Drilling Through Bitumen in Gulf of Mexico: The Shallower vs the Deeper
Authors: Han Gang , Hunter Keith C. , Osmond Jerry , Ressler Jamie Francis , Zambonini Michael John 
Published: October 2008

Abstract
Bitumen formations in deepwater Gulf of Mexico have posed significant challenges to operators such as Hess (Pony), BP (Mad Dog), ConocoPhilips (Spa Prospect), and Chevron (Big Foot). In many cases, bitumen encounters have resulted in significant delay in drilling schedule, often sidetrack or even well abandonment. In other cases, however, bitumen formations could be drilled through without sidetrack or redrill. This brings an interesting question: why some have been successful while the others have failed. Understandings of bitumen behaviors under in-situ stress and temperature conditions, bitumen formation geometry, and driving mechanisms to push bitumen into wellbore may help to reveal the myth.
This paper is a continuation of previous efforts to understand the bitumens encountered in a deepwater Gulf of Mexico field. Based on a series of lab tests that have identified the effect of temperature, pressure, and drilling fluids on bitumen mechanical behaviors, a material model is derived for the in-situ bitumen and applied in detailed 3D numerical analyses. At reservoir scale, with different bitumen geometries and in-situ stresses as inputs, the numerical models analyze the stress and deformation inside and around the bitumen formations. Around wellbore, another 3D model is assembled to study the factors of bitumen mobilization, including overburden stress, bitumen pressure, and Effective Circulating Density (ECD). The drillability of bitumen is then evaluated.
The simulations indicate there is clear distinction between the shallower bitumen adjacent to salt formation and the deeper one. The shallower bitumen is more likely to be pan-cake or lake shape spreading primarily horizontally; the deeper one has to be close to vertical to survive in-situ stresses. For lake-shape bitumen, the potential to successfully drill through is high and manipulating mud weight may help. However for vertical column bitumen, overburden stress becomes dominant, increasing mud weight may not be efficient, and challenges are more significant. Those findings are consistent with the drilling history reported in the field, and may help to develop a strategy to manage bitumen drilling.

Introduction
Deepwater Gulf of Mexico (GOM) increasingly becomes critical and strategic to supply oil and gas productions for United States. According to US Minerals Management Service (2007), deepwater GOM (water depth is deeper than 1000ft) has produced close to 1 Million of Barrels of Oil per Day (MBOD) in 2006, almost 20% of the entire nation oil production. Potentially, the 192 deepwater discoveries so far are expected to deliver 2.248 Millions of Barrels of Oil per Day (MBOD) by 2011.
While there are many challenges facing deepwater operators, recent bitumen encounter become a highlight because of the significant financial and schedule impacts. In many cases, such as Hess' Pony development (Han, 2008), BP's Mad Dog field (Romo et al., 2007), Chevron's Bit Foot (Weatherl, 2007), and ConocoPhilips' Spa Prospect (Rohleder et al., 2003), wells have to be sidetracked or even redrilled. Fig. 1 shows one of the bitumen samples rig personnel have collected at Hess Pony field, located at Green Canyon block 468/469 with 3500ft water depth. After a BHA drilled into a bitumen formation at 29249ft TVD, torque, and often mud pump pressure as well, increased so dramatically within short period that drilling had to stop. While mud might be able to circulate some bitumen out of the hole, it could not keep up with the continuous supply from the bitumen formation and a cement plug had to insert to avoid bitumen filling up the well. Not all the bitumen encounters, however, have ended up sidetracks. In one of the bitumens encountered at the depth of 22600ft TVD in Pony field, the well has been successfully drilled through with a liner and no further bitumen has shown up in the hole.
These experience leads to an interesting question: why some shallower bitumens are drillable and deeper ones seem more malicious?