Abstract

In over 300 expandable sand screen (ESS®) installations around the world it has proven exceedingly difficult to perform an equitable and rigorous comparison between the open-hole ESS system and an alternative completion technique.In this study, there is a marked apparent difference in completion performance between the systems and this is investigated in more detail in order to compare the techniques on an equal footing.

The Mokoko Abana field is situated offshore Cameroon.It is a mature heavy oil field initially developed in the 1980s.The sand is unconsolidated and all wells require sand control from start-up as evidenced by sand production if sand control is not effective.Several different types of sand control have been tried with varying degrees of success.This study is mainly focused on three different completions, all in similar configuration high angle wells in the same reservoir and sand.

The first type of completion studied was pre-packed stand alone screens (PPSAS).This showed excellent initial productivity, but was prone to a rapid decline in performance.The decline is attributed to hole collapse, either liberating fines plugging the screen, or creating a low permeability sand/shale mixture in the annular gap.

Open hole gravel packs (OHGP) were then used to support the borehole and immobilize any failed sand and fines in the annulus.The OHGP had a somewhat lower initial productivity than the PPSAS but has maintained a higher level of productivity over time.

Open hole expandable sand screens (ESS) are the most recent completion option, with the first being installed in Q4 2000. The main reasons for installing ESS were a number of reservoir simulation studies which showed better reserves recovery with a low skin completion, and hence ESS was selected in order to lower the overall skin.

As of 2005, the successful ESS wells show a high productivity that has been maintained over four years.An in-depth analysis of the productivity of the PPSAS, OHGP and ESS was undertaken.The analysis was complicated by variation in fluid and sand properties through the reservoir and masked by the different artificial lift methods used.This made an accurate assessment of the performance of each type difficult to achieve.However, even within the uncertainty in the analysis, the ESS completions have a significantly higher productivity and lower skin than the other two completion options.

The studies suggest there are production benefits in removing the annular gap. The range of sands controllable with an ESS is wide and supports the Ballard & Beare d[5] criterion.ESS completions correctly applied can be successfully produced with high water cuts and draw-downs in excess of 500psi.

Introduction

Initial sand control completions in Cameroon were installed in vertical or deviated wells, with the casing cemented, then milled out, and an external gravel pack installed.Many of these have performed well and stood the test of time.

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