Enhancing Performance and Operational Safety of Argentinean Vaca Muerta Shale Wells by Drilling High Pressure Formations Using Managed Pressure Drilling

摘要

An unconventional shale gas drilling project in the Rincon de la Ceniza onshore field is being developed to reach the Vaca Muerta formation, which is overlaid by the shallower and troublesome Quintuco formation. This formation is composed of two over-pressurized layers located in the middle and near the bottom of the formation. The Lower Quintuco formation may also be naturally fractured, containing oil and gas, trapped in an over-pressurized regime, creating an operational drilling hazard. Pore pressure prediction has a high degree of uncertainty, making the proper mud density selection a difficult task. Offset wells drilled conventionally have incurred significant Non Productive Time (NPT) due to well control events, where accurate kill mud weight definition has been challenging due to the presence of natural fractures. The objective for the drilling campaign was to define an optimal and reliable drilling method that would address the challenges of pore pressure uncertainty in a safe and cost-effective way. The first step was to implement Managed Pressure Drilling (MPD) to control over-pressurized zones and determine the more suitable mud density to control the well. The previous vertical wells were drilled conventionally having Quintuco and Vaca Muerta formations isolated from each other with two different casing strings. The drilling strategy was redefined and implemented in three horizontal wells, but in this time both formations were drilled together in one hole section. The intention was to enable drilling with the minimum mud weight possible, allowing higher Rate of Penetration (ROP) consequently. This was possible to achieve by maintaining constant bottom hole pressure conditions at variable Anchor Point depths along the open hole while drilling Quintuco formation. Before drilling the horizontal section in Vaca Muerta formation, a small and controlled constant underbalanced condition was created and fixed at the deepest Anchor Point depth which was as close as possible to the transition zone between both formations. This successful strategy allowed improving drilling performance, by reducing drilling time from 123.4 to 51.7 days per well and reducing NPT from 30.5% to 7.5% by managing well control events, eliminating one casing string and improving drilling efficiency to increase the ROP. Consequently, the cost of the well was reduced 5.5 MM USD/well when compared with the last well of the campaign without any Health Safety and Environment (HSE) issues. This paper presents the drilling strategy followed to overcome well control events and associated NPT when drilling an unconventional shale gas play with highly uncertain pore pressure profile. Planning, implementation and results from the project are presented.