March-April
Vol 2 No 2
Electrical and Hydraulic Flow Properties of Appalachian Petroleum Reservoir Rocks
W.K. Sawyer, C.I. Pierce and R. B. Lowe
ABSTRACT
Resistivty, porosity, permeability, and saturation data were obtained on core samples from the Big Injun, Clinton, Gordon, Bradford Second and Third, and Venango Second oil sands and formation factor-porosity relationships were derived using the general form of Archie's equation F=af-m, where F is the formation factor, f is the porosity, and a and m are constants that depend on characteristics of the rock. It was found that a and m varied significantly between sands, as did the exponent in the Archie equation I=Sw-n, where I is the resistivity index and Sw is the watersaturation. The average exponent, n, determined from all data was 50 percent greater at residual oil saturation than at residual water saturation. Samples from each sand exhibited differences in directional resistivity and/or permeability, The Bradford Third sand data showed correlation between resistivity and permeability anisotropy.
The Influence of Formation Anisotropy upon Resistivity - Porosity Relationships
Paul F. Worthington
ABSTRACT
Empirical relationships between electrical resistivity and porosity f are influenced by formation anisotropy, because they are derived from correlations between a tensor quantity on the one hand and a scalar quantity on the other. In particular, the derived form of the Archie relationship F=af-m shows a directional dependence because the formation factor F is a function of tortuosity. By collating data relating to horizontally and vertically orientated sandstone specimens, it has been demonstrated that computed values of the coefficient a and the exponent m can be influenced by the direction of the underlying resistivity used to obtain a and m does not approximate the corresponding intrinsic formation factor but shows a significant departure as a result of shale effects. Observed variations in a and m with orientation have not conformed to the established relationship between these two parameters. This is shown to be a consequence both of the distribution of the average values of F and O and of the mode of computation of trends through the mean points. Procedures are outlined for reducing the influence of formation anisotropy upon the Archie porosity coefficient and exponent. These adjustments must be effected before m can be meaningfully correlated with, for example, the degree of shaliness or the degree of cementation. An appropriate directional dependence must be retained, however, if m is to be correlated with a parameter such as permeability. Furthermore, if laboratory-derived values of a and m are to be used in conjunction with formation resistivities obtained from induction logs or deep laterologs in vertical wells, it is important to use a and m values that are means of their respective horizontal components.
A Porelastic Model with Azimuthal Anisotropy to Analyze Flow Unit Responses at the Siberia Ridge Field, Greater Green Reiver Basin
Jorge O. Para and Chris Hackert
ABSTRACT
A modeling scheme is presented and applied to analyze the response of reservoir flow units to a variety of acoustic/seismic measurement techniques, and to determine the frequency band in which flow units can be observed and characterized. The model estimates attenuation in a broad frequency rage to include sonic, Crosswell, VSP, and 3-D seismic scales. Because flow units in a reservoir are characterized by permeability, porosity, and fluid saturation, and because fluids are characterized by their viscosity, density, and velocity, we use the theory of poroelasticity. This theory provides the physics involved in the interactions between the fuid and the rock matrix as an acoustic wave propagates in the medium. Energy losses due to the relative movement of pore fluids and the rock matrix are described using the unified Biot and squirt-flow theory. We apply this theory to a layered poroelastic medium with azimuthal anisotropy to analyze whether flow units penetrated by a borehole can be detected at seismic scales (crosswell, VSP and 3-D seismic). We model a sandshale-coal sequence from the Siberia Ridge field, a tight gas sand reservoir located in Wyoming, as an example. The results give responses in the frequency domain containing the effect of scattering and intrinsic attenuation. By comparing total attenuation with scattering attenuation, we observe the differences associated with the flow units. Flow units can be identified because the increase in attenuation is due to the interaction of fluid flow with the rock matrix. The modeling results show scattering effects of shales and coals and demonstrate that coals control the scattering attenuation. The elastic attenuation is shown at all frequencies and the fluid flow effects are observed in the sonic and crosswell frequency measurements such as 3-D seismic would not be able to capture fluid flow in fracture-induced anisotropy. However, this modeling approach does not prevent the capture of fracture-induces anisotropy due to large fracture zones and faults that may be presented in the region using VSP and/or 3-D seismic surveys.
