Method for Evaluating Voids in a Subterranean Formation

Abstract

Voids adjacent a wellbore wall and in a region surrounding a wellbore wall can be detected by monitoring gamma rays scattered from the fractures. Gamma rays are strategically directed from a tool disposed within the wellbore and to the wall and/or the region. Some of the gamma rays scatter from the voids and are detected with detectors set a designated axial distance from the gamma ray source. In addition to identifying the presence of the voids, the location and size of the fractures/perforations is also estimated. Time lapsed imaging of the wellbore wall can yield changes in the voids that in turn can affect permeability of the well. Examples of the voids include fractures and/or perforations.

What is claimed is: 
     
         1 . A method of imaging a wellbore comprising:
 a. directing radiation from a source to a formation surrounding the wellbore;   b. detecting radiation scattered from the formation;   c. estimating a rate and energy of the detected radiation; and   d. estimating information about a change of a void in the formation based on the rate and energy of the detected radiation.   
     
     
         2 . The method of  claim 1 , wherein the void is selected from the group consisting of a fracture and a perforation. 
     
     
         3 . The method of  claim 1 , wherein steps (a)-(c) include time lapsed imaging. 
     
     
         4 . The method of  claim 1 , wherein the source comprises a gamma ray source and which is directed in a substantially conical pattern from the source and wherein the energy of the detected radiation is dependent upon an angle of scatter of the radiation. 
     
     
         5 . The method of  claim 1 , wherein the information about a change of the void is selected from the group consisting of a change in thickness of the void, a change of permeability in the void, a change in flowability through the void, and a change of density of material in the void. 
     
     
         6 . The method of  claim 1 , further comprising estimating a location of the void. 
     
     
         7 . The method of  claim 1 , wherein the void comprises a fracture that intersects a wall of the wellbore. 
     
     
         8 . The method of  claim 1 , wherein the void comprises a perforation that intersects a wall of the wellbore. 
     
     
         9 . The method of  claim 1 , wherein the wellbore comprises a well selected from the group consisting of an injection well and a production well. 
     
     
         10 . A method of imaging a wellbore comprising:
 a. providing a logging instrument having a radiation source and a radiation detector;   b. disposing the logging instrument into the wellbore and within casing that lines the wellbore;   c. directing radiation from the source to a formation surrounding the wellbore and along a path, so that at least some of the radiation scatters in a direction back from the formation towards the radiation detector,   d. detecting radiation scattered from the formation with the radiation detector; and   e. identifying a change in a void in the formation based on a change of rate of the radiation detected.   
     
     
         11 . The method of  claim 10 , wherein the void comprises an opening in the formation selected from the group consisting of a perforation and a fracture. 
     
     
         12 . The method of  claim 10 , wherein steps (a)-(d) are repeated over time so that the change of rate of the radiation detected comprises a time lapsed measurement. 
     
     
         13 . The method of  claim 10 , wherein the radiation is directed fully in a fan-shaped beam from the outer circumference of the logging instrument so that the radiation can scatter from an entire circumference of a wall of the wellbore. 
     
     
         14 . The method of  claim 10 , further comprising repeating steps (b)-(e) while moving the logging instrument to different depths in the wellbore. 
     
     
         15 . A method of imaging a wellbore comprising:
 a. directing radiation from a source to a formation surrounding the wellbore;   b. detecting radiation scattered from the formation;   c. estimating a rate and energy of the detected radiation;   d. repeating steps (a)-(c) at a later time to perform a time lapsed measurement; and   e. estimating information about a change of a void in the formation based on the time lapsed measurement.   
     
     
         16 . The method of  claim 15 , wherein step (e) is based on a rate and energy of the detected radiation. 
     
     
         17 . The method of  claim 15 , further comprising estimating a location of the void. 
     
     
         18 . The method of  claim 15 , wherein the void comprises a space in the formation selected from the group consisting of a fracture and a perforation.