Geofluids

Detailed information on the hydrogeologic and hydraulic properties of the deeper parts of the upper continental crust is scarce. The pilot hole of the deep research drillhole (KTB) in crystalline basement of central Germany provided access to the crust for an exceptional pumping experiment of 1‐year duration. The hydraulic properties of fractured crystalline rocks at 4 km depth were derived from the well test and a total of 23100 m³ of saline fluid was pumped from the crustal reservoir. The experiment shows that the water‐saturated fracture pore space of the brittle upper crust is highly connected, hence, the continental upper crust is an aquifer. The pressure–time data from the well tests showed three distinct flow periods: the first period relates to wellbore storage and skin effects, the second flow period shows the typical characteristics of the homogeneous isotropic basement rock aquifer and the third flow period relates to the influence of a distant hydraulic border, probably an effect of the Franconian lineament, a steep dipping major thrust fault known from surface geology. The data analysis provided a transmissivity of the pumped aquifer T = 6.1 × 10⁻⁶ m² sec⁻¹, the corresponding hydraulic conductivity (permeability) is K = 4.07 × 10⁻⁸ m sec⁻¹ and the computed storage coefficient (storativity) of the aquifer of about S = 5 × 10⁻⁶. This unexpected high permeability of the continental upper crust is well within the conditions of possible advective flow. The average flow porosity of the fractured basement aquifer is 0.6–0.7% and this range can be taken as a representative and characteristic values for the continental upper crust in general. The chemical composition of the pumped fluid was nearly constant during the 1‐year test. The total of dissolved solids amounts to 62 g l⁻¹ and comprise mainly a mixture of CaCl₂ and NaCl; all other dissolved components amount to about 2 g l⁻¹. The cation proportions of the fluid (X_Ca approximately 0.6) reflects the mineralogical composition of the reservoir rock and the high salinity results from desiccation (H₂O‐loss) due to the formation of abundant hydrate minerals during water–rock interaction. The constant fluid composition suggests that the fluid has been pumped from a rather homogeneous reservoir lithology dominated by metagabbros and amphibolites containing abundant Ca‐rich plagioclase.

Mỏ vàng Sanshandao, nằm ở phía tây bắc của bán đảo Jiaodong, đông Bắc Trung Quốc, là một trong những mỏ vàng lớn nhất thuộc tỉnh vàng Jiaodong. Tại đây, quặng kiểu phân tán và kiểu mạch được chứa trong các granitoid thuộc thời kỳ Mesozoi. Sự khoáng hóa và biến đổi chủ yếu bị kiểm soát bởi đứt gãy Sanshandao–Cangshang ở vùng này. Sericite trích xuất từ các đá biến đổi trong vùng khoáng hóa cho một độ tuổi isochron Rb–Sr là 117,6 ± 3,0 Ma. Các dịch lỏng hình thành quặng trong mỏ vàng Sanshandao chứa CO2-H2O-NaCl±CH4 với nhiệt độ thấp đến trung bình và độ mặn thấp. Phân tích vi nhiệt cho thấy nhiệt độ đồng nhất dần giảm từ giai đoạn khoáng hóa sớm (258–416°C) đến giai đoạn khoáng hóa chính (180–321°C) và đến giai đoạn khoáng hóa muộn (112–231°C). Nhiệt độ đồng nhất từ cùng một giai đoạn khoáng hóa gần như giống nhau và không cho thấy sự gia tăng theo độ sâu. Tính chất của các dịch lỏng hình thành quặng gần như không thay đổi trong khoảng cách sâu 2000 m.

Pre‐earthquake and postearthquake temperature changes were documented in two hot springs at Xiangcheng. Pre‐earthquake changes were documented in spring I, 13 days before and 106 km away from the M_s 5.8 Zhongdian earthquake. The 11‐year cutoff spring spouted again, and the spouted water was 24°C hotter than the former escaping gas. Postearthquake changes were documented in spring II following the 2008 M_w 7.9 Wenchuan earthquake, approximately 425 km away from the epicenter. Temperature in spring II showed a step‐like increase with a magnitude of 4°C induced by the earthquake. Spring I which is 0.3 m apart from spring II did not show a sudden change following the earthquake. However, temperatures in the two springs were identical after the Wenchuan earthquake. It indicates that the earthquake generated new hydraulic connectivity between springs I and II, and the heat transport between the two springs accounts for the postearthquake temperature changes.

