Kenzi Karasaki

Estimation of reservoir properties using transient pressure data: An asymptotic approach

An asymptotic formulation of the inverse problem for flow reveals that the inversion may be partitioned into two complementary subproblems. In the first problem the arrival time associated with the peak slope of the transient curve is directly related to reservoir properties. The second inverse problem is similar to current methods for interpreting flow data; the transient head amplitudes are related to reservoir storage and conductivity. The first subproblem, the arrival time inversion, involves much less computation than does amplitude matching. Furthermore, it appears to be more robust with respect to the starting model. Therefore the solution to the arrival time inversion provides a starting model for amplitude matching. The methodology is particularly suited to the analysis of observations from well tests. We apply the approach to observations from two interference tests conducted at the Borehole Test Facility in Oklahoma. Using the transient pressure measurements, we image a shallow conductive fracture. The existence and location of the fracture has been verified by both geophysical and borehole data. In particular, core from a slant well contains an open, vertical fracture which coincides with our conductive feature.

An inverse technique for developing models for fluid flow in fracture systems using simulated annealing

One of the characteristics of flow and transport in fractured rock is that the flow may be largely confined to a poorly connected network of fractures. In order to represent this condition, Lawrence Berkeley Laboratory has been developing a new type of fracture hydrology model called an “equivalent discontinuum” model. In this model we represent the discontinuous nature of the problem through flow on a partially filled lattice. This is done through a statistical inverse technique called “simulated annealing.” The fracture network model is “annealed” by continually modifying a base model, or “template,” so that with each modification, the model behaves more and more like the observed system. This template is constructed using geological and geophysical data to identify the regions that possibly conduct fluid and the probable orientations of channels that conduct fluid. In order to see how the simulated annealing algorithm works, we have developed a synthetic case. In this case, the geometry of the fracture network is completely known, so that the results of annealing to steady state data can be evaluated absolutely. We also analyze field data from the Migration Experiment at the Grimsel Rock Laboratory in Switzerland.

An inverse approach to the construction of fracture hydrology models conditioned by geophysical data: An example from the validation exercises at the Stripa Mine

Abstract One approach for the construction of fracture flow models is to collect statistical data about the geometry and hydraulic apertures of the fractures and use this data to construct statistically identical realizations of the fracture network for fluid flow analysis. We have found that this approach has two major problems. One is that an extremely small percentage of visible fractures may be hydrologically active. The other is that on any scale you are interested in characterizing usually a small number of large features dominate the behaviour ([1] Transport Processes in Porous Media. Kluwer Academic, The Netherlands, 1989). To overcome these problems we are proposing an approach in which the model is strongly conditioned by geology and geophysics. Tomography is used to identify the large features. The hydraulic behaviour of these features is then obtained using an inverse technique called “simulated annealing.” The first application of this approach has been at the Stripa mine in Sweden as part of the Stripa Project. Within this effort, we built a model to predict the inflow to the Simulated Drift Experiment (SDE), i.e. inflow to six parallel, closely-spaced holes, the N- and W-holes. We predict a mean total flow of approx. 3.1 (l/min) into the six holes (two-holes) with a coefficient of variation near unity and a prediction error of about 4.6l/min. The actual measured inflow is close to 2l/min.

A coupled inversion of pressure and surface displacement

A coupled inversion of transient pressure observations and surface displacement measurements provides an efficient technique for estimating subsurface permeability variations. The methodology has the advantage of utilizing surface observations, which are typically much less expensive than measurements requiring boreholes. Furthermore, unlike many other geophysical observables, the relationship between surface deformation and reservoir pore fluid volume changes is relatively well understood. Our treatment enables us to partition the estimation problem into a sequence of three linear subproblems. An application of the approach to a set of tilt and borehole pressure data from the Raymond field site in California illustrates its efficiency and utility. The observations are associated with a well test in which fluid is withdrawn from a shallow fracture zone. During the test, 13 tiltmeters recorded the movement of the ground surface. Simultaneously, nine transducers measured pressure changes in boreholes intersecting the fracture system. We are able to image a high permeability, north trending channel located within the fracture zone. The existence and orientation of this high-permeability feature is substantiated by a semiquantitative analysis of some 4000 transient pressure curves.

Joint seismic, hydrogeological, and geomechanical investigations of a fracture zone in the Grimsel Rock Laboratory, Switzerland

Author(s): Majer, E.L.; Myer, L.R.; Peterson, J.E.; Karasaki, K.; Long, J.C.S.; Martel, S.J.; Blumling, P.; Vomvoris, S.

