Tsuneo Ishido

Numerical simulation of electrokinetic potentials associated with subsurface fluid flow

A postprocessor has been developed to calculate space/time distributions of electrokinetic potentials resulting from histories of underground conditions (pressure, temperature, flowrate, etc.) computed by multi-phase multicomponent unsteady multidimensional geothermal reservoir simulations. Electrokinetic coupling coefficients are computed by the postprocessor using formulations based on experimental work reported by Ishido and Mzutani (1981). The purpose of the present study is to examine whether or not self-potential anomalies actually observed in real geothermal fields are consistent with quantitative mathematical reservoir models constructed using conventional reservoir engineering data. The most practical application of the postprocessor appears to be modeling self-potential changes induced by field-wide geothermal fluid production. Repeat self-potential surveying appears to be promising as a geophysical monitoring technique to provide constraints on mathematical reservoir models, in a similar fashion to the use of repeat microgravity surveys.

Groundwater flow and hydrothermal systems within volcanic edifices: Delineation by electric self‐potential and magnetotellurics

[1] The imaging of hydrothermal systems within volcanoes is critical in evaluating the nature and likelihood of future volcanic activity and hazard assessment. In this study, we present a conceptual model of the hydrothermal system in a volcanic edifice, as deduced from the relationship between electric self-potential (SP) and high-resolution resistivity structures. In order to develop a comprehensive model of water flow in volcanoes, we conducted the audiofrequency (10,000-0.3 Hz) magnetotelluric surveys in five large stratovolcanoes (Iwate, Iwaki, Nasu, Nantai, and Nikko-Shirane) in Japan and found that the obtained 2-D resistivity profiles have a close relationship to the previously reported SP data: good extensive conductors occur beneath areas without SP anomalies, whereas good localized conductors only occur beneath large spatial wavelength SP anomalies on the volcano side of the SP minimum. Also taking into account the locations of surface geothermal activity, the good conductors roughly correspond to the hydrothermal zone, whose upper limit is sealed by a low-permeability clay layer. The sealing layer separates an upper groundwater flow from a lower hydrothermal flow in the subsurface and controls the geothermal manifestations and river locations on the surface. We confirmed the feasibility of the proposed model based on numerical simulations of a hydrothermal system. The horizontal extent of the hydrothermal zone is highly heterogeneous even in a volcanic edifice. This heterogeneity can reflect the geological age of flanks that may be related to the occurrence of a previous large sector collapse.

Geothermal reservoir monitoring with a combination of absolute and relative gravimetry

Microgravity monitoring is a valuable tool for mapping the redistribution of subsurface mass and for assessing changes in fluid recharge from reservoir boundaries associated with geothermal exploitation. To further the development of a high-precision absolute/relative hybrid gravity-measurement technique, we conducted measurements using an absolute gravimeter in two geothermal fields in Japan. The absolute gravity measurements were performed in the central production areas to directly measure gravity changes caused by fluid withdrawal. We succeeded in measuring long-term trends within an accuracy of a few microgals in the Okuaizu and Ogiri fields, which have been producing electricity for several years. Absolute measurements in the center of the field provide reliable and local reference datum anchor points for more widely distributed relative gravity measurements. In the Ogiri field, we carried out time-lapse hybrid measurements with this combination of absolute and relative gravimetry and delineated the...

Streaming potential observations, using geothermal wells and in situ electrokinetic coupling coefficients under high temperature

Abstract In an attempt to detect streaming potentials induced by subsurface water flows, we have observed the horizontal electric field (self-potential) variations across stationary electric dipoles near geothermal wells in the Takinoue geothermal area, Japan. We observed variations of self-potential which seem to be associated with the water flows in the aquifer, induced by turning on and off the flow of the wells. Amplitudes of the variations are 3–5 mV across 60–200 m dipoles, and can be explained well with a proposed electrokinetic model: the streaming potential coefficient of − 15 mV/bar and/or the ζ-potential of −50 to −100 mV in the aquifer are appropriate to explain the observed data by the model. The obtained electrokinetic coupling coefficients are in situ ones and determined for crustal rock-water system under high temperature (~200°C) condition. The present results, together with a laboratory study by Ishido and Mizutani (1981), give fundamental information on electrokinetic coupling coefficients in the earths interior, and are very important when we make quantitative interpretations of self-potentials generated by geothermal activity on the basis of electrokinetic effects.

ζ potential measurement of volcanic rocks from Aso caldera

[1] We deduced the z potential of various rocks in Aso caldera from streaming potential measurements in laboratory. In contrast to the conventional observation that the z potential of crustal rocks is mostly negative under typical geologic conditions, almost half of the samples equilibrated with dilute KCl solution show positive values of z potential at pH 4.5–5.5. The samples showing positive z are characterized by relatively low SiO2 content and abundance of elements having high isoelectric points, and found to be localized around the summit of Mt. Takadake where the positive correlation between self-potential (SP) and altitude is observed. We emphasize that measurements of z potential of volcanic rocks are very important to give more appropriate interpretations of SP data from volcanic fields. INDEX TERMS: 0634 Electromagnetics: Measurement and standards; 0925 Exploration Geophysics: Magnetic and electrical methods; 5199 Physical Properties of Rocks: General or miscellaneous; 8499 Volcanology: General or miscellaneous; 1832 Hydrology: Groundwater transport. Citation: Hase, H., T. Ishido, S. Takakura, T. Hashimoto, K. Sato, and Y. Tanaka, z potential measurement of volcanic rocks from Aso caldera, Geophys. Res. Lett., 30(23), 2210, doi:10.1029/2003GL018694, 2003.

