Precise Localization of Offset Wells Crossed With Deep Directional Resistivity Measurements

New wells drilled in brown oil and gas fields add to the many existing cased wells or wells with liners that have been placed over the years for production and injection. For each existing well, there is an uncertainty associated with its position and the uncertainty increases with measured depth because the well position can only be derived from the survey points. A new well drilled in brown fields can therefore easily cross the ellipse of uncertainty of old horizontal wells. In addition to the risk of collision, it is then not clear how close the planned new well will cross over the existing wells. Deep directional resistivity (DDR) measurements are sensitive to resistivity variations more than 30 m away from wellbore and strongly react if there is a casing or liner within this range. Hence the measurements can be used to localize the casing or liner. However, a casing within a layered reservoir is a complex 3D problem, consequently 1D interpretation is not suited for localizing a crossed casing and only leads to an erroneous estimation of the reservoir extend and resistivity in the vicinity of a crossed casing. However, the problem can often be reasonably well approximated in 2D in certain geometries. This paper presents how existing 2D imaging inversions are adapted to estimate the position and relative azimuth of a crossed cased well. Using a specialized 2D inversion setup, validation against 3D modeling shows that the location of a casing and its relative angle can be reasonably well determined. The 2D inversion result can then serve as an initial guess for an inversion with a 3D code tuned for accurate modeling of crossed casings which further refines the estimated position of the crossed well. The outlined workflow can also be adapted in the future for the detection of a cased well before it is crossed. The outlined workflow is applied to two wells, both drilled in mature fields, and reveals the position, the distance, and the relative angle of the crossed wells with high confidence.