Abstract

One of the most challenging drilling applications in Vietnam is granitic basement drilling, where the formation is very hard (35-40Kpsi UCS), not drillable with polycrystalline diamond compact (PDC) cutters, and high vibration levels are experienced. There are many common issues and risks, but despite the challenges, Thang Long Joint Operating Company planned to drill a 3217m 3D horizontal granite section hitting all three targets: building inclination from 20 degrees to 85.6 degrees, holding the lateral, and turning from 62°to 14.3°azimuth, total depth (TD) at 6280m measured depth (MD).

A scientific approach using torque and drag modelling, relevant offset data, and an axial oscillation tool (AOT) to prove drilling to TD is achievable within the acceptable cost and time frame was proposed. A torque and drag model was built from the longest extended reach drilling (ERD) basement section drilled in 2014 (Well X, a 2D well, TD at 7300m MD, 3035m basement section) to understand drilling conditions and limitations. The results were applied in the torque and drag model of the subject well (Well Y). Bent motor bottom hole assemblies (BHAs) were then designed and optimized to minimize string buckling. The modelling was run again, and axial oscillation tools were placed in appropriate zones of each BHA.

The first eight BHAs from the beginning of the granite section at 3063m MD to 5071m MD (completed the horizontal turn at 4313m MD, then held tangent) performed well and stayed close to the planned trajectory. The torque and drag model was continuously updated after each run to predict and prepare the BHA for the next run. From 5071m MD to TD at 6280m MD, optimized bent motor BHAs in combination with two axial oscillation tools were utilized to achieve the directional drilling plan and hit all targets. Two rotary steerable system (RSS) BHAs were used to test an alternative steering solution in granite basement. Both BHAs were unsuccessful in holding the well angle, and repeatedly failed to correct the well path.

The use of axial oscillation technology provides a solution to drilling the granitic basement in Vietnam, which has proven to save operators time and reduce costs, as well as facilitate a more complex and longer basement section to connect multiple pay zones to be planned in the future.

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