Breakthrough Horizontal Drilling Performance in Pre-Khuff Strata with Steerable Turbines
Authors: Simpson Michael , Zhou Shaohua , Treece Mark , Rondon Carlos , Emmons Thomas , Martin Craig , Tahir Mohammed 
Published: January 2008

Since 1997 Saudi Aramco has attempted drilling its hard and ultra-abrasive Pre-Khuff formations typically encountered from 13,000 to 17,000 ft TVD in and about the Ghawar field of Saudi Arabia, with impregnated diamond bits run on high speed turbines, more commonly referred to as "turbodrills".

The turbodrill/impregnated diamond bit drilling assemblies were envisaged as a viable alternative to using impregnated, PDC, TSP and TCI bits on PDM's and rotary drives. The PDC bits lacked the wear resistance required to drill the hard and abrasive sections of the Pre-Khuff economically due to limited bit life and the TCI bits harbored a bleak history of excessive gauge wear and cone loss.

Three turbodrill/impregnated diamond bit runs were made in Pre-Khuff vertical wells from 1997 to 1998 all resulting in stuck pipe due to "bit sticking", where initially only the bit was stuck, as there was free rotation of the drillstring and turbodrill from the surface. Differential sticking and/or pack-off may have subsequently contributed to the pipe ultimately not being freed in each case.

After considerable review, redesign of the 8-3/8" impregnated diamond bits, shortening of the turbodrill body and BHA with the use of a box-up connection impregnated diamond bit, a respectable run of 242 ft averaging 10.3 fph was made in HWYH-961, a vertical Pre-Khuff well across the ‘Unayzah and Jauf formations with cost of $443/ft.

With this success and enhanced understanding of the results of a recent consultant firm's geomechanical analysis of the area that indicated the maximum horizontal stress axis was East-West, Saudi Aramco felt they had a good chance of successfully utilizing turbodrill/impregnated diamond bit drilling in the Pre-Khuff without getting the bit stuck if the well was drilled parallel to the field's maximum horizontal stress axis.

Finally, in 2003 the opportunity arose to test the wellbore stability hypothesis utilizing a turbodrill/impregnated diamond bit drilling assembly in well WDHY-1, a ‘Unayzah horizontal sidetrack. Two consecutive footage record breaking 5-7/8" impregnated diamond bit runs of 737 and 852 ft were made on 4-3/4" steerable turbodrills in the horizontal section of the ‘Unayzah-B siltstone and sandstone.

In 2004, four additional 4-3/4" steerable turbodrill/impregnated diamond bit runs were made in the ‘Unayzah-A and ‘Unayzah-B formations with mixed results. Three were made in HRDH-658 across an ultra hard streak at 89° in the ‘Unayzah-B, drilling with over 1500 psi overbalance to the formation with poor results contrasted by a record breaking ROP run of 7.2 fph over 360 ft on the horizontal sidetrack well TINT-3 across the ‘Unayzah-A.

This paper chronicles the development and refinement of turbodrilling practices and techniques across Pre-Khuff Strata in and around the Ghawar field of Saudi Arabia. It highlights the underlying rock mechanics principles, which allow successful application of high speed turbodrilling with impregnated diamond bits to horizontal wellbores but not in the vertical wellbores in the same field, or more specifically in the case of TINT-3, a sidetrack of the same well. Explanations are offered concerning the apparent enhanced bit, motor and turbodrill performance observed with the use of 90 pcf sodium/potassium formate brine across the target reservoir.