Located in offshore South China Sea, Ledong high-pressure/high-temperature (HPHT) gas field has entered the appraisal phase after the first discovery was announced in 2015. The pressure gradient of the main target zone is close to 2.2 g/cm3 whereas the top formation is approximately 1.8 g/cm3; the sand packages are separated by a variable shale layer thickness. To avoid kicks, mud losses, or other drilling problems, mud weight must be adjusted accordingly to preserve the well integrity. Hence, the main objective for this hole section is to stop drilling above the main target sand and set casing to isolate the formation with different pressure gradient.
An innovative look-ahead technology based on deep electromagnetic measurements was used to predict the formation change ahead of bit in real time to reduce the drilling risk. After review of the technical and geological challenges encountered in this field, this paper will discuss the successful approach taken to detect formation changes using the new technique. After the shale layer above the target sand has been identified in real-time, drilling will stop above the high-pressured sand to set casing. In addition, the authors will also describe the bottom hole assembly (BHA) configuration and measurement selection in the planning phase to ensure the success of this well.
The real-time interpretation of look-ahead measurements enables boundary detection ahead of the bit at distances ranging from 3 to 20 m in this example. The depth of detection depends on the resistivity contrast between formation, layer thickness, presence of laminations, and the transmitter-to-receiver distance. The application of the innovative look-ahead technology has helped to
Reduce the drilling risk by detecting formation change ahead of the bit
Accurately identify casing shoe position to ensure well integrity
Eliminate extra casing string, which will directly increase the well construction cost
Avoid unnecessary operation adjustments and improve drilling efficiency
Clear prediction of the resistivity profile ahead of the bit enables proactive decision making while drilling. This additional information has removed the need to consider the possibilities for different scenarios and the extra circulating time taken to make decisions among stake holders.
The successful implementation of the look-ahead technology and the application in the HPHT well has led to reduction in overall nonproductive time by reducing drilling risk and improving drilling efficiency. This innovative technique changes the real-time decision-making process while delivering a new way to manage drilling risk.
