Kazuhiko Tezuka

Modeling of low-frequency Stoneley-wave propagation in an irregular borehole

A fast modeling method is formulated for low-frequency Stoneley-wave propagation in an irregular borehole. This fast modeling method provides synthetic waveforms which include the effects of two borehole irregularities, diameter changes (washout), and formation property changes. The essential physics of the low-frequency Stoneley waves are captured with a simple 1-D model. A mass-balance boundary condition and a propagator matrix are used to express Stoneley-wave interactions with the borehole irregularities. The accuracy of the proposed method was confirmed through comparison with existing finite-difference and boundary integral modeling methods that yielded cross-correlations greater than 0.98. Comparison of synthetic records calculated for an actual borehole with field records showed qualitative agreement in the major reflections because of the washout zones, but showed some disagreements in the reflections caused by the fractures. Since the synthetic records include only information relating to the borehole geometry and the elastic properties of formation, the reflection caused by the fracture will appear only in the field record. These results suggest the possibility of distinguishing Stoneley-wave reflections caused by fractures from those caused by borehole irregularities. Further, the fast computational speed of this method--over 300 times faster than either boundary integral or finite-difference methods--makes it quite suitable for field application.

Current status of seismic and borehole measurements for HDR/HWR development

Seismic and borehole measurements provide significant information about HDR/HWR reservoirs that is useful for reservoir development, reservoir characterization, and performance evaluation. Both techniques have been widely used during all HDR/HWR development projects. Seismic measurements have advanced from making passive surface measurements during hydraulic fracturing to making passive observations from multiple boreholes during all phases of HDR/HWR development, as well as active seismic measurements to probe regions of the reservoir deemed to be of interest. Seismic data provide information about reservoir extent, locations and orientations of significant fractures, and areas of thermal drawdown. Recent advances include the ability to examine structures within the seismically active zone using statistics-based techniques and methods such as seismic tomography. Seismic method is the only means to obtain direct information about reservoir characteristics away from boreholes. Borehole measurements provide high-resolution information about reservoir characteristics in the vicinity of the borehole. The ability to make borehole measurements has grown during the course of HDR/HWR development as high temperature tools have been developed. Temperature logging, televiewer logs, and electrical property measurements have been made and shown to provide useful information about locations of fractures intersecting wellbores, and regions where water leaves and enters injection and production wellbores, respectively.

Construction of Fracture Network Model Using Static and Dynamic Data

This study reports a new process of constructing a discrete fracture network (DFN) model, which reflects various statistical properties of fractures extracted from static data such as borehole images and dynamic data such as pressure derivative curves. A DFN model is constructed by generating a number of disc-shaped fractures based on their statistical properties. The process of constructing DFN model proposed consists of two streams. One stream is for the fractures with wide apertures, which work as main flow path of fluid. Another stream is for the fractures with narrow apertures, which work as main storage of fluid. In the each stream, a DFN model is constructed by applying fractal theory and geostatistics that define the size and spatial distribution of fractures, respectively. The overall DFN model is obtained by merging the two types of DFN models. Then, the DFN model is converted to a continuum model with equivalent permeable blocks based on cubic law and is evaluated by the fluidflow behavior. Japex has been operating the Yufutsu fractured basement gas reservoir since 1996. A total of approximately 8,000meters of electrical borehole images from eleven wells is measured to understand the fracture system. According to the proposed process, the DFN models are constructed. To evaluate the DFN models by comparing the simulated pressure derivative curves with the observed curve, flow simulations are carried out as a simulation of a single porosity type. The observed pressure derivative curve shows a concave shaped curve like a dual porosity behavior, but the curve never reaches the second flat state. Through the parameter studies of the statistical properties, which relate to the size distribution and the spatial distribution of fractures, the observed curve was successfully reproduced and the quantitative relationship between the fractures with wide apertures and the fractures with narrow apertures was found.

Geomechanical wellbore imaging: Implications for reservoir fracture permeability

A field-specific geomechanical model serves as a platform for greatly reducing costs and increasing production over the life of a field. The information contained in a geomechanical model makes it possible to reduce drilling costs and production losses through fieldwide well planning that can optimize production and minimize risk. A significant value of the geomechanical model is its application to the efficient exploitation of fractured reservoirs. The essential contribution of wellbore image technologies to this exploration and production challenge is illustrated through a case study of a compartmentalized fractured gas reservoir located in Hokkaido, Japan. A growing body of evidence reveals that, in many fractured reservoirs, the most productive fractures are those that are optimally aligned in the current stress field to fail in shear. Thus, it is necessary to obtain knowledge of both the stress magnitudes and orientations and the distribution of natural fractures to determine the optimal orientations for wells to maximize their productivity. The best well intersects the maximum number of stress sensitive fractures. Applying geomechanics and the reservoir fracture distributions to model shear-enhanced permeability as the mechanism for reservoir production appears to be a promising improvement to existing reservoir flow models. Using quantitative risk assessment and realistic uncertainties in the critical parameters, it is possible to estimate the uncertainty in predictions of optimal well trajectories and of stimulation pressures to enhance natural fractures. The results indicate that the critical parameters are not always those with the most uncertainty, and that the most effective way to reduce prediction uncertainties is to calibrate against the productivity of a preexisting well.

