Abstract

This paper documents the job methodology and the remedial cementing actions taken to cure an offshore subsurface gas and oil leak in a prolific gas producer that crosses multiple, hydrocarbon-bearing horizons. It is a follow-up to ADIPEC- 0938 describing the investigative process used to diagnose the multiple leak sources. A detailed risk assessment and contingency plan preceded each operation. Seven squeezes proved necessary to control the leaks. Continuation of the leak to surface and downhole noise activity defined the success or failure of each cementing operation. Cement squeeze operations took place through two to three sets of casing, after perforating with TCP guns at shot densities from 12 to 36 shots per foot. An ultrafine cement formula ensured the smallest diameter flow conduits were penetrated. Monitoring of the annuli leak rates over a sixteen-month period with a custom designed separator, suggests the leak is cured. Final declaration of success or failure must wait until the well is place back on line following its recompletion.

Introduction

During the shallow drilling phase of the fourth well on a platform situated in the Arabian Gulf a small explosion occurred1. The fuel was a gas accumulation caught between the 30" conductor and 20" casing. Ignition occurred during hot cutting of the newly cemented 20" casing. The presence of gas was puzzling, as the well had not yet penetrated any known hydrocarbon-bearing formation. Divers sent to investigate the platform structure condition, discovered gas bubbles percolating up from a number of cracks in the cement grout covering the sea floor. Two of the wells on the platform are auto-injectors, separately pressurizing Cretaceous and Jurassic oil reservoirs. They receive gas from a deep Permian producer on the same platform. The incident well was intended as an additional gas supplier. Investigation of the platform wells with cased hole logs, annulus pressure monitoring and well history review, pointed to the Permian gas producer as the problem well. It contained pathways via cement channels and empty space between 9 5/8" × 13 3/8" and 13 3/8" × 20" casing strings for gas and oil to leak to surface from the Cretaceous and Jurassic reservoirs under gas injection. A casing diagram and the diagnosised hydrocarbon flow paths of the culprit well is presented in Figure 1.

Job Planning

A detailed Job Plan and Decision Trees were formulated before the job by a multi-disciplined team of reservoir, petroleum, drilling, engineering and safety engineers. Several months were invested in laying out the operations details and job strategy. The team reported and discussed its progress regularly to a Steering Committee composed of senior management. The mandate of the team was to repair the leak while ensuring the safety of personnel involved and the tower.

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