Using Resistivity Measurements To Determine Distance Between Wells
Authors: Mitchell F.R. , Robinson J.D. , Vogiatzis J.P. , Pehoushek F. , Moran J.H. , Ausburn B.E. , Berry L.N. 
Published: December 2007

Resistivity measurements can be used to estimate the distance between open and cased wells in the subsurface at distances to about 50 feet. Here are techniques that are useful in drilling relief wells especially when the precise position of the target well is not known as well as in preventing accidental intersections and in determining degree of departure preventing accidental intersections and in determining degree of departure in sidetracking or redrilling operations.

Introduction
Techniques have been developed that facilitate the direct estimation of distance between an open and a cased well in the subsurface utilizing resistivity measurements. This capability was developed specifically for the drilling of an intermediate-depth relief well at Piney Woods, Miss., where the actual intersection of two wells was required. However, these techniques have been found useful in conventional relief-well drilling at Bay Marchand, La.; and other applications also appear feasible.
The success or failure of the intermediate-depth relief well at Piney Woods greatly depended upon the ability to make the intersection. The wild well had been drilled to a total depth of 21,122 ft and cased to 20,607 ft, which was far below the 9,000- to 13,000-ft depth interval where intersection was desired. Normal methods of directional well surveying, it was felt, would not provide the degree of control necessary in this case. Accordingly, other methods were sought that might provide the needed "homing in" capability. The use of resistivity devices was one of the approaches tried.
We will give a brief account of the evolution of the techniques employed that will include (1) empirical approaches, (2) experimental studies, and (3) analytical solutions. We well also show the characteristic response of the various electrical devices in proximity to casing for typical conditions of interest and will show examples of successful applications.

Empirical Approach
The use of electrical logs for locating various drillpipe and casing fishes in wellbores is well known, and many examples could be cited. The use of resistivity devices to infer proximity to sidetracked casing is also well known. However, from a survey of the literature and from informal contacts with many outside companies, we concluded that little effort had been expended by the industry to utilize resistivity devices as a direct means of determining distance between wells in the subsurface. The few examples of intersecting wells that we did locate have been most helpful.
Fig. 1 shows response of the standard electrical survey in an intersecting well along with normal measured resistivities for the same interval from a twin well. Intersection between the two wells actually occurred over a 75-ft interval. Apparently the SP electrode was scratched at 470 ft, thus changing the potential markedly. Note the systematic decrease in potential markedly. Note the systematic decrease in measured resistivities above and below the intersection. Fig. 2 shows fractional response of two of the standard devices as a function of distance between wells. Distances were calculated from gyro and multishop surveys normalized at the intersection. The 16-in. normal was omitted because of the large bore-hole involved. Other particulars as to log spacings, angle of incidence between wells, casing size and weight, etc., are indicated on the illustrations.
JPT
P. 723