Real-time fracture detection and fracture orientation estimation using tri-axial induction measurements

Patent metadata

Field	Value
Publication number	US-9606257-B2
Application number	US-201113217983-A
Country	US
Kind code	B2
Filing date	Aug 25, 2011
Priority date	Sep 15, 2010
Publication date	Mar 28, 2017
Grant date	Mar 28, 2017

Abstract

A method for determining existence of a fracture in a formation surrounding a wellbore drilled through subsurface rock formations includes calculating vertical resistivity, horizontal resistivity, apparent formation dip, apparent formation azimuth and axial resistivity for a plurality of longitudinal instrument spacings using measurements from a triaxial induction well logging instrument disposed in the formation. A spread in the axial resistivity values is determined and the axial resistivity spread threshold therefrom. Fracture indicator values and fracture orientation values are calculated from transverse components of the triaxial induction measurements. Presence of a fracture is indicated when at least one of the fracture indicator value exceeds a selected threshold, the axial resistivity spread exceeds the spread threshold and when the apparent formation dip exceeds a selected threshold.

First claim

What is claimed is:

1. A method for determining existence of a fracture in a formation surrounding a wellbore drilled through subsurface rock formations, comprising:
deploying an electromagnetic logging tool in the wellbore, the electromagnetic logging tool including a triaxial transmitter and a triaxial receiver longitudinally spaced apart from one another;
causing the triaxial transmitter to transmit electromagnetic energy into the subsurface rock formations;
causing the triaxial receiver to receive said transmitted electromagnetic energy to obtain electromagnetic measurements such that the obtained electromagnetic measurements comprise responses between the triaxial transmitter of the electromagnetic logging tool and the triaxial receiver of the electromagnetic logging tool;
determining a fracture orientation value of the rock formations from only two measured responses of the electromagnetic logging tool in accordance with the following equation:
θ=0.5·tan −1 [(σ xx+σyy− 2σ xx 45 )/(σ xx−σyy )]

wherein the two measured responses are transverse with respect to one another, and wherein θ represents the fracture orientation value, σxx represents an apparent conductivity of the one of the two measured responses, σyy represents an apparent conductivity of the other of the two measured responses, and σxx 45 represents an apparent conductivity of the one of the two measured responses rotated by 45 degrees about an axial direction of the logging tool;
determining a fracture indicator value of the rock formations based at least partially upon the two measured responses and the fracture orientation value; and
determining the presence of a fracture in the rock formations based upon the magnitude of the fracture indicator value.

2. The method of claim 1 , wherein the fracture indicator value is determined in accordance with the following:
B= 0.5·(σ xx−σyy )/[δ+cos(2θ)]

wherein B represents the fracture indicator value, and δ represents a constant.

3. The method of claim 1 , wherein x-direction magnetic moments of the triaxial transmitter and the triaxial receiver are coplanar, and wherein y-direction magnetic moments of the triaxial transmitter and the triaxial receiver are coplanar.

4. The method of claim 1 , wherein the triaxial transmitter is actuated using continuous wave alternating current at at least one frequency.

5. The method of claim 1 , wherein the fracture orientation value is determined from the two measured responses without inversion to obtain at least one of horizontal resistivity, vertical resistivity, dip, or dip azimuth of the formation.

6. A method for determining existence of a fracture in a formation surrounding a wellbore drilled through subsurface rock formations, the method comprising:
(a) moving an electromagnetic logging tool along a depth axis of the wellbore, the electromagnetic logging tool including a triaxial transmitter and a triaxial receiver longitudinally spaced apart from one another;
(b) causing the electromagnetic logging tool to obtain triaxial conductivity tensor logs as a function of depth in the wellbore while moving in (a);
(c) inverting the triaxial conductivity tensor logs to obtain horizontal resistivity, vertical resistivity, and formation dip angle logs;
(d) processing the conductivity tensor logs to compute fracture orientation, fracture indication value, and axial resistivity logs of the subsurface rock formations;
(e) processing the fracture orientation and fracture indication value logs with a depth filter to compute an average fracture indication value;
(f) processing the axial resistivity log to compute a spread in axial resistivity values between various axial resolution values; and
(g) flagging a fracture zone at depth ranges in which the formation dip angle obtained in (c) is greater than 75 degrees, the average fracture indication value computed in (e) is greater than a first threshold, and the spread in axial resistivity values computed in (f) is greater than a second threshold.

7. The method of claim 6 , wherein the fracture orientation is computed in (c) using the following equation:
θ=0.5·tan −1 [(σ xx+σyy− 2σ xx 45 )/(σ xx−σyy )]

wherein θ represents the fracture orientation, σxx represents a first member of the conductivity tensor, σyy represents a second member of the conductivity tensor, and σxx 45 represents an apparent conductivity of the first member rotated by 45 degrees about an axial direction of the logging tool.

8. The method of claim 6 , wherein the fracture indication value is computed in (c) using the following equation:
B= 0.5·(σ xx−σyy )/[δ+cos(2θ)]

wherein B represents the fracture indicator value, δ represents a constant that prevents the denominator from being zero, θ represents the fracture orientation, σxx represents a first member of the conductivity tensor, σyy represents a second member of the conductivity tensor.

Assignees

• Wu Peter T
• Weixin Dong
• Barber Thomas D
• Homan Dean
• Schlumberger Technology Corp

Inventors

• Wu Peter T
• Weixin Dong
• Barber Thomas D
• Homan Dean

Classifications

• G01V3/28 Primary

  using induction coils · CPC title

• G01V3/38

  Processing data, e.g. for analysis, for interpretation, for correction · CPC title