Estimation of production sweep efficiency utilizing geophysical data

Patent metadata

Field	Value
Publication number	US-9405026-B2
Application number	US-201213610530-A
Country	US
Kind code	B2
Filing date	Sep 11, 2012
Priority date	Dec 12, 2011
Publication date	Aug 2, 2016
Grant date	Aug 2, 2016

Abstract

Method for estimating fluid heterogeneity in a subsurface region from seismic wave attenuation or velocity dispersion that is either measured or extracted from geophysical data ( 210 ). A rock physics model in the form of a mathematical relationship is selected that relates attenuation or velocity to frequency and to physical properties that are related to fluid heterogeneity ( 220 ). The model, or asymptotes ( 230 ) representing the model's behavior at frequency extremes, is inverted ( 240 ) and that relationship is used to obtain ( 250 ) one or more of the physical properties related to fluid heterogeneity, such as characteristic length scale for fluid saturation heterogeneities ( 270 ) and relative volume fractions of the fluids saturating the pore space ( 280 ).

First claim

The invention claimed is:

1. A computer implemented method for estimating fluid heterogeneity in a subsurface region from compressional wave attenuation or velocity dispersion, comprising:
measuring compressional wave attenuation or velocity, or extracting it from geophysical data, for at least one frequency;
selecting a frequency-dependent, mathematical rock physics model for attenuation or velocity, said model pertaining to or containing one or more model parameters that pertain to heterogeneous features of fluid distribution in the subsurface region;
using the rock physics model to predict frequency dependence of compressional wave attenuation or velocity; and
using the predicted frequency dependence, the attenuation or velocity measured or determined from the geophysical data for at least one frequency, and measured or estimated rock and fluid properties of the subsurface region, called subsurface properties, to determine, using a computer, at least one unknown subsurface property, including fluid saturation, that is related to fluid heterogeneity, each such subsurface property corresponding to a parameter in the mathematical rock physics model.

2. The method of claim 1 , wherein the rock physics model is for a selected type of spatial distribution of fluids and includes at least the following parameters:
a characteristic length for fluid patches in the heterogeneous distribution, sometimes referred to as “heterogeneity length scale”; and
relative percentage of each of two or more fluids, this being the fluid saturation parameter.

3. The method of claim 2 , wherein the using the rock physics model to predict the frequency dependence comprises determining at least two asymptotes in the rock physics model's frequency-dependent behavior.

4. The method of claim 3 , wherein a high-frequency asymptote and a low-frequency asymptote are determined, and then used in the determining of at least one unknown subsurface property related to fluid heterogeneity.

5. The method of claim 2 , wherein the two or more fluids are brine and hydrocarbons.

6. The method of claim 2 , wherein the selected type of spatial distribution of fluids is random, periodic, regularly layered, random layered or other type of distribution, and the selection is made, at least partly, according to which type better fits the measured or extracted attenuation or velocity values or their asymptotic behavior, where the fit is assessed by visual comparison or through a mathematical fitting routine having a confidence interval defined by a mathematical function that describes the difference in the attenuation or velocity predicted by the rock physics model and the measured or extracted attenuation or velocity.

7. The method of claim 1 , wherein the method is applied both to compressional wave attenuation measurements and to compressional wave velocity measurements, using the same rock physics model to analyze each, followed by combining results of each application to determine the at least one unknown subsurface property related to fluid heterogeneity.

8. The method of claim 1 , wherein the geophysical data comprise at least one of seismic data, cross well tomography, vertical seismic profile data, sonic logs, and laboratory measurements on core plugs.

9. The method of claim 1 , wherein the measured or estimated rock and fluid properties of the subsurface region are selected from a list comprising porosity, permeability, grain density, grain bulk and shear moduli, dry rock frame bulk and shear moduli, fluid bulk modulus, fluid viscosity, and fluid density.

10. The method of claim 4 , wherein using the predicted frequency dependence, the attenuation or velocity measured or determined from the geophysical data for at least one frequency, and measured or estimated rock and fluid properties of the subsurface region, called subsurface properties, to determine at least one unknown subsurface property, including fluid saturation, that is related to fluid heterogeneity comprises:
inverting said asymptotes for said at least one unknown subsurface property as a function of either attenuation or velocity, and frequency.

11. The method of claim 10 , wherein two model parameters corresponding to two unknown subsurface properties related to fluid heterogeneity are determined by steps comprising:
plotting the two inverted asymptotes on a cross-plot wherein the two model parameters are plotted versus each other for input data comprising the attenuation or velocity value or values and corresponding frequency or frequencies measured or extracted from geophysical data, and recording their intersection point coordinates as the determined values of the two unknown subsurface properties related to fluid heterogeneity.

12. The method of claim 11 , wherein the input data are extracted from geophysical data that are selected such that (i) the attenuation peaks at a frequency within and preferably centered in the geophysical data's bandwidth, or (ii) a corresponding shift in velocity versus frequency occurs at a frequency within and preferably centered in the geophysical data's bandwidth.

13. The method of claim 12 , wherein determination of where the peak frequency for attenuation or mid-shift frequency for velocity is relative to the input data's frequencies is made by plotting the two asymptotes on a plot of attenuation or velocity vs. one of the two model parameters for one of the input data's frequencies, then repeating the plot for other input data frequencies, identifying whether the input data frequency is located on the high-frequency asymptote or the low-frequency asymptote according to which curve is flatter.

14. The method of claim 12 , wherein the two model parameters, as well as the two unknown subsurface properties related to fluid heterogeneity, are heterogeneity length scale and fluid saturation, and wherein determination of where the peak frequency for attenuation or mid-shift frequency for velocity is relative to the input data's frequencies comprises:
substituting heterogeneity length scale and fluid saturation relationships determined from the inverted asymptotes into the rock physics model along with the input data's frequencies, resulting in substituted relationships, one corresponding to each asymptote, that relate attenuation to fluid saturation or attenuation to heterogeneity length scale;
using the substituted relationships to cross-plot predicted attenuation or velocity versus at least one of the two model parameters, for each asymptote; and
examining said cross-plots for constant attenuation or constant velocity behavior, and associating constant attenuation or velocity behavior with location of the peak frequency for attenuation relative to the input data's frequencies.

15. The method of claim 14 , wherein an intersection of the cross-plot for the high-frequency asymptote with the cross-plot for the low-frequency asymptote is an estimate of the model parameter's value in the subsurface region.

16. The method of claim 14 , further comprising generating a plurality of the cross-plots, each for a different frequency asymptote relationship, and:
upon a finding that the cross-plot for the low-frequency asymptote for a certain frequency exhibits flat behavior, concluding that that frequency is below the peak frequency;
upon a finding that the cross-plot for the high-frequency asymptote for a certain frequency exhibits flat behavior, concluding that that frequency is above the peak frequency; and
upon a finding that for a certain frequency, neither the cross-plot for the high-frequency asymptote nor the cross-plot for the low-frequency asy

Assignees

• Toms Julianna J
• Krohn Christine E
• Liu Enru
• Exxonmobil Upstream Res Co

Inventors

• Toms Julianna J
• Krohn Christine E
• Liu Enru

Classifications

• G01V1/306 Primary

  for determining physical properties of the subsurface, e.g. impedance, porosity or attenuation profiles · CPC title

• G01V2210/624

  Reservoir parameters · CPC title