When hydrocarbons are produced from a subsurface reservoir, pore pressure in the reservoir is reduced. This reduction (termed depletion) causes a redistribution of stress in the reservoir and surrounding rocks, leading to a variety of potential issues such as compaction and subsidence, fault reactivation and other forms of strain localization. It generally also leads to a reduction in the minimum horizontal stress (Sh) and therefore to a potential reduction in the fracture gradient (FG) in the depleted interval. The reduction in FG can be one of the most notable challenges when attempting to drill and complete new wells and is the focus of this paper.
The level of depletion at which the reduced FG becomes an operational issue is a function of the geomechanical properties of the reservoir and surrounding rocks. It is also a function the effectiveness of so called “wellbore strengthening” techniques which aim to locally increase the FG by the addition of an appropriately sized concentration of lost circulation material (LCM). Within BP, reservoirs with depleted sand drilling challenges range from just a few hundred psi of depletion (for example in deep water Angola fields with shallow reservoirs) to several thousand psi (in deeper reservoirs as in the Gulf of Mexico) and in some cases to 10,000 psi or more of depletion in certain High Pressure/High Temperature settings.
Quantifying the FG of the depleted interval and the effectiveness of wellbore strengthening techniques is key to field development planning and safe drilling operations. For a safe and successful development, several issues need to be considered. These include the variation of original (prior to depletion) FG across a field (where experience has shown that traditional 1-D models are inadequate for the complex structures drilled today), the change of FG with depletion (is the “Stress Path” linear and uniform across the field, or is structure and potential stress arching important?), and by how much we can increase the near-wellbore FG using wellbore strengthening techniques such as StressCage.
In this paper, we present an integrated workflow undertaken to mitigate the risks of drilling depleted sands. Field examples are used to demonstrate the utilization of multiple sources of information including drilling data, rock mechanics, and stress analysis, along with the size distribution and concentration of particulate materials in the mud to achieve an optimum solution.