Monitoring the real-time down hole equivalent circulating densities (ECD) of parasite aeration can be done quite easily and accurately with a simple spreadsheet calculation. The method is described and field examples are presented comparing the results of this using procedure with pressure while drilling (PWD) data. Situations where using this method may not be accurate will also be discussed.
Parasite aeration involves running a small diameter string of tubing down the outside of the casing string and injecting air to aerate the mud column in the casing to drill pipe annulus to reduce bottom hole pressure. Parasitic aeration is a highly effective method for underbalanced drilling operations which has many advantages over down the drill pipe aeration. These include a lower air injection pressure, independent control of the mud and air injection rates, aeration can continue during connections, different dual gradient pressure profile more like the pore pressure profile for high altitude locations, lower pressure surges after trips or connections, and the unique ability to monitor in real-time the down hole pressure with a simple spreadsheet calculation.
Aerated drilling involves injecting both a liquid and a gas into the wellbore as circulating fluids to reduce the pressure being exerted on subsurface formations. Aerated drilling is used primarily to combat lost circulation1–7, to reduce formation damage8–10 and for underbalanced drilling (UBD) situations where an ECD in the 4 to 11 lbm/gal equivalent range is needed.
Water, brine, water-based muds, refined and produced oils, and oil-based emulsion muds have all been used as liquids for aerated (or gasified) drilling operations and compressed air, cryogenic nitrogen, membrane generated nitrogen, and natural gas have all been used as the gas. There are several different methods used for aerated drilling such that the liquid and gas are injected into the wellbore at different locations using different equipment. All forms of aeration have a non-linear pressure profile for the aerated interval with the majority of the ECD reduction occurring the last few thousand feet below the surface11, 22.
The three common methods of aerated drilling in use today are;
drill pipe injection at the standpipe,
annular injection with a parasite tubing string, and
annular injection with a temporary concentric casing string.
Both annular injections methods generate a dual-gradient pressure profile which more closely matches the pore pressure profile encountered in mountainous areas where the surface location is at an altitude significantly higher than the surrounding basin11 or in deepwater applications.
Drill pipe injection at the standpipe is the most common aeration method used and involves injecting the compressed gas directly into the high pressure liquid flow at the standpipe such that two-phase flow occurs throughout the wellbore. Drill pipe injection at the standpipe is also referred to as direct circulation. Aeration using drill pipe injection produces an annular pressure profile which is nonlinear over the entire wellbore, especially the last few thousand feet below the surface11, 22.