document

Abstract

  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 350∞F 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 295∞F 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 302ºF 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 323∞F 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.

Introduction

  Pre-Khuff horizontal drilling at 13,000-17,000 ft in Saudi Arabia presents a number of major challenges including, hard and abrasive sandstone drilling with unconfined compressive strengths ranging from 3000 to 34,000 psi, AIT’s from 25 to 63∞ and high bottom hole temperatures in the 250 to 350ºF range, as formally addressed in recent publications on the subject 1, 2. The specific problems directly related to required drilling fluid properties are:

• Differential sticking i.e., stuck pipe due to large overbalances (820-1600 psi) sometimes required to stabilize sloughing shales interbedding the main gas reservoirs of the ‘Unayzah A, B, C and Jauf formations.

• Downhole fluid and barite losses near the end of the horizontal section and during the completion phase when the KCl/polymer drilling fluids lose their suspension properties at high bottom hole temperatures (in excess of 240∞F) due to fluid loss and viscosifying polymer degradation.