The deepwater Gulf of Mexico (GoM) Lower Tertiary Wilcox group is still considered an area with many geological uncertainties. In this geological environment, core samples are crucial for geologists, geoscientists, and reservoir engineers to better understand and characterize the reservoir.
This paper will focus on the longest 12 ¼-in. coring application in a deepwater Gulf of Mexico lower tertiary well. A total of 360 ft of lower tertiary Wilcox sand was cored and recovered in a single run. 12 ¼-in. core barrels were assembled for this application along with a fit-for-purpose 12 ¼-in. core bit equipped with abrasion-resistant polycrystalline diamond compact (PDC) cutters and an integral stabilization sleeve for better bit and bottomhole assembly (BHA) stabilization throughout the operation. Extensive preplanning was done in order to ensure flawless execution in this challenging environment.
In deepwater coring operations, maintaining weight on bit (WOB) is crucial for good results. Lack of rig compensation and heavy rig heave can be a disastrous combination. Bit bounce associated with high rig heave and core barrel jamming are some of the hazards of having insufficient weight reaching the core bit, which leads to increased nonproductive time (NPT). In order to have better control of the downhole drilling parameters, such as downhole weight on bit, torque and vibration, a comprehensive drilling mechanics and dynamics measurement tool was placed on top of the core barrel. The benefit of implementing this tool is the ability to optimize the coring parameters in real time in order to maximize core recovery.
Additionally, the section was drilled using a constant rheology drilling fluid, and hydraulics simulations were performed to guarantee equivalent circulating density (ECD) management, optimized hole cleaning and reduced operational risks. Geomechanical modeling included fracture characterization analysis to select the proper particle size distribution for bridging and wellbore strengthening formulations.
The planning and collaboration between the service company and the operator, in a multidisciplinary team, was the key for the accomplishment of this record run. The record run eliminated two extra coring runs (the standard coring application in the Gulf of Mexico is 120 ft), accounting for significant reduction in operational cost.