Abstract

In the pressure depleted section of the Safah Field in Oman, use of conventional overbalanced drilling techniques resulted in complete lost circulation of drilling fluid, differential sticking of the drill string and inability to drill horizontal sections.

Underbalanced drilling techniques were successfully utilized to add horizontal lateral sections to existing wells, complete the Shuaiba Carbonate Reservoir waterflood pattern and increase Safah Field production.

Introduction

To complete the waterflood pattern, it was required that dual horizontal lateral sections be drilled in the existing Safah 158 and Safah 170 wells.

In July 2000 Safah 170 well was drilled. The Safah 170 well was programmed to have two horizontal laterals drilled into the Shuaiba Limestone using a conventional overbalanced drilling fluid system. Due to Shuaiba Reservoir pressure depletion in Safah 170, complete lost returns and differential sticking of the drill string occurred while attempting to drill horizontally. Attempts to produce Safah 170 were unsuccessful. Safah 170 was shut in.

The Safah 158 well, a single lateral horizontal, was drilled in 1998. Safah 158 is also in the pressure depleted section of the Safah Field Shuaiba limestone and was producing 200 BOPD prior to reentry to add two drilling horizontal lateral sections.

In midyear 2000 an engineering study was conducted to evaluate the feasibility of using underbalanced drilling techniques to add required horizontal lateral sections to Safah 158 and Safah 170 wells, so that the Safah Field waterflood pattern could be completed.

In late year 2000 and early 2001 underbalanced drilling techniques were utilized to drill horizontal lateral sections in the Safah 158 and Safah 170 wells to complete the Safah Field waterflood pattern. With the added horizontal laterals, Safah 158 production has increased from 200 BOPD to 800 BOPD. With the addition of horizontal laterals, Safah 170 (which initially was nonproductive and was shut in) now produces 1500 BOPD.

Theory

To complete the Safah Field Shuaiba reservoir drain pattern horizontal laterals were required in the pressure depleted Safah 158 and Safah 170 wells. Underbalanced drilling modeling for these two wells was conducted.

The top of the Shuaiba production limestone in Safah 158 and Safah 170 wells is at 6500 ft. The estimated Shuaiba formation pressure in these wells was 1300–1400 psi (0.21 psi/ft). Modeling conclusions resulted in the below plan to achieve underbalanced drilling conditions.

Safah Lease crude (42 degree API) was selected for the underbalanced drilling fluid.

Pumping lease crude and lease gas as a two phase fluid system down the drill string was considered but rejected due to additional costs and limited availability of a gas tight kelly hose, and additional costs and limited availability of electronic telemetry MWD tools. Modeling predicted that underbalanced drilling conditions could be achieved by utilizing a single phase drilling fluid – lease crude, and using gas lift in the drill string annulus.

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