Detailed and very accurate measurements of compressional Vp and shear Vs wave velocities and their attenuation in dry, partly saturated and fully saturated clay-bearing sandstone were carried out at 20, 145, and 198 degrees C, under variable confining and pore pressures.The velocity results show that water saturated rock has high Vp and relatively low Vs not only at room temperature but also at 145 and 198 degrees C. Varying the pore pressure reveals that the transition from the fully saturated to the dry state involves a gradual transition of Vs and a very irregular transition of Vp - with a minimum in Vp relative to either the dry or the fully saturated cases.The attenuation of S and P waves - Qs(-1) and Qp(-1), respectively - shows similar patterns: in dry rock, both Qs(-1) and Qp(-1) are low. In fully saturated rock Qp(-1) is low and Qs(-1) is high. In partly saturated rock, Qs(-1) changes gradually with saturation, but Qp(-1) shows a very strong maximumsuggesting strong energy dissipation. The same behavior is observed at 145 degrees C, with the steam/hot-water transition. It was also found that attenuation of shear waves increases with permeability in some sandstones, consistent with squirting attenuation mechanism.The results suggest diagnostic characteristics for determining the degree of saturation in situ from seismic logs, with particular applicability to evaluation of steam content in geothermal reservoirs and of gas content in oil reservoirs. Introduction In recent years seismic attenuation has become of increasing interest to seismologists, including the study of bright spots in hydrocarbon exploration, reservoir evaluation, and well logging.Seismic methods are particularly important in exploration for and evaluation of geothermal reservoirs, including physical property determinationssuch as Poisson's ratio and seismic wave attenuationfor delineating the boundaries and state of reservoirs. The latter measurements are particularly interesting for the possible distinction between steam-bearing and hot-water domains. To make full use of seismic data, it is necessary to interpret attenuation in terms of the physical properties of rocks, and the mechanisms responsible for loss of energy in seismic waves. In this paper we show some of the important effects of fluids on wave velocities and attenuation in porous and cracked rocks. We further show that a fundamental physical link may exist between seismic wave attenuation in rocks and the hydraulic permeability in these rocks.The effects that pore fluids have on seismic velocities are well-documented. It is natural to suppose that pore fluids will also influence seismic attenuation, but very little experimental work has been done in this area. There is, however, no shortage of theoretical models of fluid loss mechanism. Flow models based on partial saturation of individual cracks have been developed by White and by Mavko and Nur, and a thermoelastic partial saturation mechanism has been presented by Kjartansson and Nur. The models show invariably that attenuation is related to the local hydraulic diffusivity. Thus, if the local and global hydraulic properties can be related, we may eventually be able to infer permeability from seismic measurements. SPEJ P. 450＾

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Gas permeability has been measured on a suite of cores from the Spirit River tight gas sand of western Alberta and on two samples from the Cotton Valley formation of east Texas. Using nitrogen as the mobile fluid, we have measured permeability as a function of partial water saturation at in-situ levels of pore pressure and confining pressure. Samples from both locations show strong dependence of permeability (k) on effective pressure and on the degree of water or brine saturation. The validity of Darcy's law in the microdarcy range has been verified in a dry Spirit River sample. Extensive thin-section, x-ray diffraction, and scanning electron microscope (SEM) studies have been conducted. The primary clays in Spirit River and Cotton Valley cores are chlorite and illite. In one sample we measured k vs. saturation first with distilled water and then with a 2 % KCl brine solution and saw no significant change in permeability behavior. By observing the effects of pressure, partial saturation, and salinity on permeability in these samples, we can deduce several important characteristics of the pore structure and can evaluate the relative importance of clay content.

