Interbedding of hard shale and sandstone sequences across the Lower Permian ‘Unayzah and Jauf formations of Saudi Arabia which usually occur between 12,900 to 14,600 ft tvd, present unique drilling fluid density requirements such that 90-102 pcf (12.0-13.6 ppg) drilling fluids are often required to mechanically stabilize the shales while only 66-89 pcf (8.8-11.9 ppg) is needed to balance the ‘Unayzah and Jauf reservoir pressures.

Review of offset wells drilled with similarly high overbalances indicated that the differential sticking forces effected drilling 5-7/8" horizontal hole with conventional KCl-polymer water based drilling fluids or halide brines weighted up with calcium carbonate can easily exceed the allowable overpull on 5" drillpipe if as little as 200 ft of high permeability virgin reservoir is traversed and drilling fluid HTHP filtrate, HTHP filter cake; bridging material particle size distribution and lubricity are not properly maintained; resulting in differentially stuck pipe, expensive fishing and or sidetracking operations.

The requirement to drill 1500 to 5000 ft horizontal sections across the pre-Khuff with 250 to 3508F BHST's which severely degrade the aforementioned fluids properties; catalyzed Saudi Aramco's search for an alternative drilling fluid which was also non-damaging to the sensitive pre-Khuff sandstone reservoirs.

Formate brine drilling & completion fluids appeared to meet all of the above criteria and were therefore selected for field trials.

This paper describes the use of a 90 pcf (12.0 ppg) sodium/potassium (Na/K) formate brine to drill a 1556 ft build to horizontal section across the ‘Unayzah-A formation at 13,900 ft tvd, with 820 psi overbalance and 2958F BHST in TINT-3, a 92 pcf (12.3 ppg) potassium (K) formate brine to drill a 1761 ft horizontal section across the Jauf formation at 14,500 ft tvd with 302°F BHST in HWYH-201 and the use of a 102-105 pcf (13.6-14.0 ppg) Na/K-formate system with calcium carbonate used as a bridging and supplementary weighting agent to drill a 1646' horizontal section across the ‘Unayzah-B formation at 13,900 ft tvd, with maximum overbalance of 1600 psi and BHST of 3238F in HRDH-658.

One of the wells exhibited greatly improved drill string/wellbore lubricity, bit performance, reduced torque and drag, ECD's and pump pressures whilst inadequate drilling fluid density masked the full benefits of formate brine use on the other two. However, differential sticking did not occur in any of the wells.

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