Well instability is a common challenge while drilling through complex formations in Argentina. These complex formations are characterized by a small operating pressure window which typically requires the use of managed pressure drilling (MPD) to enable safe drilling operations. Most of the challenges are related to either loss circulation or gas influx into the wellbore, due to both initial fluid invasion and the interconnection of existing natural micro-fissures and fractures which eventually lead to increasing problems with gas invasion and more well instability. In addition to MPD, another possible solution, commonly implemented by the operator in this area, is the use of an additional casing string to isolate those unstable formations.
The two main formations discussed in this analysis are the Quintuco-Vaca Muerta, which is a naturally fractured/fissured limestone formation, and Sierras Blancas, a depleted sandstone. Vaca Muerta Formation is one of the major gas producing formations in the Neuquén Basin, Argentina. The Quintuco-Vaca Muerta Formation group has been treated in the past using a stress-caging technique, which utilizes the mix of conventional drilling fluid materials such as calcium carbonate, graphite, asphalt, among other bridging materials. Those solutions routinely provided insufficient stable sealing while drilling, thus requiring the need of MPD, a common practice to handle wellbore instability issues in the region.
The root cause leading to this challenging scenario is existing micro-fractures in the Quintuco-Vaca Muerta section. The problem is acerbated by the pressure exerted on the formation by the drilling operation which opens communication between micro-factures facilitating the flow of gas toward the wellbore and leading to gas inflows into the drilling fluid during circulation. The standard operating procedure to stop influxes would be to increase the mud density, however the experience in Loma La Lata Field has shown that increasing the mud density tends to result in additional fluid invasion due to the inefficient seal. And the higher pressure opening leads to additional connections between existing fractures, causing even more gas invasion into drilling fluid and the need for denser fluids to control the instability. The costly cycle of wellbore instability led the operator to look for alternative solutions to be able to drill continuously while minimizing non-productive time.
Irrespective of the advantageous performance provided by the MPD, the continuous search for an improved and cost-effective solution drove the operator to try an alternative sealing technology to minimize well instability in a safe and effective manner. This alternative sealing technology creates a strong shielding layer at the wellbore wall, instead of using higher density treatments which require a complex material loading in the drilling fluid system. This shielding technology creates a strong, thin, mechanical barrier at the wellbore wall, is used in low concentrations, and is also not affected by the shearing effect caused by circulation of the drilling fluid. These Wellbore Shielding Technology properties enable a low equivalent circulation density (ECD) to be maintained while minimizing the fluid invasion due to the strong mechanical properties of the thin barrier, or "shield", at the wellbore wall.
After the first application, the drilling efficiency significantly improved as compared to offset wells, even without using MPD. The operator was able to drill a long 2700-m (11,680-ft) section of open hole. The drilling string was pulled out of hole later reaching total depth (TD) without issues. Two log runs were conducted as planned, and the well was finally cased and cemented without operational issues.