Introduction

Located in the Emirate of Sharjah, United Arab Emirates, the Sajaa onshore gas field is a Thamama limestone reservoir ranging from 11,000 to 13,000 feet true vertical depth (3350 to 3960 meters true vertical depth). The bottomhole temperature is in the 260 to 310 degrees Farenheight (126 to 154 degrees Celcius) with bottomhole pressures of 1100 to 1900 psi (75 to 129 atmospheres). This paper will discuss the process used in basis of design, selection of bidders, pre tender qualifications, underbalance modeling, pre qualification, assurance testing of equipment, initial rig up, two phase nipple milling, window milling and the drilling of the first six wells in the Sajaa Underbalanced Coiled Tubing Drilling project.

Background

In April of 2002 a Production Technical Limit (PTL) meeting reviewed the wells and reservoir and identified the prize of unlocking additional permeability and height to gain additional rate and reserves. The PTL also identified the work required for sub surface evaluation of the reservoir with the basic questions of how much gas is left and where is the gas located. Idea's were then put forward on various options of snub drilling, conventional underbalance drilling as in the 2000 campaign, and coiled tubing drilling. All the options had one thing in common and that was the use of multi lateral wellbores to connect gas zones not fully connected with the vertical well mother bores. Coiled tubing drilling was evaluated as the best choice because of the need for real time directional steering where mud pulse is not possible due to the two phase fluids being pumped down the coil.

In June of 2002 a Peer Assist was held in Sharjah with invited BP quests and distinquished underbalance/CTD/snubbing experts from Alaska, the North Sea, BP Houston's non conventional wells team, Service Company expertise from BP's experts in Colombia and underbalance consulting firms. The result of the three day meeting was a Risk Assessment with the Underbalance Coiled Tubing Drilling. Key areas of risk were:

  • Personnel (right people, right skill sets, doing the right thing)

  • High temperature equipment (300 degrees F assurance testing)

  • Staffing levels identified (supervisory and operators)

  • Two phase milling of nipples and windows

  • Equipment availability

  • Nitrogen (liquid versus membrane)

  • Coil purchase, owner and lead-time

  • Formation deliverability

  • Loss of mother bore

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