Method and apparatus for estimating distance to or from a geological target while drilling or logging
A system and method estimates the distance between a borehole and a subsurface boundary of interest in a geophysical region. In one embodiment, available existing sensor data for the geophysical region is used to create a resistivity model of the region, with the model reflecting changes in resistivity across the boundary. A hypothetical borehole has a number of segments along its length that are spaced-apart from the boundary by different, preselected distances. The ratio between two selected resistivity curves in each of the respective spaced-apart segments is computed, and these ratio values are plotted as a function of distance from the boundary. A curve-fitting algorithm is applied to derive an equation, which may be applied to actual sensor data from a sensor package.
1. A method for estimating distance between a borehole and a subterranean geophysical boundary within a geophysical region, comprising:
defining a resistivity model of the resistivity characteristics of said geophysical region based on available resistivity data for said region;
defining a hypothetical borehole having a trajectory extending through said geophysical region, said hypothetical borehole trajectory at a plurality of discrete locations along the length of the borehole being spaced apart from said geophysical boundary by a plurality of selected distances;
deriving from said resistivity model a plurality of hypothetical resistivity sensor values each corresponding to one of said plurality of discrete locations along said hypothetical borehole;
deriving an equation approximating a mathematical relationship between said plurality of resisitivity sensor values and said plurality of selected distances;
wherein said equation defines a relationship between actual resistivity sensor data and quantified estimates of distance between an actual borehole and said geophysical boundary.
2. A method in accordance with claim 1 , wherein said resistivity model evidences a change in resistivity at said geophysical boundary.
3. A method in accordance with claim 1 , wherein each of said plurality of hypothetical resisitivity sensor values comprises a ratio between a resisitivity phase value and a resistivity amplitude value when said hypothetical borehole trajectory is one of said plurality of predefined distances away from said geophysical boundary.
4. A method in accordance with claim 1 , wherein said plurality of discrete locations along the length of said borehole comprises at least two discrete locations.
5. A method in accordance with claim 4 , wherein said plurality of selected distances comprises distances ranging from less than one-half meter and as great as one and one-half meters.
6. A method in accordance with claim 1 , wherein said available resistivity data is obtained from prior drilling in said geophysical region.
7. A method in accordance with claim 1 , wherein said deriving an equation comprises deriving a polynomial approximation of a relationship between said plurality of hypothetical sensor values and said plurality of predefined distances.
8. A method in accordance with claim 3 , wherein said available resistivity data includes data sets corresponding to at least two depths of investigation.
9. A method in accordance with claim 8 , further comprising selecting said resistivity phase value from a data set corresponding to a first depth of investigation and selecting resistivity amplitude value from a data set corresponding to a second depth of investigation.
10. A method for estimating distance between a borehole and a subterranean geophysical boundary within a geophysical region, comprising:
defining a resistivity model of the resistivity characteristics of said geophysical region based on available resistivity data for said region;
defining a hypothetical borehole having a trajectory extending through said geophysical region, said trajectory being such that the distance between said hypothetical borehole and said geophysical boundary varies along the length of said hypothetical borehole;
deriving at least two hypothetical resistivity sensor curves corresponding to said trajectory and said resistivity model;
selecting two of said at least two hypothetical resistivity sensor curves having a desired correlation with said trajectory's distance from said geophysical boundary;
computing ratios between said selected two hypothetical resisitivity sensor curves at a plurality of points along said trajectory;
deriving an equation approximating a mathematical relationship between said computed ratios and distances from said geophysical boundary at said plurality of points;
said equation being applicable to actual resistivity sensor values from a downhole sensor tool to permit estimation of actual distance of a borehole from said geophysical boundary.
11. A method in accordance with claim 10 , wherein said at least two hypothetical resistivity sensor curves comprise at least one resistivity amplitude curve and at least one resistivity phase curve.
12. A method in accordance with claim 10 , wherein said resistivity model evidences a change in resistivity at said geophysical boundary.
13. A method in accordance with claim 10 , wherein said available resistivity data is obtained from prior drilling in said geophysical region.
14. A method in accordance with claim 10 , wherein said deriving an equation comprises deriving a polynomial approximation of a relationship between said plurality of hypothetical sensor values and said plurality of predefined distances.
15. A method in accordance with claim 10 , wherein said available resistivity data includes data sets corresponding to at least two depths of investigation.
16. A method in accordance with claim 15 , wherein said selecting two hypothetical resistivity curves comprises selecting a resistivity curve corresponding to a first depth of investigation and selecting a resistivity curve corresponding to a second depth of investigation.
17. A machine-readable medium that provides instructions, which when executed by a machine, cause said machine to perform the method of any of claims 1 through 16 .
18. A computer-based system for estimating the distance between a borehole in a geophysical region having a boundary therein between formations having different resistivity characteristics, comprising:
a modeling application, executed by a computer, for generating a resistivity model of said geophysical region based on existing sensor data from said geophysical region;
a user input mechanism for defining a hypothetical borehole in said resistivity model;
a display device for displaying a plurality of resistivity curves corresponding to hypothetical borehole;
a first computation application, executed by said computer, for computing ratios between a selected two of said resistivity curves at a plurality of selected locations along the length of said hypothetical borehole;
a second computation application, executed by said computer, for plotting said ratios as a function of distance of said hypothetical borehole from said boundary;
a curve-fitting application, executed by said computer, for deriving an equation defining a correlation between the ratio between said selected two resistivity curves and distance from said boundary.
19. A system in accordance with claim 18 , wherein said hypothetical borehole has at least one segment that is spaced-apart from said boundary by a preselected distance.
20. A system in accordance with claim 19 , wherein said hypothetical borehole has at a first segment that is spaced-apart from said boundary by a first preselected distance and a second segment that is spaced-apart from said boundary by a second preselected distance greater than said first preselected distance.
21. A system in accordance with claim 20 , wherein said first computation application computes a ratio between said selected two resistivity curves in said first segment and said second segment.