Dynamic BHA Modeling of Hole Enlargement While Drilling Leads to ROP Improvement in Gulf of Mexico
Authors: Barrett Myles , Compton Molly T. , Agnihotri Mukul , Verano Fernando , Mitchell Simon , Fitzmorris David Lawrence 
Published: July 2010

Abstract
Salt is typically underreamed in the deepwater Gulf of Mexico wells to manage equivalent circulating density (ECD) and creep. Rapidly drilling the long salt sections is an opportunity to significantly reduce the overall drilling time and cost of deepwater wells. Rapid penetration rates while under-reaming through salt have been achieved by running near maximum torque with high weight on bit (WOB) and rotation speeds (RPM). However, the consequence of these parameters is high vibration and an increased potential for downhole tool failure. The operator's well was located in over 3000 feet of water in Mississippi Canyon and the well plan included a hole-enlargement-while-drilling (HEWD) interval opening a 12-1/4?? pilot hole to 14-3/4?? from 13,000ft MD to the base of the salt at 21,222ft MD while building angle at 1.5°/100ft, holding tangent for 4500ft and dropping back to vertical.
The formation was evaluated using sonic and gamma ray offset data to confirm the interval was homogeneous salt. To optimize the penetration rate while maintaining low vibration, the service provider utilized proprietary technologies to assess the drill bit, BHA and drilling parameters using laboratory data derived from physical testing of salt formations. The BHA was assessed using finite element simulation software to model the behavior of the drillstring and BHA, including the bit and reamer as a drilling system in the geometry of the wellbore. An optimization process was followed to determine sequentially the allowable surface torque, fastest penetrating drill bit, most stable BHA and finally optimized parameters. The process involves running hundreds of iterations using different BHA configurations and parameter combinations in a virtual environment. The parameters were checked for optimization at specific measured depths representing build, tangent and drop sections of the interval.
In the simulated BHA, the recommended weight on bit was limited to maintain the neutral point below the heavy weight drill pipe (HWDP) and the torque was restricted to a factor of safety below the make-up torque of the drill pipe. Two candidate PDC bits were modeled for ROP, torsional oscillation and surface torque. Four BHA combinations were simulated using different stabilizer configurations. The optimal BHA was selected based on evaluation of lateral bit/reamer vibrations, stick-slip and torsional oscillation. The dynamic parameter analysis used permissible bit and reamer vibration levels, torsional oscillations and surface torque to determine the maximum recommended parameters of 30 klbs WOB and 150 RPM in salt. The parameter analysis was conducted for the vertical section which encountered transitions of sub-salt and parameters of 20klbf WOB & 125 RPM were recommended
The operator ran the recommended BHA and documented an increase of 24% in ROP compared to the vertical offset well. Excluding the directional portions, the increase in penetration rate was 43%. This resulted in 13.4 hrs of reduced drilling time saving the operator $558,000USD.

Introduction
Sub-salt exploration in the Gulf of Mexico (GoM) routinely requires drilling through salt sections in excess of 6000ft. As these salt-sections account for a significant portion of the total drilled depth, it is necessary to drill them efficiently. Due to the higher fracture gradient of salt, it is possible to drill these long sections without intermediate casings. However, the length of these sections coupled with the tendency of salt to creep can introduce problems with tight spots while running casing.