Using the Properties of Carbon Composite Rod to Decrease the Uncertainty of Along Hole Depth Measurement, Leading to Increased Confidence in Field Wide Geological Models and Well Placement.

Abstract

Depth is probably the most important measurement that can be made in the petroleum industry because it is the reference for all other measurements that may be recorded. The discrepancies between drillpipe depth and wireline depth are well documented and both suffer from limitations. The properties of carbon composite rod, including low stretch coefficient and negligible thermal expansion, when combined with advanced well entry modelling allow very precise determination of absolute depth. Carbon composite rod is an enabling technology for intervening in extended reach wells, and part of the qualification process has been the evolution and verification of the predictive simulation software, which has proven to be very accurate and reliable. Part of the output of the simulation model is the calculation of a tension/compression profile along the entire length of the rod with the BHA at any position along the wellbore, which allows the extremely accurate calculation of the total cumulative stretch at any point along the wellbore, and hence the absolute position of any measurement devices. Carbon composite rod does not behave like drillpipe, wireline or coiled tubing so traditional modelling software cannot be used to simulate well entry. A bespoke mathematical technique was developed to fully describe the behaviour of the rod in a downhole environment and was initially used to predict well entry parameters, assisting in planning. Following each intervention, predictions were compared with actual performance for both surface weight and downhole cable head tension. The software's facility to assign individual friction factors to the rod and individual BHA components, and also to any area of the completion, was used to build extremely detailed well models in order to overlay calculated and measured parameters. With the model built, cumulative stretch with the measurement BHA at any point can be calculated with great accuracy. The results showed that the trajectory of the well bore and any areas of isolated high friction can have a very large effect on stretch and can render traditional stretch corrections liable to extreme errors. Further modelling in a variety of different well trajectories was undertaken to build further understanding of localised stretch effects and showed that even in complex wells stretch may be described by fourth order relationships with depth. The paper will demonstrate that the physical properties of carbon composite rods allow very precise modelling of cumulative stretch along the entire wellbore even in complex trajectories. This raises the prospect of being able to record a reference log of absolute depth which can then be used correct any previous data. Should the reference log be a continuous gyro survey, the absolute position of the well bore in 3 dimensions may be derived.