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Review of drilling well path directions relative to local structural features and geology in gas fields throughout The Cooper Basin, S.A.

Kotomski, Robert

Engineering Honours Degree 2009

University of Adelaide


Well trajectories are paramount to the success of a drilling project and should the trajectory deviate too much it is possible that a reservoir target could be missed or wellbore collision could result. The primary objective, therefore, of this study was to review previous gas wellbore drill trajectories in the Big Lake and Tirrawarra fields which will be subject to an infill drilling development strategy. The study ultimately aimed to correlate wellbore deviation to local structural features and geology within the Cooper Basin.

A regional analysis was first performed by plotting tophole and bottomhole well locations throughout the two fields. This analysis showed no correlation between the existence of faults within the fields and a preferred drilling direction. It was also discovered that the location of a well within a field did not impact the magnitude of deviation or the direction of deviation. While Tirrawarra was shown to have a largely random scatter of well paths, Big Lake showed a clear northwest/ south‐east drilling direction.

A more detailed inclination and directional analysis was then carried out on each well within the two fields. A basin‐wide inclination build, and subsequent drop, tendency when drilling out of the surface casing shoe was discovered. This trend was found to be caused by two primary factors. Firstly, the effective dip angle of the formations drilling out of the surface casing shoe is considerably higher than those of deeper formations. This leads to a large formation deviating force and hence a build of angle. The deviating force is then either reduced along the depth of the well, and control drilling can be employed to make the well drop back to vertical, or the effective dip angle becomes negative such that the bit is subjected to a large dropping force and naturally drops back to vertical. Secondly, aggressive drilling with a high weight‐on‐bit was found to exacerbate the build of angle. Further analysis showed that the inclination trend occurred irrespective of the type of drill bit used to drill the production hole and it is also recognised that the type of bottom‐hole‐assembly used to drill the production hole may contribute to the inclination trend.

From a directional perspective, it is possible that the field‐wide north‐west/south‐east preferential drilling direction in Big Lake is caused by the stress orientation within the field. Big Lake is documented to have a maximum horizontal stress orientation in the north‐west/south‐east and it is postulated that a wellbore could exhibit a natural predisposition to drill in this direction due to the formation of drilling‐induced‐tensile‐fractures which may cause a weaker plane for the bit to drill in.

Overall it is presented that the magnitude of deviation in the Cooper Basin is minimal and will not impact the infill drilling development strategy. Nevertheless cost savings can still be made by minimising any degree of deviation by reducing drilling time and footage drilled. It is recommended that more conservative drilling out of the surface casing shoe will minimise the build of angle and eliminate the need for control drilling. An experimental pendulum production hole BHA has also been suggested to quantify the effect of the bottom‐hole‐assembly type in Cooper Basin drilling.

Australian School of Petroleum



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