Drilling the 6-5/8" vertical sections in the Tarim oilfield, in China's western Xinjiang Uygur Autonomous Region is challenging. The sections are a very hard and abrasive formation, typically a 300m interval starting at a depth of 7000m. The Cretaceous Bashijiqike formation consists of layers of quartzitic fine sandstone (35% quartz) and silificatedclaystone with conglomeratic sandstone stringers both at the top and bottom of the Section. Low rate of penetration (ROP) and short bit runs are typical for drilling this formation. With bottom hole temperatures at 148°C and mud density around 1.9sg, the application of roller cone bits and downhole tools with moving parts is limited.

Historically the section has been drilled with a simple rotary BHA consisting of a bit, 468m of 5-inch spiral drilling collar, 2565m of 4-inch drill pipe and 5-inch drill pipe to the surface. Bits used have been a variety of locally sourced 6 bladed matrix PDC bits with cutters from 8mm to 16mm. the average interval per bit was 30m at 0.47m/hr.

The increase in PDC bit performance was very critical to the overall improvement of drilling efficiency and economics in this application. A target of 50m interval at 0.5m/hr was set for the development of new PDC drill bits.

The new 6 bladed bits, using 16mm and 13mm cutters respectively, exceeded the objectives. This success is a combination of in depth analysis of the application and past performance, use of new high abrasion and impact resistant cutters and optimized cutting structure design. In the first well the new bits drilled 206% further and 62.7% faster than competitor bits used at shallower depth in the same well. On the second well, the new bits outperformed competitors by 159% on interval drilled and 38% on ROP.

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