This paper focuses on capillary pressure, Pc, as a parameter characterizing the sealing performance of rock, and reviews the current status of research on its evaluation method. Fluid sealing due to Pc is one of the most fundamental mechanisms controlling overall mass transport in underground formations. The petroleum exploration community has attempted to gather knowledge on evaluating sealing performance, but currently there is no measurement method that is both accurate and efficient. Moreover, the theoretical model of Pc does not go far beyond an ideal system with a simple and homogeneous internal structure. This aspect makes a theoretical approach to the sealing performance of natural rocks with complex structures difficult. On the other hand, in recent years, the importance of sealing performance has attracted growing interest in risk analyses of CO2 geological sequestration. As a result, the establishment of a methodology applicable to a CO2 system, besides resolving the above issues, is desired. Especially from the standpoint of CO2 geological sequestration, special attention should be paid to estimating the range of variations of Pc in cap rocks because that option requires an assessment over a large injection site in spite of the lack of available core samples. The authors' new approach, which attempts to quantify the effects of various factors on Pc using an artificial sample whose internal structure is fully controlled, might have the potential to contribute to those requirements.
Enhanced Oil Recovery (EOR) is used to increase the production of oil from geological reservoirs. EOR technology often involves injecting water into rock formations to recover oil remaining in rock pores. Therefore, an evaluation of the wettability of the oil–mineral–water 3-phase interfacial system is needed. In particular, the contact angle of water on the mineral surface in the presence of oil is the essential parameter governing wettability. In this study, the contact angles in the decane–muscovite–water interfaces were measured using an Atomic Force Microscope (AFM). The topologies of small water droplets (mostly less than 15 μm) in a liquid decane solution on muscovite plates were acquired with an AFM. The height and contact width obtained from the profiles of small droplets provide the contact angles between decane–muscovite–water interfaces. To correct for the effect of scanning pressure on topologies, contact angle measurements were carried out at different scanning pressures and force curves were measured on the decane–water interface. The corrected contact angles of water droplets showed around 20 degrees over a contact width of about 6 μm, which were in agreement with macroscopic contact angle data, while they decreased to about 15 degrees at a smaller contact width than about 2 μm. The relationship between corrected contact angles and droplet sizes is explained well by the modified Young's equation with a line-tension force of -2.1 × 10-9 N. The results indicate that the wettability of oil–mineral–water interfaces differs between nano/micro scale and macro scale. Therefore, microscopic wettability should be considered when evaluating EOR in real rock systems.
A set of constitutive equations for poroelastic materials saturated by multi-phase fluid is important for modeling subsurface geoengineering activities such as carbon dioxide sequestration into subsurface formations and land subsidence related to the production of water dissolved methane. Difficulties in such modeling mainly arise from the fact that the pressures of each phase fluid differ due to the difference of wettabilities of each fluid. Thus, the contributions of each fluid pressure to bulk strain and to the increment of porosity need to be rigorously expressed in the constitutive relations. In addition, the partial porosity of each fluid should be expressed separately. In this article, existing models are reviewed to discuss how these two difficulties have been treated; then, the state-of-the-art understanding of constitutive equations and remaining problems are presented. In the case of poroelasticity and the two-phase fluid condition, the Bishop's effective stress parameter is considered appropriate to express the contribution of each fluid pressure to bulk strain. For the expression of increment of partial porosity, thermodynamically consistent equations expressed by experimentally determinable parameters have been proposed recently, and the importance of introducing these equations is explained.
It has been observed that seismic stimulation of oil reservoirs has an impact on oil production, and we can apply seismic stimulation for enhanced oil recovery (EOR). Recently, many laboratory experiments and field tests have been conducted. Seismic stimulation can contribute to the coalescence/dispersion of oil droplets, and make them move through the pore-throat; however, the detailed mechanism of seismic stimulation is not fully understood. We focus on the behavior of oil droplet flow at the pore-throat, and we analyze trap and flow mechanisms at the pore-throat under the influence of seismic waves. We model the pore-throat in reservoir rocks, and adopt the lattice Boltzmann method (LBM) for oil droplet flow simulation. LBM is a well-known computational simulation method in fluid dynamics and is preferred to analyze microscopic flow phenomena. Using LBM, we can simulate an oil and water two-phase flow under complex boundary conditions including wettability of a solid surface. From our research, we conclude that the phenomenon of oil flow forced by seismic stimulation at the pore-throat depends on various factors, for instance, capillary pressure induced by interfacial tension between two-phase fluids.