The Koryaksky-Avachinsky volcanogenic basin, which has an area of 2530 km², is located 25 km from Petropavlovsk-Kamchatsky City and includes five Quaternary volcanoes (two of which, Avachinsky (2750 masl) and Koryaksky (3456 masl), are active), and is located within a depression that has formed atop Cretaceous basement rocks. Magma injection zones (dikes and chamber-like shapes) are defined by plane-oriented clusters of local earthquakes that occur during volcanic activity (mostly in 2008–2011) below Koryaksky and Avachinsky volcanoes at depths ranging from −4.0 to −2.0 km and +1.0 to +2.0 km, respectively. Water isotopic (δD, δ¹⁸O) data indicate that these volcanoes act as recharge areas for their adjacent thermal mineral springs (Koryaksky Narzans, Isotovsky, and Pinachevsky) and the wells of the Bystrinsky and Elizovo aquifers. Carbon δ¹³С data in СО₂ from CO₂ springs in the northern foothills of Koryaksky Volcano reflect the magmatic origin of CO₂. Carbon δ¹³С data in methane CH₄ reservoirs penetrated by wells in the Neogene-Quaternary layer around Koryaksky and Avachinsky volcanoes indicate the thermobiogenic origin of methane. Thermal-hydrodynamic TOUGH2 conceptual modeling is used to determine what types of hydrogeologic boundaries and heat and mass sources are required to create the temperature, pressure, phase, and CO₂ distributions observed within the given geological conditions of the Koryaksky-Avachinsky volcanic geofluid system.

Due to active actions of groundwater and geothermal, the stability of underground engineering is important during geological structure active area. The damage mechanical theory and statistical mesoscopic strength theory based on Weibull distribution are widely used to discuss constitutive behaviors of rocks. In these theories, a statistical method is used to capture mesoscopic properties of rocks in order to generate a realistic behavior at a macroscopic scale. Based on the above theories, this paper aims at establishing a constitutive relation of brittle rocks under thermal-mechanical coupling conditions. First, a statistical damage constitutive model was established by considering the thermal effects and crack initiation strength. Subsequently, the parameters of the model were determined and expressed according to the characteristics of stress-strain curve. Third, the model was verified by conventional triaxial experiments under thermal-mechanical actions, and the experimental data and theoretical results were compared and analyzed in the case study. Finally, the physical meaning of the parameters and their effects on the model performance were discussed.

We derive the discrete fracture network (DFN) of a Lower Cretaceous carbonate platform succession exposed at Mt. Faito (Southern Apennines), which represents a good outcrop analogue of the coeval productive units of the buried Apulian Platform in the Basilicata oilfields. A stochastic distribution of joints has been derived by sampling at two different scales of observation. At the outcrop scale, we measured fracture attributes by means of scan lines. At a larger scale, we extracted fracture attributes from a 3D model. This multiscale survey showed the occurrence of an arresting bed for through-going fractures, which is characterized by a low relative permeability, determining a vertical compartmentalization. The DFN model, obtained by integrating fieldwork and numerical modelling by means of the 3D-Move® software, shows a well-defined relationship of permeability and fracture porosity with the relative connectivity of the fracture network. The latter is influenced by the length and aperture and to a lesser extent by the fracture intensity. The permeability distribution obtained for our outcrop analogue can be used to inform modelling of the Basilicata oilfield reservoirs, although the different burial history between the exposed Apennine Platform and the buried Apulian Platform must be taken into account.

The capillary‐sealing efficiency of intermediate‐ to low‐permeable sedimentary rocks has been investigated by N₂, CO₂ and CH₄ breakthrough experiments on initially fully water‐saturated rocks of different lithological compositions. Differential gas pressures up to 20 MPa were imposed across samples of 10–20 mm thickness, and the decline of the differential pressures was monitored over time. Absolute (single‐phase) permeability coefficients (k_abs), determined by steady‐state fluid flow tests, ranged between 10⁻²² and 10⁻¹⁵ m². Maximum effective permeabilities to the gas phase k_eff(max), measured after gas breakthrough at maximum gas saturation, extended from 10⁻²⁶ to 10⁻¹⁸ m². Because of re‐imbibition of water into the interconnected gas‐conducting pore system, the effective permeability to the gas phase decreases with decreasing differential (capillary) pressure. At the end of the breakthrough experiments, a residual pressure difference persists, indicating the shut‐off of the gas‐conducting pore system. These pressures, referred to as the ‘minimum capillary displacement pressures’ (P_d), ranged from 0.1 up to 6.7 MPa. Correlations were established between (i) absolute and effective permeability coefficients and (ii) effective or absolute permeability and capillary displacement pressure. Results indicate systematic differences in gas breakthrough behaviour of N₂, CO₂ and CH₄, reflecting differences in wettability and interfacial tension. Additionally, a simple dynamic model for gas leakage through a capillary seal is presented, taking into account the variation of effective permeability as a function of buoyancy pressure exerted by a gas column underneath the seal.