Shear Wave Anisotropy Measurement Using Cross-dipole Acoustic Logging: an Overview
X.M. Tang and D. Patterson
ABSTRACT
Recent advances in cross-dipole logging technology give rise the several important applications in the exploration and production of hydrocarbon reservoirs. The cross-dipole acoustic toll is designed to measure azimuthal shear-wave anisotropy around the borehole. This azimuthal anisotropy can be caused by (1) fractures that are parallel to or inclined with the borehole, (2) unbalanced formation stresses, and (3) depositional beddings in a deviated borehole. Therefore, determining the anisotropy from cross-dipole acoustic logging allows us to obtain these important formation characteristics. The examples give an overview of cross-dipole logging and data interpretation along with application cases. These applications cover three major areas: (1) Fracture delineation in open and cased boreholes; (2) Hydraulic-fracture evaluation through casing; and (3) Formation stress diagnosis and orientation determination.
Calculating water Saturation in Electrically Anisotropic Media
W. David Kennedy, David C. Herrick and Tingting Yao
ABSTRACT
The influence of resistivity anisotropy upon field electrical measurements was described at least as early as 1920. Electrical logging instrument responses in vertical boreholes drilled through horizontal layers were, fortuitously, not affected by anisotropy, not because the formations were isotropic, but because commercially available logging instruments responded to the same tensor component of resistivity that is typically measure in core analysis, and the other tensor component of anisotropy remained unsampled and could be ignored in formation evaluation. Log responses significantly influenced by anisotropy were rare enough that there was not much concern with, or appreciation of, this problem. Consequently Archie (1942) formulated his "law" relating water saturation to resistivity in terms of the resistivity of an isotropic medium. Although his resulting formulation is a relatively complicated power law, it nevertheless has remained the industry-standard paradigm for quantitative formation valuation for (so far) 59 years. With the advent of highly deviated and horizontal drilling, reservoir anisotropy can no longer be ignored in interpretation theory. Since it has been customary to compute water saturations from resistivity observations obtained from logging instruments based on a scalar equation, the question naturally arises as to how anisotropy will influence the computation of water saturation. The usual saturation equations can be cast into a form that contains both tensor factors, such as the conductivity, and scalar factors such as porosity and water saturation. The scalar water-saturation factor has solutions in terms of the tensor factors. However, by diagonalizing the conductivity tensor its components uncouple and the familiar scalar Archie equation can be applied separately to the tensor components.
The Effect of Crossbedding Anisotropy on Induction Tool Response
B.I. Anderson, T.D. Barber and S.C.Gianzero
ABSTRACT
Previous studies of resistivity anisotropy have neglected crossbedding effects. This article analyzes induction response in crossbedded reservoirs using a new computer modeling code. The code computes the response of an induction logging tool as it orthogonally traverses many beds, each of which possesses different crossbedding characteristics. The crossbedding in each medium is described by a uniaxial conductivity tensor whose principal axes have strike and dip angles oriented arbitrarily with respect to the bedding planes. The code is numerically efficient; response for a tool logging through several beds can be generated in less then 15 minutes on a modern workstation. Results show that, for anisotropy coefficients less that 5, computed responses for both two-coil and multicoil devices vary in a continuous manner as the sondes cross a singe bed boundary separating two infinitely thick beds. Furthermore, after correction for skin effect, the limiting log values far from the bed boundary are entirely predictable from a previously published formula. However, in vertical wells, when the crossbedding dip angle is 75degree or greater and the anisotropy coefficient greater than or equal to 5, anomalously large readings appear in the vicinity of the bed boundaries. These large readings are similar to the polarization horns that occur in dipping beds at high-contrast isotropic interfaces. In the case of a thin bed (e.g., < 5 ft) located between two massive shoulder beds, the large anomalies from the bed boundaries merge into a single anomaly at the center of the bed. This behavior was not expected and can be quantified only by modeling. Modeled results are also used to analyzed the Schlumberger AIT Array Induction Imager instrument response in a crossbedded reservoir in the Nugget formation where we expect different values of Rv and Rh.