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

Numerical modeling of boiling due to production in a fractured reservoir and its field application

Numerical simulations were carried out to characterize the behaviors of fractured reservoirs under production which causes in-situ boiling. A radial flow model with a single production well, and a two-dimensional geothermal reservoir model with several production and injection wells were used to study the two-phase reservoir behavior. The behavior can be characterized mainly by the parameters such as the fracture spacing and matrix permeability. However, heterogeneous distribution of the steam saturation in the fracture and matrix regions brings about another complicated feature to problems of fractured two-phase reservoirs

Numerical investigation of pressure transient responses of a well penetrating a deep geothermal reservoir at super-critical conditions

Numerical simulations were carried out to predict pressure transients in a hypothetical deep geothermal well which penetrates a reservoir at super-critical conditions. Production at about 4000 m depth was assumed. In many cases, two-phase conditions develop due to high temperature and production-induced pressure decrease. Several cases in which single-phase conditions are maintained were studied in detail. Pressure transients are influenced by the reservoir temperature distribution - in particular, a temperature distribution with subcritical conditions at the well but supercritical conditions farther away causes a characteristic nonlinear pressure response whlch is influenced by the large compressibility and small kinematic viscosity near the critical point

Pressure-transient behavior during cold water injection

During injection testing, pressures in geothermal wells sometimes decline after an initial period of increase despite continued injection. The injection tests carried out at the Yutsubo geothermal field in Kyushu, Japan exhibit this peculiar behavior. During injection testing of Yatsubo well YT-2, 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 due both to cooling and to pressure buildup, 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 Simulation of Underground Electrical Signals Caused by Hydrofracturing Operations

ABSTRACT When fluids are injected at high pressure into geothermal wells to create and/or enlarge fractures in the reservoir and stimulate production, the resulting fluid leakage away from the fracture surface into the relatively impermeable country rock can cause detectable transient electrical signals (voltages) to propagate outward into the reservoir through the mechanism of electrokinetic coupling (or "selfpotential"). If nearby observation wells are equipped with appropriate sensors, these signals may be characterized and reveal information concerning fracture geometry and other fracture properties that is otherwise unobtainable or ambiguous based on microseismic monitoring alone. A new numerical simulator ("SPFRAC") has been developed to forward-calculate the electrical signals that will emanate from a fracture (or network of fractures) when pressurized in this manner. A representation of the fracture network as a collection of interconnected triangular planar elements is superimposed upon a conventional regular three-dimensional Cartesian computational grid describing the low-permeability country rock. The theoretical approach is presented, and the capabilities and limitations of the SPFRAC simulator are discussed. SPFRAC itself has been configured for operation on either a Windows PC or a Unix/Linux workstation, and is available through the United States Department of Energy

Numerical simulation of electrokinetic potentials associated with subsurface fluid flow

Report Number: SGP-TR-151-21; OSTI ID: 889745A 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

Effects of Heterogeneous Seal Layer Property on The Long-Term Behaviour of CO2 Injected into Deep Multilayer Systems

Heterogeneity in mudstone/shale layers has significant effects on seal layer integrity. The presence of intralayer sandstone channels in a seal layer may allow the buoyant CO2 to escape from the reservoir, even if the globally averaged permeability of the seal layer seems low enough. On the other hand, multi-layered structures are known to work often as baffles for the upward migration of CO2 in formations. In this paper, we investigate the storage capacity of multilayer formations with discontinuous seals. Numerical simulations are carried out to study the effects of seal layer discontinuity on the long-term behaviour of CO2 injected into deep saline aquifers. To represent a seal layer composed of low permeability rocks intersected by sandstone channels, ‘MINC' doubleporosity model is adopted. Also conducted is sensitivity analysis to investigate the effects of key parameters such as capillary pressure, relative permeability, temperature, and the thickness of the formations. The results show that CO2 injection into a sufficiently deep multi-layered reservoir enables CO2 to be stored and trapped in and around the reservoir without reaching to a shallow aquifer, even though seal layers have discontinuities. The upward movement of CO2 is greatly affected by capillary pressure of sandstone channels in seal layers. The relative permeability and the temperature-dependent CO2 properties have a significant effect on the final plume spread and the amount of CO2 dissolved or fixed by residual gas trapping

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

Abstract 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