Abstract

This paper discusses the implementation of dynamic underbalance perforating in a subsea development, which helped address environmental concerns related to flaring during perforating. Use of a new perforating system implemented on recently completed wells has resulted in a significant reduction in flaring while maximizing underbalance during gun detonation. The elimination of flaring has also simplified perforating operations, resulting in significant time savings.

The Terra Nova offshore field is located 350 km off the East Coast of Canada. It is a subsea development producing from a multi layered, highly faulted oil reservoir to a Floating, Production, Storage and Offloading vessel (FPSO). The development plan utilizes high productivity producer-injector pairs in the individual fault blocks to optimize recovery. Initially, standard practice was to perforate wells using multiple runs of wireline conveyed guns, up to 6 runs per well. In order to maximize completion efficiency the wells were perforated in an underbalance condition. The underbalance was generated by flowing back and flaring the well at the Mobile Offshore Drilling Unit (MODU) during each wireline run. The net results were multiple flowbacks per well, with diminishing underbalance for each successive perforating run due to flare rate limitations. The multiple flowbacks increased the risk for environmental incidents due to unintentional releases while flaring. Alternative gun deployment methods had been considered but deemed undesirable due to sump limitations and potential for formation damage.

In order to minimize flaring an innovative perforating technique was implemented. This technique generates dynamic underbalance during gun detonation, mitigating the need to flare the well to create underbalance. Fast pressure gauges were run on wells perforated using this method and the data confirmed that dynamic underbalance was achieved. Successful implementation of the dynamic underbalance system has resulted in elimination of flaring related to perforating operations, and has significantly reduced the potential for unintentional environmental releases.

Introduction

Wireline conveyed perforating in conjunction with well flowback evolved as the preferred methodology for well completions in the Terra Nova Field. This methodology provided effective results while accommodating requirements for:

  • Perforating with an under balanced wellbore

  • Maintaining a monobore completion

  • Avoid perforating with kill pill across reservoir

A consequence of this approach was the need to flare the well at the MODU to generate the required underbalance. A typical well would require up to 6 wireline runs to perforate the entire interval, which resulted in an equal number of flaring events. These flaring events created environmental risk due to the potential for unintentional liquid discharges. The objectives of this paper are to discuss how a new approach to perforating in the Terra Nova Field significantly reduced the environmental risk associated with perforating, while also reducing rig time required for perforating operations without compromising well productivity/injectivity.

Field Overview

Terra Nova is a Petro-Canada operated subsea development located off the East Coast of Canada (Figures 1&2). First oil was achieved in January 2002. The field consists of an FPSO vessel and 4 associated subsea drill centers connecting 3 main producing areas. The target reservoir sands are Jurassic in age and located in the Jeanne d'Arc formation (Figure 3). 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 177.8mm monobore design to accommodate high flow rates and future intervention requirements. Reservoir properties for two Terra Nova wells are presented in Table 1.

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