Novel Relief Well Design for the Alleviation of Gas or Fluid Pressure in Shallow Permeable Layers
Authors: Strout James Michael , Poudens Olivier , Colliard David 
Published: April 2012

Relief wells have been deployed by Total Indonesie at the Sisi field as a riskmitigation measure to address the potential hazard for the bearing capacity ofa jack-up foundation due to shallow gas in a shallow permeable soil layer at17–27m below mudline (bml). A novel design approach was proposed and developedby NGI for this purpose. The technical solution is based on the application ofgeocomposites, commonly used in soil improvement applications on land (verticalstrip drains for consolidation). This solution utilizes common materials whichcan be procured locally at very low cost, and can be installed by any competentoffshore geotechnical contractor. 8 relief wells to 35m bml were completedduring a November 2011 campaign. The relief wells form a proactiverisk-reduction strategy implemented by Total in connection with planned wellintervention in 2012.

Introduction

The Sisi asset owned and operated by Total offshore Indonesia is located in aregion where shallow gas is a potential threat for offshore installations. Thewater depth in the area is between 50 and 85 m, and the location of the assethas therefore been repeatedly above sea level during the glaciations of theQuaternary period, when sea water was trapped on land as ice at high latitudes. These fluctuations of sea level have produced a layering consisting of claysand sands that have been cut by erosional fluvial or tide water channels. Capillary effects associated with sediment grain size make sand and sand/corallayers as potential deposits in which to find gas. At Sisi these sand/corallayers are present at 17 to 27m below mudline (bml) as well as at 87-91m bml. Although shallow gas has been identified in the deep layer, the shallow layerhas to date not shown any indication of free gas or overpressurized fluid. Thepossible effect of shallow gas on soil strength was considered in the bearingcapacity calculations for the platform foundations.

Annulus pressure measurements in 2008–2009 identified abnormal excess pressurein the well annuli. This triggered an initial site investigation campaign in2010 where no free gas or overpressurized fluid was detected from 0 to 78.5mbml. However, risk assessments for the jack-up installation for a planned wellintervention in 2012 identified potential free gas in the shallow sand/corallayer as a possible hazard for the jack-up, as the presence of free gas willinfluence effective stresses and reduce bearing capacity for the spud cans. Migration of gas along the CP was identified to be the most likely source ofgas leakage into this shallow layer.

As a risk mitigation measure Total chose to implement shallow relief wellsaround the Sisi installation to maintain hydrostatic pressures and alleviateany potential future gas or fluid migration into the layer. The relief wellswere installed during the geotechnical campaign preparing for the jack-upinstallation. The relief wells provide an active mitigation of a potentialshallow gas risk, and in addition can provide a means for direct visualindication of no shallow gas using a few moments of ROV time at opportunitywhen other work is done at the location.

A fundamental set of design constraints were imposed by Total for the design ofthe relief wells. The relief wells shall:prevent gas accumulation by providing a migration/seepage pathway to theseabedeffectively alleviate fluid pressure in excess of hydrostatic in the targetlayerbe durable and resilient to last the anticipated operational lifetime of theinstallationbe possible to install using a standard geotechnical drill ship without ROVsupportbe designed using as much standard, locally available components aspossiblenot introduce any risk to the platform or seabed installations, or interfereunduly with future operations