum groundwater temperature anomaly of around 7 K. It is demonstrated that groundwater temperatures in Paris can be well predicted with a root mean squared error of below 1 K by means of satellite-derived land surface images. This combined approach is shown to improve existing estimation procedures that are focused either on rural or on urban conditions. While they do not detect local hotspots caused by small-scaled heat sources located underground (e.g., sewage systems and tunnels), the findings for the city of Paris for the estimation of large-scale thermal anomalies in the subsurface are promising. Thus, the new estimation procedure may also be suitable for other cities to obtain a more reliable insight into the spatial distribution of urban ground and groundwater temperatures.