HPHT Horizontal Sand Control Completion
Authors: Law D. 
Published: December 2007

The Marnock field is a retrograde gas condensate accumulation in the central area of the North Sea. Compared to conventional North Sea reservoirs it is near HPHT with a pressure of 9123 psia and a temperature of 310°F.

The development plan comprised six horizontal wells, with average reservoir sections of 750m length. Drilling was conducted in two phases and has delivered four wells capable of the original plateau rate expected from six wells.

Well completion design for this field presented significant challenges. The main ones were; installing downhole sand control in a horizontal well, flow back of 1.9 sg drilling mud through sand screens and the use of an isolation valve to allow the upper completion to be installed under balance.

Completion operations were initially delayed by equipment failures, and difficulties achieving the hole quality required for installation of sand screens. The main elements of the completion design were revised to a modular approach, reducing interdependency and adding flexibility to the operation. Improvements in the drilling and preparation of the reservoir section were also implemented.

The four wells completed are capable of delivering sand free production, with individual well rates of up to 120mmscf/d.

Introduction

Marnock is a gas condensate accumulation, containing ca. 1 tcf gas and 110 mmstb condensate in-place, located within the Eastern Trough in the UK Central North Sea, some 130 miles east of Aberdeen in water depths of around 95 metres (Fig. 1).

The accumulation comprises a gently tilted fault block with hydrocarbons contained within Triassic fluvial sandstones, sealed predominantly by Cretaceous mudstones and argillaceous limestones. Hydrocarbon fluid properties vary considerably across the structure with relatively lean gas present in the eastern terrace and condensate-rich fluids contained within the western area of the main accumulation. Compared to conventional North Sea reservoirs it is near HPHT with a pressure of 9123 psia and a temperature of 310°F (Table 1).

Initial plans for a stand alone field development with 22 conventional wells failed to clear economic hurdles. It was not until the mid-90's when a project bringing together several medium sized fields using the one infrastructure (Eastern Trough Area Project) and advancements in horizontal well technology, that the development became economically viable.

The development was based on the Central Processing Facility (CPF) being placed over the Marnock field, and 6 horizontal production wells drilled through the platform by a heavy duty jack-up (HDJU) rig. Sanction to proceed with the field development was given prior to the well concept being proven, as no analogues were known at this time. As HPHT horizontal wells with sand control were somewhat unique, additional slots were included on the platform as contingency.

The completion design for this field has presented significant challenges, primarily;Installing down hole sand control in a horizontal wellFlow back of 1.9 sg drilling mud through the sand screensthe use of an isolation valve to allow the upper completion to be installed under balanceProvision of a big bore (5" ID) high pressure TRSCSSVThe use of cement filled ECP's for future water shut offThe introduction of HP coil tubing to the North Sea

This paper addresses the key areas of work and significant changes implemented throughout the project to take the completion from concept to an operationally deliverable result.