Dynamic-Underbalance Perforating Overcomes Environmental Challenges
Authors: Bybee Karen 
Published: February 2015

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 108167, "Overcoming Environmental Challenges Using Innovative Approach of Dynamic- Underbalance Perforating," by Dennis Baxter, SPE, Harold McCausland, and Brian Wells, SPE, PetroCanada, and Vinay K. Mishra, SPE, and Larry Behrman, SPE, Schlumberger, prepared for the 2007 Offshore Europe, Aberdeen, 4-7 September. The paper has not been peer reviewed.
Wireline-conveyed perforating in conjunction with well flowback evolved as the preferred method for well completions in the Terra Nova field. A consequence of this approach was the need to flare the well at the mobile offshore drilling unit (MODU) to generate the required underbalance. The full-length paper details how a new approach to perforating significantly reduced the environmental risk associated with perforating, while also reducing rig time required for perforating operations without compromising well productivity/injectivity.

Introduction
Terra Nova is a PetroCanada-operated subsea development off the east coast of Canada. First oil was achieved in January 2002. The field consists of a floating production, storage, and offloading (FPSO) vessel and four associated subsea drill centers connecting three main producing areas. The target reservoir sands are Jurassic and are in the Jeanne d'Arc formation. There are currently 27 wells in the field, including 15 oil producers, 9 water-injection, and 3 gas-injection wells. All wells were drilled and completed from a MODU. A typical well incorporates a 7-in. monobore design to accommodate high flow rates and future intervention requirements.

Previous Approach
The approach to perforating at Terra Nova evolved over the course of development drilling. As part of the well-planning process, an assessment of perforating alternatives typically was completed for each well to identify opportunities to reduce rig time and flaring requirements. Priorities for perforating included the following.Perforating in an underbalance condition to optimize perforating efficiency.Maintaining a monobore completion through the perforated interval to facilitate future interventions (retrieval of spent guns).Avoiding use of kill pills to prevent near-well damage.
In addition, sump availability usually was limited because of overpressure zones and low penetration rates below the reservoir interval. These priorities and limitations typically eliminated the use of tubing-conveyed or stackable gun systems. As a result, use of wireline-conveyed guns, detonated in an underbalance condition, was the typical approach. Underbalance was generated by use of static underbalance for the initial gun run and flowing the well to the MODU for subsequent runs. The wells were flowed to the MODU vs. the FPSO vessel primarily because of concerns regarding the effect of perforating debris on process equipment. To accommodate flowback, the MODU was fitted with a well-test package consisting of the following equipment.High and low stage separators and associated equipment.Dual flare booms with burner heads rated at 2 544 m3/d.Air compressors for oil atomization at burner heads.Zone 1 office.Computerized data-collection network and associated sensors.