Abstract

Extended offset horizontal wells (out to a kilometer or more) are becoming common in field developments. In areas where overburden velocity varies laterally, it becomes difficult to know where the well is relative to the top of the reservoir away from the top reservoir penetration point. This is important to know for depth conversion and volumetrics and to know what part of the reservoir is sampled by the well to build a good simulation model for reservoir characterization.

We examine the use of the conventional offset VSP method but it can only image reflectors below the reservoirs in the well making receivers in the horizontal section only good for imaging the base reservoir. The top reservoir over the horizontal section could only be imaged by a very far offset source recorded by receivers in the vertical section giving raypaths with too much moveout, poor signal quality and little control on overburden velocity. Instead, we have applied a checkshot solution to the problem where we can use the seismic time and the checkshot time to estimate borehole location relative to the top reservoir and the overburden velocity for depth conversion.

Examples from onshore North Sumatra illustrate the ideal and practical methods and the accuracy of our estimates. The marine applications are very accurate (± 20 feet in the estimate of well position relative to the top reservoir) due to more homogeneous near surface and the ability to easily locate the source directly over the receiver. The land applications are less accurate but still more accurate than no information (± 60 feet), due to more variable near surface statics, datuming velocities and the inability to easily locate the source over the receiver.