Drilling Office

Close Approach

Close Approach (Cont’)

Standard Proximity Calculations Normal Plane: MD interval is

recommended because in horizontal wells, TVD interval may have several positions within the object well, creating discontinuous results.

Close Approach (Cont’)

Travelling cylinder plots are undistorted and depict the true 3-D relative position of surrounding wells;

these plots are especially useful for forward projections.

Will not detect a close well that is passing just beyond the end of the subject wellbore.

Scanning down the offset survey ensures that no portion of the offset well is missed. This ability is especially important for detecting close approaches between wells oriented perpendicularly.

Strengths Weaknesses

Close Approach (Cont’)

Standard Proximity Calculations Horizontal Plane: TVD interval is

recommended because in highly deviated wells (with inclinations at or exceeding 90°) it is possible to have multiple penetrations with the horizontal plane and multiple distances from the same point in the subject well

Close Approach (Cont’)

Useful for determining relative positions offormation penetrations that are essentially flat.

Limited use as an anticollision tool with high-angle, designer wells. A spider plot provides more information for anticollision, while depicting a horizontal perspective.

Extremely distorted travelling cylinder plots.

Strengths Weaknesses

Close Approach (Cont’)

Standard Proximity Calculations 3D Least Distance: (Reverse Normal

Plane) Computes the radius of a sphere centered on the subject survey that just touches the offset survey.

Close Approach (Cont’)

Provides the true closest distance between wellbores for a given position, which is useful in additional

applications (well injection/fluid flow analysis).

Anticollision uses are for blowout intersection for well killing procedures and for active magnetic

ranging anticollision procedures.

Distorted travelling cylinder plots.

Strengths Weaknesses

Close Approach (Cont’)

CALCULATION OUTPUTS center-to-center distance ellipse of uncertainty (EOU) distance separation factor alert radii

Close Approach (Cont’)

center-to-center distance The straight-line distance between the

subject survey and an offset survey No error analysis is incorporated in this

calculation.

Close Approach (Cont’)

ellipse of uncertainty (EOU) distance The ellipse of uncertainty (EOU) distance

is equal to the ct-ct distance minus the sum of the ellipse semimajor axes of the subject well survey and the offset well intersection point survey.

Close Approach (Cont’)

separation factor Separation factor = (ct – ct distance)/(Sum

of EOU semimajor axes) S.F. > 1, the error ellipses do not overlap S.F. = 1, the error ellipses just touch S.F. < 1, the error ellipses overlap

Close Approach (Cont’)

alert radii Major Risk

The major risk radii are defined as the smallest cones of the three available alert zones. The major risk zone specifies close approaches that come so close to the nearby well that a dangerous situation exists, even if the offset well is shut in.

Close Approach (Cont’)

alert radii Minor Risk

The minor risk radii are defined as a warning zone, so that any wells coming within the warning zone can be shut in; however, these wells are not as critical as a well entering the major risk radii.

Close Approach (Cont’)

alert radii Drilling Buffer

The drilling buffer radii is a zone defined to ensure enough clearance from nearby wells to allow for normal deviation from a plan during drilling operations. This buffer ensures efficiency in the drilling operation, without adding any unnecessary expenses for anticollision purposes.

Close Approach (Cont’)

alert radii The alert radii are defined by an initial radius at

the surface, and a table of user-defined, growth rate information. The growth rate information defines the TVD range for a cone. The cone has an initial radius that is equal to the parameter set at the surface and grows at the user-defined rate, down to the specified depth. The next TVD range starts where the previous range left off and grows according to next level of user-defined specifications. The zone sizes can be set by the user and are generally defined by the operator’s anti-collision policies or rules.