Drilling into a Potential Losses Zone with 20ppg MW: WBM or OBM? * Micromax experience*

01 November 2011 ** June 2015: Supplementary question added to this thread to explore global experience with Micromax mud **

Hello, just looking at some feedback...
I have currently been tasked with designing an exploration HPHT well in 100' of water in the Arabian gulf region. What makes the well challenging from a design standpoint is a pressure ramp at 10,000' TVD and then a potential losses zone at 11,500´ TVD. The PP increases from 13.5ppg to 19.5ppg EMW in a 300ft interval starting at 10,000´ and at 11,500´, a potential fractured dolomite formation is encountered. On 1 close offset well partial losses were seen with 20ppg MW when the fractured dolomite section was penetrated and from regional offset wells total losses with sour water influxes were seen with only 3 ft of the fractured dolomite being penetrated. From the pore pressure prediction, the fractured dolomite formation has a pore pressure of 17.7ppg EMW and to further complicate the well design, at ~13,000ft, the pore pressure further reduces to ~15.7ppg and partial losses were seen in the 1 close offset well with 16.4ppg MW below 13,000´. In addition, there is a potential second PP ramp/regression at ~16000´ (Up to 19ppg EMW and then back down 17.7ppg) and TD is 21,000´. Below 15,000´ in this field is complete wildcat exploration with no close offset penetrating below 15,000´.
Although it is not clear from the old close offset the nature of the losses, I have assumed that these losses are not drilling practice related and are due to a narrow mud weight window. From a well design standpoint, a casing pt will be required at:

• In the pressure ramp at +/- 10,000´
• If Losses occur or can´t be cured at 11,500´ - Contingency Liner
• At 13,000´ to allow drilling of the 15.7ppg PP formations.
• At 16,000´ before the 2nd Ramp is penetrated.
• At 17,000´ below the 2nd ramp.

At this time, the fracture gradient supplied is sketchy and thus the use of regional known drilling hazards for the well design.
My question:

• Due to the logistics (Offshore) of heavy OBM and the potential for total losses at 11,500, can I consider WBM for drilling the section from 10,000´ to 13,000´? The temperature at this depth is 220-240°F.
• Once the string at 13,000´ is cemented, the mud system can be changed to OBM for the HPHT phases.
• The liner size that will be set at 11,500´ is 11 ¾” and thus there will be huge challenges with getting a good shoe for drilling ahead (Hole Opening, Running of the liner, cementing, etc in 20ppg MW)

Anyways, I would appreciate any feedback on drilling with WBM at 20ppg and running of a liner with slim clearances in 20ppg MW.
Thxs
Jeff

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