Bitumen formations have posed significant challenges to deepwater Gulf of Mexico drilling operations. Many operators such as ConocoPhilips (Spa Prospect), Chevron (Big Foot), BP (Mad Dog) have reported bitumen encounters and associated significant costs. The current consensus is to avoid bitumen formations as much as possible. However, fundamental questions still remain unclear and controversial. For example, what bitumen behaves at in-situ conditions (i.e. high stress and high temperature), what shape is bitumen formation, what mechanisms drive bitumen into wellbore.
This paper highlights a part of comprehensive efforts to drill through the bitumen encountered in a deepwater Gulf of Mexico field. A series of lab tests have been done to investigate the effect of temperature, pressure, and drilling fluids on bitumen mechanical behaviors. A material model for in-situ bitumen is therefore derived and applied in detailed 3D numerical analyses. At reservoir scale, the numerical models analyze the stress and deformation inside and around the bitumen formations with different lateral extensions and thickness, and evaluate the stability of various bitumen shapes at in-situ conditions. At wellbore scale, 3D models are used to study the factors of bitumen mobilization, including overburden stress, bitumen stress, and borehole pressure. The relative importance of each factor has been quantified.
One of the key findings is the role of overburden in bitumen mobilization. We find that, for the bitumen encountered adjacent to the salt body, the radial stress gradient (i.e. horizontal if well is vertical) is dominant to flow the bitumen at the beginning after borehole introduction. The overburden effect becomes evident later. Further, both mechanisms result in stress concentrations within short distance around the wellbore. Another finding contradictory to previous publications is the mud pressure required to stop bitumen movement. The simulations indicate for the bitumen studied in this paper as long as the mud pressure at the bitumen formation reaches the level of in-situ minimum horizontal stress (i.e. fracture gradient instead of overburden gradient), the bitumen stops moving. However, even 0.25ppg mud pressure fluctuation may trigger the mobilization again.
These studies improve the understandings of in-situ bitumen and near-wellbore bitumen flow conditions, and may help to develop a successful strategy to drill through.
Bitumen, Tar, and Asphalt
Before discussion, it is necessary to clarify and differentiate the following three carbon composites: tar, bitumen and asphalt. Based on Webster's Dictionary (1995), bitumen is "any of various mixtures of hydrocarbons … often together with their nonmetallic derivatives that occur naturally or are obtained as residues after heat-refining natural substances …", while tar is "a dark brown or black bituminous usually odorous viscous liquid obtained by destructive distillation of organic material". Krishnan and Rajagopal (2003) define asphalt as "a limestone that contains bituminous matter in a sufficient proportion, usually 7-12%, to become plastic when heated, and to cement the powdered limestone firmly when it sets on cooling."
Interestingly but clearly, we should assign the term "bitumen" instead of "tar" to the material encountered in deepwater GOM as it is not a residue after refining. While most previous publications have referred bitumens to tars, we will use exclusively the term "bitumen" in this paper.
Introduction
The challenges associated with drilling into bitumen formations are not new to the operators in deepwater Gulf of Mexico (GOM). Many field developments, such as ConocoPhilips' Spa Prosepct at Walker Ridge (Rohleder et al., 2003), BP's Mad Dog field at Green Canyon (Romo et al., 2007), and Chevron's Bit Foot at Walker Ridge (Weatherl, 2007), have been affected. The problems encountered include spike in torque, increase of mud pressure, adhesion to BHA, closure of hole, etc.