Development of Hydrologic Characterization Technology of Fault Zones

Author(s): Karasaki, Kenzi | Abstract: Through an extensive literature survey we find that there is very limited amount of work on fault zone hydrology, particularly in the field using borehole testing. The common elements of a fault include a core, and damage zones. The core usually acts as a barrier to the flow across it, whereas the damage zone controls the flow either parallel to the strike or dip of a fault. In most of cases the damage zone is the one that is controlling the flow in the fault zone and the surroundings. The permeability of damage zone is in the range of two to three orders of magnitude higher than the protolith. The fault core can have permeability up to seven orders of magnitude lower than the damage zone. The fault types (normal, reverse, and strike-slip) by themselves do not appear to be a clear classifier of the hydrology of fault zones. However, there still remains a possibility that other additional geologic attributes and scaling relationships can be used to predict or bracket the range of hydrologic behavior of fault zones. AMT (Audio frequency Magneto Telluric) and seismic reflection techniques are often used to locate faults. Geochemical signatures and temperature distributions are often used to identify flow domains and/or directions. ALSM (Airborne Laser Swath Mapping) or LIDAR (Light Detection and Ranging) method may prove to be a powerful tool for identifying lineaments in place of the traditional photogrammetry. Nonetheless not much work has been done to characterize the hydrologic properties of faults by directly testing them using pump tests. There are some uncertainties involved in analyzing pressure transients of pump tests: both low permeability and high permeability faults exhibit similar pressure responses. A physically based conceptual and numerical model is presented for simulating fluid and heat flow and solute transport through fractured fault zones using a multiple-continuum medium approach. Data from the Horonobe URL site are analyzed to demonstrate the proposed approach and to examine the flow direction and magnitude on both sides of a suspected fault. We describe a strategy for effective characterization of fault zone hydrology. We recommend conducting a long term pump test followed by a long term buildup test. We do not recommend isolating the borehole into too many intervals. We do recommend ensuring durability and redundancy for long term monitoring.

Evaluation of uncertainties due to hydrogeological modeling and groundwater flow analysis: Effective continuum model using TOUGH2

Starting with regional geographic, geologic, hydrologic, geophysical, and meteorological data, we develop an effective continuum model to simulate subsurface flow and transport in a 4 km by 6 km by 3 km thick fractured granite rock mass overlain sedimentary layers. Individual fractures are not modeled explicitly. Rather, continuum permeability and porosity distributions are assigned stochastically, based on well-test data and fracture density measurements. Large-scale features such as lithologic layering and major fault zones are assigned deterministically. We employ the TOUGH2 simulator for the flow calculation. The model simulates the steady-state groundwater flow through the site, then streamline analysis is used to calculate travel times for particles leaving specified monitoring points to reach the boundary of the model. Model results for the head distribution compare favorably with head profiles measured in several deep boreholes and the overall groundwater flow is consistent with regional water balance data. Predicted travel times range from 1 to 25 years.

Multicontinuum description of flow in composite heterogeneous media

Adequacy of the description of flow and transport processes in subsurface depends on how well a model represents the heterogeneity. One of the simplest models to describe the heterogeneity structure is a so-called composite system. Flow and transport simulation in composite systems can be reduced to solving equations and averaging the solutions. A different approach related to averaging the equations leads to new equations. This description is designated as monocontinuum description. If the homogeneous components of a composite system, so-called phases, have different hydrodynamic and/or geometric parameters, it is natural to study averaging on the individual phases along with the global averaging. This approach takes into consideration the mean fields in the individual continuum phase as well as the cross flows and cross forces between continua. However, this description is nonclosed. To overcome this difficulty, the phenomenological theory usually postulates a special interaction mechanism for closing the equations. This paper presents the exact equations of mass balance and moment balance for each phase. The exact sense of exchange terms in the multicontinuum models is explained. We demonstrate that joint consideration of the monocontinual and multicontinual systems in the case of two-phase random composite leads to a closed description and that we can find the exchange terms. For a periodic composite system the same approach leads to a closed description for any number of phases. The terms describing the interactions between continua for systems with a random and periodical structure are calculated. We examine the hypothesis customarily made in the phenomenological models.

Using an effective continuum model for flow and transport in fractured rock: the H-12 flow comparison

LBNL-44966 Using an Effective Continuum Model for Flow and Transport in Fractured Rock: The H-12 Flow Comparison Christine Doughty and Kenzi Karasaki Earth Sciences Division E.O. Lawrence Berkeley National Laboratory December, 1999 This work was supported by Japan Nuclear Fuel Cycle Corporation (JNC) and JGC Corporation. through the U.S. Department of Energy Contract No. DE-AC03-76SF00098.

Feature Detection, Characterization and Confirmation Methodology: Final Report

LBNL-1358E Feature Detection, Characterization and Confirmation Methodology F inal Report Kenzi Karasaki, John Apps, Christine Doughty, Hope Gwatney, Celia Tiemi Onishi, Robert Trautz, and Chin-Fu Tsang Earth Sciences Division March 2007 NUMO-LBNL Collaborative Research Project Report This work was supported by the U.S. Department of Energy under Contract DE-AC02-05CH11231.