Drilling Evolution in Changning Shale Gas Block of Central China

Abstract

Abstract Changning shale gas block is located in the southern part of the Sichuan basin. It is one of the first pilot experimental shale gas blocks in China. The paper will elaborate the drilling challenges in the field and the overall drilling performance with conventional approaches. In 2014, new drilling tools and methods were tried and applied. These significantly improved the performance, further elevating the drilling speed to a new level. The paper will review the drilling performance of horizontal wells completed previously in the field. It will focus on demonstrating formation characteristics, well trajectory and geometry design, drill bit and directional bottomhole assembly (BHA). One of the biggest challenges is the hard rock on top of the shale formation that constrains the drilling speed, especially when designing the curve interval at the hard rock. By analyzing the drilling challenges, a new drilling system and method were introduced to the project in 2014 to combat the challenges. It has been proven successfully with actual drilling data. Initially, as a pilot project, conventional motor technology was applied to six horizontal wells in Changning block. The average well drilling cycle was approximately 66 days for 1000 m of lateral section. The main challenge with motor drilling was high drag at the curve and lateral due to extreme friction as high mud weight was applied to balance the formation stability. This resulted in a poor on- bottom penetration rate during sliding mode. Directional work at the hard rock formation posed an additional challenge. In early 2014, a hybrid high-build-rate rotary system was applied to deepen the kickoff point and avoid the steering work in the hard zone. Air drilling was used to drill vertically across the hard rock. A power rotary steering system was also implemented to further improve the drilling speed in the lateral section. The combination of technical approaches led to significant drilling improvement. The rate of penetration (ROP) on bottom at the curve section has improved 200% over conventional drilling. The well drilling cycle was cut in half, which was considered as a breakthrough achievement in the field. With the application of hybrid high-build-rate rotary steering system, the new well design can be executed. Air drilling is chosen to drill through the hard rock in the shortest distance. Using the power rotary steering system for the lateral further promotes the drilling performance to a new level.