Norio Arihara

Abnormal transmission attenuation and its impact on seismic-fracture prediction — A physical modeling study

Fracture-induced anisotropy can lead to observable azimuthal variations of seismic attributes that then can be used for characterizing a fracture system. Unfortunately, abnormal transmission losses along raypaths also can result in similar azimuthal variations leading to uncertainty in such fracture determination. Using a physical model containing gas-filled fractures, we investigate the impact of abnormal transmission loss on fracture detection from ultrasonic data in a laboratory setting. Recorded reflection amplitudes and traveltimes are used to study ultrasonic responses to the presence of the gas-filled fractures and to understand observed azimuthal attribute anomalies. Experimental results from this study highlight the pitfalls in using azimuthal attribute variations as indicators of the presence of fractures when abnormal transmission attenuation is significant.

A Streamtube Approach for Modeling Mass and Heat Migration for Thermal Recovery Methods

This paper presents a new reservoir simulator for mass and heat migration based on the streamtube technique. The current version of the program focuses on characterizing migration of the injected fluid. The method is heavily based on the distribution of the pore volume along each streamtube calculated by using the Time-of-Flight method. This information and the 1D solution of mass and energy equations are used to determine thermal migration as well as temperature changes at production wells. The streamtubes are derived from an underlying velocity field, which is obtained from the solution of the pressure equation under the assumption of incompressibility. The compressibility effects are accounted for in the 1D solutions along periodically changing streamtubes. Simulations are carried out for a five -spot well pattern in 2D areal domains with various types of heterogeneity under different injection scenarios. We confirmed that the results from the streamtube approach had a reasonable balance between its ability to provide an insight into the behavior of mass and heat flow and the computational efficiency. Comparisons with a commercially available simulator both validated the method and illustrated the cases in which this method is more useful to reservoir engineers.

3-D Analysis of Complex Fracture Systems

It is now well-known that stress changes the microstructure of rock and causes the development of fractures. In rocks with vertical fractures, seismic amplitudes, velocities and frequencies vary with the direction of seismic wave propagation (Nur, 1971; Thomsen, 1986, 1988). That is, such rocks are effectively anisotropic to the propagation of seismic waves, and this knowledge can be applied to the remote detection of vertically aligned fractures.

Accurate simulation for strongly heterogeneous reservoirs

Accurate simulation for highly heterogeneous reservoirs requires two model functions, abilities to accurately calculate both pressure and velocity and to efficiently handle permeability tensors. A flux continuous model was formulated incorporating full tensor permeability for two-phase flow of immiscible and incompressible fluids in a two-dimensional domain. Flow equations were split into two systems of similar configurations, one for pressure and velocity and the other for saturation, and solved sequentially by means of the mixed finite volume element method. The former pressure equations, expressed as two coupled first-order partial differential equations, were solved simultaneously for pressure and velocity. Numerical examples of simulating flow in a sand-shale system and fractured reservoirs are presented to demonstrate the performance of the proposed simulator. The mixed finite volume method could accurately approximate the flow variables, and generate more realistic streamlines than the finite difference method for a discontinuous permeability field. The examples also validated the use of effective permeability of full tensor form for accurate simulation of naturally fractured reservoirs.

Effective permeability estimation for naturally fractured reservoirs.

Appropriate modeling of naturally fractured reservoirs is one of the most important and challenging issues in reservoir characterization. In simulation, a dual porosity or dual permeability model is applied when fractures are well developed to form a fracture network. On the other hand, the single-continuum approach, where the fracture system is represented by effective permeability, is commonly used if fractures are discrete or disconnected. Focusing on the latter case, this paper proposes a semi-analytical technique to evaluate effective permeability for periodically or randomly fractured media including infinitely thin, infinite-conductivity fractures.The complex variable boundary element method is used to compute potential fields and streamlines in the two-dimensional space for discretely distributed fracture systems under the periodic boundary conditions. Effective permeability is evaluated first for discrete fracture systems of regular patterns to demonstrate the validity of the method and to examine the sensitivity of the effective permeability to the variations in the basic fracture parameters. With a constant total length of fractures, systems of varied fracture lengths show higher effective permeability than systems of uniform fracture length. 500 distributions of stochastic fractures are next generated to establish correlation between effective permeability and the fracture parameters, total length L, mean length m, and standard deviation of fracture length ?D. Sensitivity to the parameters shows that non-zero a increases effective permeability, that the incremental gain of effective permeability is proportional to L, and that the larger m, the larger effective permeability. The effective permeability tensors are also determined for oriented fractures. Analyses by non-parametric regression show that the diagonal elements, kxx and kyy, are highly affected by the angle between the oriented fractures and the pressure gradient, while the off-diagonal elements, kxy and kyx, are strongly affected by both the total length and the angle.

Experimental and Modeling Studies of Two-Phase Flow in Pipelines

The objectives of this study are to develop and evaluate a mechanistic model for gas/liquid two-phase flow in pipelines. A mechanistic model has been developed by combining currently available models and correlations. The approach of the modeling study was based on the work by Xiao et al. Modifications have been made on the annular flow model by implementing the currently developed film-thickness-distribution model. An experimental database has been developed for model evaluation. Seventy-five runs of steady-state air/water flow tests in horizontal and slightly inclined pipes were conducted using a large-scale experimental facility. The experimental program was set up in a wide range of experimental conditions to cover the intermittent, dispersed bubble, and annular flow patterns. An evaluation of the model was carried out for each flow pattern, namely, intermittent, dispersed bubble, and annular flow. The comparisons between the measured and calculated pressure drops show good agreement for each flow pattern. Also, overall evaluation revealed that the proposed model provided the best performance among the commonly used empirical correlations, such as Beggs and Brill, Mukherjee and Brill, and Dukler et al.