Pressure-transient behavior during cold water injection into geothermal wells

During injection testing, the pressures in geothermal wells used for reinjection sometimes initially increase but then decline as injection continues. Injection tests carried out at the Yutsubo geothermal field in Kyushu, Japan, exhibit this peculiar behavior. During injection testing of Yutsubo well YT-2, the observed downhole pressures eventually began to decline despite sustained injection rates. We have carried out numerical simulation studies using a radial flow model to examine this behavior. Double porosity (MINC) models are adopted, in which the fracture porosity increases as a result of both cooling and pressure build-up, and the permeability is very sensitive to porosity changes. This extreme sensitivity of fracture permeability to porosity appears to be necessary to reproduce the late-time pressure decline, and suggests that fractures were opened by injection-induced cooling near the well.

Numerical investigation of production behavior of deep geothermal reservoirs at super-critical conditions

Abstract Numerical simulations have been performed to predict pressure transients in deep geothermal reservoirs at sub- and super-critical temperatures. First, pressure drawdown and buildup tests of reservoirs with different initial conditions were simulated. The calculated pressure responses are dominated by non-linear changes of fluid kinematic viscosity and compressibility. The pressure of a super-critical zone is shown to cause complex behavior. Short- and long-term production tests in both unbounded and bounded reservoirs were then simulated. Unbounded and bounded reservoirs exhibit very similar short-term production behavior near or above the critical temperature (375–400°C). Unbounded reservoirs of low transmissivity ( kh = 1 darcy-meter) exhibit long-term production behavior that depends on whether the reservoir is sub-critical (300–375°C) or super-critical (400°C); substantial increases in flowing enthalpy and declines in feedpoint pressure occur at early times in the super-critical reservoir.

Characterization of Fractured Reservoirs Using a Combination of Downhole Pressure and Self-Potential Transient Data

In order to appraise the utility of self-potential (SP) measurements to characterize fractured reservoirs, we carried out continuous SP monitoring using multi Ag-AgCl electrodes installed within two open holes at the Kamaishi Mine, Japan. The observed ratio of SP change to pressure change associated with fluid flow showed different behaviors between intact host rock and fractured rock regions. Characteristic behavior peculiar to fractured reservoirs, which is predicted from numerical simulations of electrokinetic phenomena in MINC (multiple interacting continua) double-porosity media, was observed near the fractures. Semilog plots of the ratio of SP change to pressure change observed in one of the two wells show obvious transition from intermediate time increasing to late time stable trends, which indicate that the time required for pressure equilibration between the fracture and matrix regions is about 800 seconds. Fracture spacing was estimated to be a few meters assuming several micro-darcies (10-18 m2) of the matrix region permeability, which is consistent with geological and hydrological observations.

Permeability-control on volcanic hydrothermal system: case study for Mt. Tokachidake, Japan, based on numerical simulation and field observation

We investigate a volcanic hydrothermal system by using numerical simulation with three key observables as reference: the magnetic total field, vent temperature, and heat flux. We model the shallow hydrothermal system of Mt. Tokachidake, central Hokkaido, Japan, as a case study. At this volcano, continuous demagnetization has been observed since at least 2008, suggesting heat accumulation beneath the active crater area. The surficial thermal manifestation has been waning since 2000. We perform numerical simulations of heat and mass flow within a modeled edifice at various conditions and calculate associated magnetic total field changes due to the thermomagnetic effect. We focus on the system’s response for up to a decade after permeability is reduced at a certain depth in the modeled conduit. Our numerical simulations reveal that (1) conduit obstruction (i.e., permeability reduction in the conduit) tends to bring about a decrease in vent temperature and heat flux, as well as heat accumulation below the level of the obstruction, (2) the recorded changes cannot be consistently explained by changing heat supply from depth, and (3) caprock structure plays a key role in controlling the location of heating and pressurization. Although conduit obstruction may be caused by either physical or chemical processes in general, the latter seems more likely in the case of Mt. Tokachidake.Graphical abstract.

Contention between supply of hydrothermal fluid and conduit obstruction: inferences from numerical simulations

We investigate a volcanic hydrothermal system using numerical simulations, focusing on change in crater temperature. Both increases and decreases in crater temperature have been observed before phreatic eruptions. We follow the system’s response for up to a decade after hydrothermal fluid flux from the deep part of the system is increased and permeability is reduced at a certain depth in a conduit. Our numerical simulations demonstrate that: (1) changes in crater temperature are controlled by the magnitude of the increase in hydrothermal fluid flux and the degree of permeability reduction; (2) significant increases in hydrothermal flux with decreases in permeability induce substantial pressure changes in shallow depths in the edifice and decreases in crater temperature; (3) the location of maximum pressure change differs between the mechanisms. The results of this study imply that it is difficult to predict eruptions by crater temperature change alone. One should be as wary of large eruptions when crater temperature decreases as when crater temperature increases. It is possible to clarify the implications of changes in crater temperature with simultaneous observation of ground deformation.