Fractures distributed in crystalline rock inevitably influence fluid transport and solute migration. Most evaluations of fluid-conducting features and contaminant migration processes have been conducted with the present hydrological characteristics of fractures for deep underground usages (e.g., for high level radioactive waste (HLW) disposal, and LPG and CO2 storage). Relatively little attention has been given to the possible long-term behavior and evolution of these features, and their influence on fluid flow and geochemical interaction after installation of engineered materials underground. In the orogenic field of Japan, there are large areas of crystalline rock. The rocks in each area have a distinctive history, which is partly reflected in the characteristics of the fracture systems and associated mineral fillings that occur. These characteristics generally imply that fluids can flow through fracture networks, except in the cases of fault zones or crushed zones. Structural and mineralogical features readily illustrate how certain contaminants might react and be retarded by fracture fillings and open pore geometry, due to chemical sorption and/or physical retardation. The study reported here seeks to provide geological evidence that natural long-term physical and chemical processes are unlikely to significantly change the overall transport and retardation properties of rock. Hence, the study improves confidence in the currently adopted evaluation methodology and its long-term applicability. This paper, with the present understanding, describes fracture systems that are developed in intrusive crystalline rocks of different ages within the Japanese orogenic belt, and the fluid transport properties of these fracture systems. The aim is to build a synthetic model for the long-term fracturing process and hence evaluate fracture stability. Mineralogical studies and dating analyses of fracture fillings also suggest that structurally the fractures are relatively stable. Studies on fluid-conducting fractures show the unique characteristics of the fracture-forming process and the relatively stable geometries of fracture network systems in crystalline rocks distributed within the orogenic belt. This geological evidence also enables us to provide a model to build confidence in a technical approach that is applicable to hydrogeological and geological modeling over long time scales under the orogenic stress field present in Japan. The model might also be useful for other host rocks, as well as for characterizing a site in crystalline rocks at a continental margin, in order to allow the underground environment to be exploited.
The residence time and origins of groundwater are key factors accounting for its behavior in deep stratum. In this paper, various groundwater behaviors are discussed by focusing on dissolved noble gases and natural radionuclides with a long half-life as geochemical tracers. The concept and history of noble gas hydrology, which is a field of hydrology using noble gases dissolved in groundwater as tools to trace groundwater movements in strata, are summarized by comparing past studies. Current applications and future studies are presented. The main subjects of noble gas hydrology are groundwater dating and estimating the origins of groundwater. The residence time ranging over million years can be determined using excess dissolved 4He concentration and the accumulation rate of 4He calibrated with 36Cl (half-life t1/2 = 3.01 × 105 y). On the other hand, dissolved noble gases (i.e., 3He or 85Kr) should also be used to determine a short range of groundwater residence time of less than 100 years to exploit groundwater resources and overcome water shortages in the 21st century. The origins of groundwater can be estimated from characteristic changes of 3He/4He ratios in regional groundwater flows. Furthermore, paleotemperature, which aims at reconstructing paleo-climate information, is another key subject in noble gas hydrology.
The generation and migration of geofluids in subduction zones are discussed for the subducting slab and the overlying mantle wedge and crust in terms of theoretical models and observations. Theoretical models include several mechanisms of fluid migration, e.g., Rayleigh-Taylor instability, Stokes ascent, channel flow, and porous flow, whose characteristic lengths and velocities differ significantly. As a result, these mechanisms may occur in different settings within subduction zones. We compare seismic and geochemical observations with the model of fluid migrations, based on which a typical fluid fraction within the mantle wedge is estimated to be 0.1 to 1 vol.%. Accordingly, it is suggested that fluid migration within the mantle wedge is driven by the buoyancy of the fluid, rather than being dragged by the flow of solid matrix. This suggests the fluid rises vertically. In the shallow part of the mantle wedge and within the arc crust, in particular the upper crust, the channel flow seems to be dominant. However, the relationship between these channels and the surface exits observed as volcanoes and hot spring systems is unclear. To better understand fluid distribution and migration, we need to incorporate more observations (e.g., electrical conductivity structure) and models (e.g., models of petrological and thermal structures).
The mechanisms that generate the three main types of earthquake in subduction zones are discussed addressing their relations to geofluids. Studies on the spatial distribution of earthquakes and seismic velocity structure within the subducted slab provide evidence that strongly supports the dehydration embrittlement hypothesis for the generation of intermediate-depth intraslab earthquakes. Detailed imaging of the seismic velocity structure in and around plate boundary zones suggests that interplate coupling is mainly controlled by local fluid over-pressure. Seismic tomography studies show the existence of inclined sheet-like seismic low-velocity zones in the mantle wedge, not only in Tohoku but also in other areas in Japan, which perhaps correspond to the upwelling flow of the subduction-induced convection system. These upwelling flows reach the Moho directly beneath the volcanic areas, suggesting that those volcanic areas are formed by the upwelling flows. Aqueous fluids derived from the slab are probably transported up through the upwelling flows to the arc crust, where they might weaken the surrounding crustal rocks and finally cause shallow inland earthquakes. All of these observations suggest that geofluids expelled from the subducting slab play an important role in the generation of earthquakes in subduction zones.