Improved Productivity Justifies World Record Underbalanced Perforating Operation
Authors: Johnson A.M. , Bakker E.R. , Hungerford K.E. 
Published: March 2010

A Dutch onshore gas well was drilled to a total depth of 5,136 m along hole (3,700 m true vertical depth) with a maximum deviation of 89 degrees. The well was completed with 193.7 mm (7 5/8 in.) 13 ﹪ chrome (Cr13) tubing with a 177.8 mm (7 in.) cemented liner across the reservoir providing fullore access to the pay zone. Due to the very stratified nature of the reservoir, the long-term sustained production depended on achieving vertical connectivity with all the layers. In order to achieve this and maximize productivity, a single run underbalanced perforating operation was required. Due to coiled tubing limitations in this deep, high pressure well, a hydraulic workover unit was selected to deploy and retrieve the guns. A total gross interval of 1,026 m was perforated in one run utilizing 85.725 mm (33/8 in.) deep penetrating guns and thereafter snubbed from the well under live conditions (46 MPa at surface). To date, it is understood that this is the longest section of guns conveyed, fired underbalanced, and deployed out of a live well. This paper addresses the perforating technique selection, the planning and execution process, the major operational learning points, and discusses final productivity results which demonstrate the validity of this technique.

Introduction

The Anjum reservoir is a Rotliegendes sandstone at a true vertical depth of 3,700 m. Reservoir pressure and temperature are high for onshore Netherlands at 56 MPa and 137 °CDATA[C, respectively. Following discovery of the prospect in 1992 with the Anjum 1 exploration well, Anjum 4 was drilled as the second well in the field. It was designed as a combined appraisal/development well with the objective of providing capacity in the region of six to seven million m3/day.

The reservoir is of a highly stratified nature with a number of high permeability features, some of which are on a centimetre scale. The Upper-Slochteren gas-bearing interval is split up into six discrete units, which are correlatable across the whole of the North Netherlands area. The upper three sands dominate in terms of reservoir quality and productivity. There exist two significant and continuous high permeability streaks (HPS) which, if targeted, would provide initial high productivity. However, the longterm sustained production depends on achieving vertical connectivity with all reservoir zones. Therefore, in order to maximize exposure across all intervals, and to provide the maximum connectivity with the individual layers, a sub-horizontal well was designed. A cemented and perforated liner was selected, since it provided the opportunity to effectively bypass near wellbore drilling induced damage. Secondary benefits of this design are that it provides maximum selectivity (water shut off, stimulation etc.) and through the utilization of deep penetrating perforating guns reduces non-Darcy turbulent effects.

The well was completed with a 177.8 mm (7 in.) Cr13 liner across the pay zone and a 193.7 mm (7 5/8 in.) Cr 13 monobore completion with a tubing retrievable safety valve to provide maximum wellbore access (Figure 1).