A hydrogeological conceptual model of the Caldas do Moledo geothermal site is proposed that shows mixing between geothermal waters and local shallow groundwaters. Stable isotope values of Caldas do Moledo geothermal waters indicate recharge areas located at relatively high altitudes (850–1250 m a.s.l.). The NW–SE Vigo–Régua shear zone plays an important role in fluid recharge and circulation towards the NNE–SSW Régua–Verin fault system, forming a path for ascent of geothermal fluids. The apparent ¹⁴C age of geothermal fluids (15.66 ± 2.86 ka BP) was estimated in the total dissolved inorganic carbon (TDIC). Geothermometer calculations indicate that, assuming a conductive temperature gradient of 32°C per kilometer for northern Portugal, the maximum depth of circulation is roughly 1.8 ± 0.4 km. The K, Ca and SO₄ concentrations found in some Caldas do Moledo geothermal spring waters show mixing between deep geothermal and shallow groundwater systems. Local shallow groundwaters showing the highest SO₄ concentrations were found at low elevation areas, originating from fertilisers and pesticides applied to the Port wine vineyards in the Douro River valley. Geothermal waters from boreholes AC1 and AC2 do not show evidences of direct pollution from the spreading of such agrochemicals.

Permeability and diffusivity are critical parameters of tight reservoir rocks that determine their viability for commercial development. Current methods for measuring permeability and/or diffusivity may lead to erroneous results when applied to very tight rocks including gas shales, coal, and tight gas sands, as well as rocks considered as seals for nuclear waste repositories and strata for geological sequestration of CO₂. The use of He as routinely applied to measure porosity, permeability, and diffusivity may result in non‐systematic errors because of the molecular sieving effect of the fine pore structure to larger molecules such as reservoir gases. Utilizing gases with larger adsorption potentials than He, such as N₂, and including all reservoir gases to measure porosity or permeability of rocks with high surface area is a viable alternative, but requires correcting for adsorption in the analyses. This study expands several approaches to measure permeability and diffusivity with considerations for gas adsorption, which has not been explicitly considered in previous studies. We present new models that explicitly correct for adsorption during pulse‐decay measurements of core under reservoir conditions, as well as on crushed samples used to approximate permeability or diffusivity. We also present a method to determine permeability or diffusivity from on‐site drill‐core desorption test data as carried out to determine gas in place in coals or gas shales. Our new approach utilizes late‐time data from experimental pressure‐decay tests, which we show to be more reliable and theoretically (and practically) accurate than the early‐time approach commonly used to estimate gas‐transport properties.

Laboratory experiments were performed to help understand the fluctuations and the spike‐like anomalies of Rn in a time series that was recorded continuously at monitoring stations. One of the experiments indicated that Rn is adsorbed on the surface of sand particles and can be liberated with minor changes in the physical conditions of the containing medium. Another experiment indicated that the liberation of ultra‐trace Rn, adsorbed on the surface of sand, was not very sensitive to small temperature and pressure changes but was responsive to the flow of carrier gases. Among the carrier gases tested, CO₂ was preferred because it has a boiling point similar to that of Rn. However, all other gases that are inert to Rn can also be carrier gases. Temperature variation in the supra soil layer can be measured fairly accurately inside double‐insulated PVC pipes that also house the Rn detecting system. Temperature variation appears to be related to localized strain heating that is a part of the earthquake energy variation cycle. Up‐flow of soil gas, caused by the strain heating, induced the sudden release of Rn, which thus appears as a spike‐like anomaly. The migration of soil gases is expected to follow the thermal cycle corresponding to each earthquake cycle. Therefore, the spike‐like anomalies can be used, in conjunction with the temperature variation cycle, as time markers to forecast the time, place, and magnitude of a coming earthquake.