Advanced Casing and Mud Line Suspension Designs for an UAE Khuff/Pre-Khuff Offshore Exploration Keeper-Well

Abstract

Abstract Khuff and pre-Khuff exploration wells drilled in the United Arab Emirates (UAE) are geological challenging and technically complex. Contributors to the geological challenges are unstable shale formations like Nahr Umr and Sudair, massive limestone formations interbedded with Anhydrites and the shallower reservoirs of the Jurassic likethe Arab. In addition, abnormal pressured and H2S bearing formations are common as well as catastrophic loss prone formations. Extensive formation evaluation (FE) requirements, contingency strings and the wish of temporary suspension of the well for possible future cost savings rather than abandoning it add to the technical complexity of these deep wells. The operatordesigned an exploration well targeting the Upper and Lower Khuff as primary and the deeper and likely over-pressured pre-Khuff formations as secondary well objectives. All possible Khuff reservoir formations should be cored, logged and, if hydrocarbons (HC) being present in upper and lower Khuff, tested. In case of commercially recoverable HC the exploration well should be temporary suspended and turned into a producer in the future. To safely drill the well to TD and to meet above objectives the operator defined certain requirements for the well design. These criteria cover essential casing seats, H2S resistant materials, well barrierenvelopes and implementation of a Mud Line Suspension system (MLS) for a future re-entry. The operator identified a robust casing design being essential to successfully meet well objectives and requirements. A standard design resulting in a 6" reservoir section was considered not being suitable. An open hole in this size would compromise valuable formation evaluation deliverables in this exploration well. In addition, the design does not cater for a contingency string which might be required for likely different pressure regimes within Upper, Lower and pre-Khuff. Possible wellbore stabilityand loss/gain challenges higher-up in the wellmay end in setting a casing much shallower than planned. Implementing another string to reach the primary target in 8-1/2" and thus maintaining the contingency of a 6" section was deemed necessary. In the select phase the operator compared different designs used in offset wells, all including a contingency casing or liner string. These wells faced great challenges with under-reaming for intermediate 11-3/4" and/or 16" strings. Non-premium or semi-flush casing connections, small annular clearances and spool type wellheads have been notorious weak points in well barrier envelopes of previous designs. Standard MLS systems are not suitable for the re-entry phase of such a complex well. The operator engineered a new casing design by introducing a 22" surface and 18" intermediatecasing in addition to the standard 13-3/8" intermediate casing, 9-7/8" production casing and 7" reservoir liner. A 4-1/2" liner is kept as a contingency string. The newly developed wellhead and MLS design suitable for this four casing scheme requires only two BOP installations (21-1/4" and 18-3/4") without any further BOP or well head lifting in-between sections. The newly designed MLS system caters for the four casing strings and tolerates a 7"x 8-3/8" liner tie-back to surface contingency for the future production well, if required. The operator then compared and ranked this new casing scheme against designs used in off-set wells. This novel and advanced casing and MLS design exceled the other designs and is considered fit-for-purpose, while reducing operational and nonproductive time (NPT) for such a well.