Applied Rock Mechanics in Drilling of Depleted Reservoirs in Deepwater Gulf of Mexico

摘要

This paper describes the method used in one Deepwater Gulf of Mexico field for analyzing rock properties in order to define a stable pressure window to maximize the efficiency of the drilling process. The method combines wellbore stability modeling, formation evaluation, log-derived and laboratory-derived rock properties, well site pressure integrity testing, geophysical data, and acoustic log analysis. As a complement to a carefully structured drilling program, this synergistic approach resulted in extended reach wells that were drilled with no wellbore-related down time. The process begins with a definition of the pore pressure cells, both in magnitude and position, generally delineated by studying the seismic profile along the projected wellbore and analyzing acoustic log and pressure testing data. The pore pressure data, earth stress field information, and rock properties are used to model the minimum wellbore pressure for stable drilling. A study of the changing seafloor profile along the well path is used to adjust the overburden stress, which is then combined with rock properties to estimate the in-situ stresses, resulting in an estimate of the fracture gradient, or the maximum allowable wellbore pressure to avoid drilling fluid losses. In our use, the term "fracture gradient" represents the minimum horizontal stress at a given depth. Modeling tools are discussed along with a commentary on the validity of log-derived mechanical properties. The approach is applied to drilling operations on two extended reach wells and the paper documents the entire design and analysis process.