This article, written by Senior Technology Editor Dennis Denney, contains highlights of paper OTC 19242, "An Evaluation of Perforating Techniques and Use of Cesium Formate Kill Pills To Maximize Productivity in HP/HT Gas Wells and Minimize HSE Risks," by Alistair Roy, SPE, Allan Twynam and Jeff Parke, BP; Angus Morrison, Schlumberger; and John Downs, Cabot, prepared for the 2008 Offshore Technology Conference, Houston, 5-8 May. The paper has not been peer reviewed.

The BP-operated Rhum field is a North Sea high-pressure/high-temperature (HP/HT) lean-gas field. A key design requirement was to evaluate overbalance- and dynamic-underbalance-perforating techniques to optimize productivity while reducing the risk of producing hydrocarbons to surface during downhole operations. This case history focuses on available perforation techniques with the aim of matching or exceeding the flow performance achieved on the appraisal-well drillstem test (DST).

Introduction

The Rhum field is 45 km north of the Bruce field (Fig. 1). It has three subsea development wells tied back to the host Bruce production platform. The water depth is approximately 108 m. The field has several challenges.

Bottomhole temperature and pressure (BHP) were 300°F and 12,500 psi, respectively.

The lean-gas reservoir fluid leads to high surface pressures (>10,000 psi).

The rock strength varies from 8,000 to 30,000 psi unconfined compressive strength (UCS).

Middle sour environment is expected with H2S levels of 10 to 20 ppm and CO2 levels between 4 and 8%.

Appraisal Well 3/29a-4 was spudded in August 2000, and testing was completed in February 2001. This well confirmed that the field was an HP (12,418 psia at 4745 m true vertical depth subsea), lean-gas discovery with approximately 1 Tcf of gas initially in place.

Completion Objectives

From the start of the project, a desired objective for the Rhum completions was to maintain at least one mechanical barrier and one fluid barrier while running the completion. This combination was essential risk mitigation for running completions in a subsea environment. Only two clear fluids were capable of delivering the required 1.9- to 2.2-specific-gravity (SG) brine weight: cesium formate and zinc bromide. The environmental risks of working with zinc bromide were deemed unacceptable; therefore, cesium formate was selected.

It was decided to evaluate whether cesium formate could be used during perforation to avoid the complicated rig ups and multiple runs required for deploying the required number of perforations by use of coiled tubing (CT) or electric line (E line).

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