The Kuqa Fold-Thrust Belt in Tarim Basin is verified as the most challenging geological structure for ultra-deep hydrocarbon development in China onshore. Its average drilling cycle is larger than 450 days with an average well depth of about 21000ft, which is far unable to meet the E&P requirements of oil company, due to its unique drilling challenges caused by the high-steep structure (maximum dip angle 80°) and the thick gravel layer (maximum thickness 17933ft, maximum buried depth 26074ft) with high abrasiveness in upper formations, the complicated thick gypsum salt layer and interbedded unpredictable abnormal high pressure zone in middle formations, as well as the poor drillability and high crustal stress along the lower small-geometry borehole.
These drilling bottlenecks have promoted years of efforts for technical innovations, including: 1) Automatic vertical drilling technology for upper high-steep layer; 2) Gas drilling technology for suprasalt abrasive gravel layer; 3) A unconventional TaBiao series of casing program to isolate multilayer salt and to guarantee subsequent stimulation operations; 4) Precise determination of salt bottom to prevent losing isolation of salt layer; 5) High temperature resistant and high density oil-base drilling fluid for safe drilling through salt and hydrocarbon-bearing formations; 6) Enhanced PDC bit and downhole motors for ROP improvement; 7) Other cutting-edge technologies.
These drilling technologies have been integrated and extensively applied in the Dabei-Keshen block with a tremendous success. The ultra-deep well drilling cycle is sharply shortened by half with highly improved drilling speed and dramatically reduced downhole accidents and complexities. The drilling breakthrough delivers strong assurance for more safe and cost-effective production in ultra-deep formations in Tarim.
This paper will highlight the ultra-deep well drilling challenges, matching drilling technologies respectively for suprasalt layer, salt stratum, and subsalt target layer, as well as the application results of technical integration.