Abstract: Directional Surveys in a Digital Age

John Sharry

The accuracy with which we spot well locations and well paths is becoming more and more critical as we attempt to exploit smaller and more precisely defined targets. With 3-D seismic survey resolution approaching 6 meters, the accuracy with which we position wells needs to keep pace. The question: "Has this target been tested?" is still valid. There are several problems with directional survey data that must be dealt with before a definitive answer can be given. Two of the more significant problems; old data calculated with obsolete methods and coordinate system standardization are discussed below.

Before computers became widely available in the mid 1970's straight line methods of computation, such as tangential and average angle were used to compute bottom hole locations and well paths which then were posted on maps and entered into databases. These methods produce results that differ by tens to hundreds of feet from the more accurate radius of curvature and minimum curvature methods used today. Old wells are an important source of data for the redevelopment of old fields. However, before we can use the older surveys we need to recalculate the paths of these wells using modern algorithms.

A second problem is cartographic in nature. Directional surveys can be reported in three coordinate systems: magnetic north, grid north, or true north. There are no standards with regards to the north reference to be used. In the Gulf of Mexico grid north is reported most often, although at least 15 percent are true north. Onshore it is the opposite. Elsewhere around the world true north is more common. In the High Island Area the difference between the grid north and true north location of a well with a 5000^prime offset is 230^prime. Can we afford such an error m our work? What coordinates do our workstations require? Some require true north others grid north. Its decision time. Do you know where your wells are?

AAPG Search and Discovery Article #90960©1995 AAPG Southwest Section Meeting, Dallas, Texas