Rotary Application of Low Matrix Strength Impregnated and TSP Cutter Bits to Unayzah Sandstone Drilling
Authors: Simpson M. A. , Roed A. H. , Al-Shammari H. A. , Hoekstra Dirk 
Published: December 2007

The Unayzah is a Lower Permian, hard and abrasive sandstone formation, interbedded with shale layers encountered at about 13,000 ft throughout most of Saudi Arabia. The Unayzah reservoirs hold substantial sweet gas and condensate reserves, which serve as important feedstock for Saudi Arabia’s gas plants. The Unayzah’s unconfined compressive strength (UCS), which typically ranges from 10,000 to 43,000 psi and internal angle of friction, from 25º to 75°, make it a formidable drilling challenge. A hard and abrasive section which has UCS from 25,000 to 30,000 psi with internal angle of friction from 55 to 75º, encountered from 200-400 ft into the Unayzah is extremely hard to drill. ROP’s in this section are commonly limited to 0.5 to 1.5 fph. Up to 21 TCI, PDC and TSP bits have been required to drill the longer Unayzah sections, which can be up to 1300 ft. in thickness.

Numerous bearing failures, undergauge bits and hole, lost cones and required fishing operations have occurred with the IADC Code 537-837 TCI bits which have been used.

Downhole temperatures across the Unayzah of 260º F have impeded the use of high speed PDM’s due to softening and expansion of conventional stator elastomers and the resultant loss of downhole efficiency, chunking or motor failure. The recent availability of high temperature stators, good to 320º F and special clearance rotor/stator assemblies has more less alleviated this problem and motor runs through the Pre-Khuff are again being made.

Attempts to apply high-speed turbine/impregnated bit drilling techniques, successful across the Unayzah in neighboring Bahrain and Oman, have met with a recurring bit sticking problem due to the high in-situ stresses across the Unayzah in Saudi Arabia. The high in-situ stresses are envisaged as initiating breakout with an associated reduction in hole size; 0.0476 in. in the 5-7/8 in. hole and 0.0684 in. in the 8-3/8 in. hole; immediately after the hole is drilled with Turbine/Fixed Cutter bit assemblies.

Expected value footage cost analysis of 1241 5-7/8" and 8-3/8 in. Unayzah bit runs indicated that diamond enhanced TCI bits run on rotary would offer the lowest cost; however, TSP and Impreg bits run on rotary were almost as cost efficient and harbored less inherent risk.

Numerous trial tests were made with 5-7/8" and 8-3/8" impregnated and TSP cutter bits on packed rotary assemblies. The most successful runs showed the average drilling cost across the Unayzah could be reduced significantly when fixed cutter bits were run on rotary under their optimal set of operating conditions, viz., hydraulics, bit weight and rotary speed. For the 5-7/8" impregnated bits, this was 160 rpm with 10,000 lbf weight on bit (WOB). For the 5-7/8" surface set TSP cutter bits, similar performance was achieved at lower rotary speeds, 100-120 rpm and higher WOB’s, in the 12,000 to 18,000 lbf range. Ongoing modifications to the size, shape and cutter layout of the TSP bits and matrix material hardness, and diamond concentration/distribution for the Impregs have been made and tested.