The Effect of Survey Station Interval on Wellbore Position Accuracy

Abstract

Abstract Operational reasons exist for surveying with measurement-while-drilling (MWD) instruments at station intervals greater than the recommended maximum of 100 ft (30 m). The objective of this study is to quantify the degradation of position accuracy with longer station intervals and to enhance the MWD survey error model by adding a survey-station, length-dependent term in a standard format. The source data for this study include 150 continuous gyroscope surveys run in drillpipe in the open hole. The surveys are calculated to determine the change in bottomhole location from the original complete survey to the same survey calculated with the regular stations removed to simulate longer survey intervals. The resulting displacements are normalized by survey length, analysed statistically, and correlated to various factors, including interval length, hole angle, and overall angle change between stations. The results of the study show a strong correlation between the length-dependent error and the angle change between survey stations. A minimum angle change per length was observed on each of the surveys, which is presented as a minimum statistical tortuosity. Therefore, vertical or inclined sections of wells that are planned or surveyed with little angle change still have a length-dependent error. The results of the study agree with perceived knowledge that highly curved sections of wells improve their accuracy through extra surveying, and vertical or straight sections do not degrade significantly when surveying with longer intervals. This study provides a simple formula for calculating the additional survey error in terms of units per 1,000 units of drilled depth. The formula is inserted into the MWD error model, sensitizing it to the length between survey stations. The study also contains an analysis of misalignment error at low angles, as it contributes to the interval-dependent error. Survey comparisons in near-vertical wells indicate that over short sections, the misalignment error causes a larger lateral error than is currently estimated using the MWD model, which has an impact on the collision-avoidance separation near the surface for platform wells. It also indicates that for longer survey intervals (>1,500 ft), the misalignment is actually smaller than what is estimated using the current MWD error model. If this revised misalignment error is adopted, the wellheads will need less separation at the surface or less effort to nudge them apart to maintain minimum separation rules on land pads. The quantification of the survey-position error, which is dependent on the survey interval length, has not been previously presented and provides valuable information for well planning and accounts for collision-avoidance distances compared to wells that were previously drilled with surveys at infrequent intervals.