Formation pressure and sampling measurements in low mobility formations under dynamic filtration can lead to measurements influenced by continuous mud circulation. Generally, active mud circulation inhibits mud cake growth, promoting filtration and invasion of mud filtrate into the reservoir. The resulting invasion adds its own pressure to the actual formation pressure. This is more pronounced in low mobility formations where pressure or sampling measurements made with mud circulation show higher than expected reservoir pressures and/or extended clean up times as a result of dynamic filtration and invasion.
We focus on formation pressure acquisition and present data sets where pressure acquisition was done with active mud circulation. The data is then compared with measurements acquired in a pseudo-static and static mud column.
The measured near wellbore formation pressures acquired with active mud filtration are significantly higher (in some cases, > 400psi) compared to those obtained with a static mud column (assumed to be reading closer to the true formation pressure). The additional pressure is often referred to as supercharging, i.e., the excess pressure superimposed on the original formation pressure by the viscous flow of mud filtrate. The difference depends amongst other factors primarily on the formation mobility and surface pump flow rate during the pressure acquisition. For higher mobilities, there is generally little appreciable difference between active mud circulation and zero mud circulation. Secondary factors like pipe movement, pipe diameter, mud composition and reservoir wettability also influence the degree of the extra pressure measured.
Best practices for formation testing while drilling in low mobility carbonates are discussed. Lessons are drawn from experience where ignoring such best practices result in questionable data.