Fractured Reservoir Characterization Incorporating Microseismic Monitoring and Pressure Analysis During Massive Hydraulic Injection

The fractured basement gas reservoir in Yufutsu field was characterized by integrating static fracture information obtained from borehole micrtyo-resistivi images and dynamic information recorded during a massive hydraulic injection operation. Yufutsu gas reservoir is so called “fractured reservoir” seated in Cretaceous granite and Eocene conglomerate formation at around 4 - 5km depth in the Southern Ishikari Plain, central Hokkaido, Japan. One of the most important issues in the development of this reservoir is how to model the fracture system which contributes to the hydrocarbon migration.

Passive seismic monitoring of a stimulation of HDR geothermal reservoir at Cooper Basin, Australia

A team of Japanese researchers, with long experience in passive seismic monitoring in the area of development of hot dry rock (HDR) and hot wet rock (HWR) geothermal systems, carried out seismic monitoring at the Australian HDR site being developed by Geodynamics Limited in the Cooper Basin. The seismic network consisting of 4 near-surface stations and 4 downhole stations detected approximately 32,000 triggers during injection of 20,000 m3 of liquid into granitic basement over 3 weeks. The authors located the events on a semi-realtime basis and the locations were fed back to the pumping side. The locations of seismic events showed sub-horizontal extension of the reservoir to 1,800 m away from the injection well at a depth of around 4,500 m. The heterogeneous source migration suggest that the reservoir was close to a critically stressed state.

Use of a computational fault-extraction process with calibrations to characterize a fractured basement reservoir, Yufutsu, Japan

The Yufutsu oil and gas field, located in Hokkaido, northern Japan, produces hydrocarbon from a typical fracture-type reservoir composed of very tight Cretaceous granitic basement and overlying conglomerate of the Eocene. Here, delineations of faults accompanying large open fractures are essential for optimal developments. To capture fault distribution objectively, various seismic-attribute estimates ascribed to fault distributions are derived from three-dimensional seismic data by computational procedures. However, to use the estimates properly, calibrating them with independent observations other than seismic data is important. We present a calibration scheme by coupling seismic data with microseismic data and in-situ stress data to delineate active faults under a strike-slip faulting stress regime in the Yufutsu field. Active faults are interpreted to be fluid pathways formed by shear dilation. In the calibration, two kinds of parameter sets are tuned. One controls linkages of fault responses fragmented by artificial noises caused by seismic acquisition and processing to adjust spatial continuities of fault surfaces properly. Another limits extracted fault strikes with respect to Mohr-Coulomb failure criterion to highlight active faults. The calibrated seismic-attribute estimates show a qualitative consistency with the microseismic hypocenter distribution observed during a massive hydraulic stimulation. In addition, a large difference in gas productivity observed at four wells, from very high productivity to no gas flowing, is clearly related to an existence of distinct lineation with strong magnitudes of the calibrated seismic attributes. It suggests that the calibrated estimate becomes a criterion to judge an economic viability of a well.

Discrepancies Between Sonic Log And VSP Velocities In Volcanic Rocks

VSP technique is one of the effective tools to correlate seismic data with borehole measurements. However, sonic log and VSP velocities are often found to disagree. Several authors point out on these discrepancies that the VSP travel times are commonly delayed to the sonic integrated times in deeper part of sedimentary rocks. Our investigation on these discrepancies is not only for clastic rocks but also for volcanic rocks because volcanic rocks are one of the main petroleum reservoirs in Japan. Positive drifts (VSP travel times being greater than sonic integrated times) of 8 ms/km on the average are also found in our study in clastic rocks, which is in good agreement with the previous studies. These drifts are well explained by the effect of wave propagation with dispersion. In volcanic rocks, however, negative drifts are found common in deeper depths. It is concluded that the dispersion effect is small in volcanic rocks (high Q) and drilling-induced alteration is a major cause of the negative drift, while the dispersion effect is dominant in clastic rocks.

Reservoir Structure At Yufutsu Gas Field, JAPAN Determined By Analysis of Induced Microseismic Multiplets

Summary We examine induced microseismicity from a hydraulic stimulation at the Yufutsu gas field on Hokkaido Island, Japan, managed by Japan Petroleum Exploration Co., Ltd. (JAPEX), to delineate the fracture system in the reservoir. Over 3,700 seismic events were detected by a network of 4 deep downhole and 3 shallower seismic sensors. 701 of these events showed clear onsets of P and S waves and were located. The source distribution determined using conventional location methods was too diffuse to reveal detailed reservoir structure. We therefore re-examined the dataset by clustering the events using waveform similarity, and subsequently relocated using relative arrival times. 576 events (82% of located events) were identified as multiplet events based on waveform coherence. We relocated the multiplets using a double differential (DD) technique, and obtained a source distribution that delineates tens of sub-parallel streaks composed of multiplets. A synthetic test of the DD location technique with our station distribution suggests that the streaks are artifacts arising from the co-planar network arrangement.

Fracture detection by downhole measurements.

This article provides a selected review on recent methods of fracture detection by downhole measurements. Following methods are discussed here: (1) imaging tools of borehole wall, (2) detection of permeable fractures using Stoneley wave propagation, and (3) monitoring of acoustic emission during hydraulic fracturing.