A new analytical solution is presented for the laboratory pulse decay permeability problem. With this solution, pulse decay permeability problem. With this solution, permeability of a core sample can be calculated from the permeability of a core sample can be calculated from the decay rate of a pressure pulse applied to one end of the sample. This development permits rapid. accurate measurement of permeability in samples such as tight gas sands, limestones, and shales. Introduction Because of its usefulness in measuring very low permeability. the pulse decay technique has been permeability. the pulse decay technique has been discussed often in the literature. In this technique, a small pore pressure pulse is applied to one end of a jacketed sample, and the pressure vs. time behavior is observed as the pore fluid moves through the sample from one reservoir to another. Brace et al. cave an approximate solution to this problem with the assumption of a linear pressure gradient at all times. This simplification leads to a predicted exponential pressure vs. time decay. By means of numerical solutions, Lin and Yamada and Jones have shown that the Brace solution can lead to significant errors in calculating permeability. These numerical solutions. however. are inconvenient to use and require considerable computer programming time. We present an analytical solution based on realistic assumptions and boundary conditions. Experimental Technique To understand the theoretical problem more thoroughly, a short description of the experiment is desirable. Fig. 1 is a schematic of the system. Initially, both valves are open and pressure is constant throughout the system. Next, Valve 1 is closed, and the pressure is changed slightly in the large Reservoir 1. Valve 1 remains closed for a few minutes to allow thermal effects to diminish (particularly important if the pore fluid is (as). Valve 2 then is closed, and, at time equal zero. Valve 1 is opened. A small differential pressure between the reservoirs will be indicated by the p transducer and will decrease with time. Pressure in Reservoir 1 remains constant during the decay. After the differential pressure has decreased by approximately 20%, Valve 2 is opened to terminate the decay. This accelerates the equilibration of pressure so that the next measurement can be made. pressure so that the next measurement can be made. Theory As stated earlier, the pressure in Reservoir 1 remains essentially constant during the decay (t 0) because the volume of Reservoir 1, V1, is much greater than the pore volume, Vp, or the volume of Reservoir 2, V2. It can be assumed that fluid viscosity, is independent of position, x, in the sample and that fluid density, p, position, x, in the sample and that fluid density, p, permeability, k, and porosity, are dependent only on permeability, k, and porosity, are dependent only on fluid pressure, P. By combining Darcy's law with the one-dimensional diffusion equation we obtain ,..................(1) where B is fluid compressibility, Bs, is rock compressibility, and Bk is the dependence of permeability on pore pressure. The magnitude of the nonlinear terms pore pressure. The magnitude of the nonlinear terms with respect to the linear ones is equal to (Bk + B)P0, where P0 is the pressure pulse amplitude. Because (Bk - B ) = 10 -2 bar - 1 (Ref. 8) and P0=1 bar, the product is small, and, hence, nonlinear terms can be product is small, and, hence, nonlinear terms can be ignored. If we further assume that the equation of flow is ------- = --- --------, ......................(2) SPEJ P. 719

Seismic Reservoir Characterization, also known as reservoir geophysics, has evolved over the past several years into a multi-disciplinary, business-critical function in most ED&P organizations. Sheriff defines reservoir geophysics as "The use of geophysical methods to assist in delineating or describing a reservoir or monitoring the changes in a reservoir as it is produced." Reservoir geophysics is applied across a wide spectrum of the oilfield life cycle from discovery and early development to tertiary recovery. One critical part of this process is careful analysis and understanding of petrophysical properties from well logs and core data (seismic petrophysics). The purpose of this paper is to illustrate why seismic petrophysics is so important and to show how carefully constructed synthetic models can help the geoscientist interpret acoustic and elastic impedance inversion from seismic data. Introduction Well logs are sometimes viewed by geophysicists as "hard data" and not subjected to the same level of scrutiny as the seismic data. This can be a mistake because well logs are susceptible to errors from a number of sources. In this presentation we will examine some of the processes and procedures that allow well logs to be correctly used in Seismic Reservoir Characterization. The basic steps in seismic petrophysics analysis are:Collect and organize input dataPerform geophysical log interpretation for volume minerals, porosity, and fluidsDetermine fluid properties (oil API, brine salinity, etc.) and reservoir pressuretemperaturePerturb reservoir properties using rock physics effective medium models (pseudo-well modeling)Compute synthetic seismic tracesGenerate trend curves and crossplotsCreate graphics and digital output files. Geophysical Well Log Analysis Well log analysis for geophysics differs in several important ways from standard log analysis. In most cases well logs are obtained for the purpose of estimating recoverable hydrocarbon volumes. Therefore the zone of interest is mainly the producing interval(s). For geophysics, well logs form the basis for relating seismic properties to the reservoir. While we are still concerned about producing intervals, we also need good information about all of the rock through which the seismic waves have passed. Therefore our zone of interest is much larger and encompasses basically everything from the surface to total depth. In all cases the log data will require some editing, normalization, and interpretation before they can be used in a reservoir study. Several specific analysis steps will be followed:De-spike and filter to remove or correct anomalous data pointsNormalize logs from all of the selected wells to determine the appropriate ranges and cutoffs for porosity, clay content, water resistivity, etc.Compute the volumetric curves such as total porosity, Vclay, and SwCalibrate the volumetric curves to core data if availableCorrect sonic and density logs for mud filtrate invasion if neededCompute Vshear on all wells. Missing log curves can often be computed with a reasonable degree of certainty. There are two major ways this is done. The first is through application of modern rock physics principles.