Abstract

This paper describes a new gravel packing technique for extended-reach open-hole horizontal completions. The method enables gravel packing of long horizontal completions without excessive Beta-wave placement pressure.

The physical characteristics of gravel packing horizontal wells have been studied intensively in the past few years. The Alpha- and Beta-wave model has been developed to predict the pattern of sand deposition in the well during the gravel pack operation. The Beta-wave (return gravel wave) placement pressure is the main factor in determining the maximum length of a horizontal gravel pack. This pressure is limited by the requirement to install the gravel pack without exceeding formation fracture pressure, which could result in loss of control of the gravel pack and adverse effects on well productivity.

To address the Beta-wave pressure limitation, a new technique has been developed and operationally verified in field installations in the North Sea. The technique uses a special valve, or series of valves, in the inner washpipe that automatically operate to re-direct return fluid flow into the washpipe during placement of the Beta-wave. The valve eliminates the need for the return fluid to travel down the length of the screen to enter through the end of the washpipe. This prevents the accumulation of friction pressure in the screen/washpipe annulus as the Beta-wave propagates from the toe to the heel in the open-hole horizontal section of the well. Strategic placement of these valves in the washpipe string regulates the friction pressure applied against the formation below the fracture limit. Effective design and placement of the valves in the string are made possible with the aid of computational models. The new method essentially eliminates the Beta-wave pressure as a limiting factor in gravel packing extended lateral completions, and makes extended gravel packs possible that would not have been feasible using conventional methods.

This paper will describe the concept, discuss the verification-testing program used to validate the theory and review actual field operations that have been performed using this technique.

Introduction

Gravel packing is a commonly applied technique to control formation sand production from open-hole oil and gas wells. In a gravel pack completion, a screen is placed in the well across the productive interval and specially sized, high permeability gravel pack sand is mixed in a carrier fluid and circulated into the well to fill the annular space between the screen and the formation. The size of the gravel pack sand is selected to prevent formation sand invasion and the size of the screen openings are selected to retain the gravel pack sand. A complete gravel pack in the open-hole/screen annulus creates a very stable, long lasting downhole environment where only well fluids (not formation sand) are produced. Gravel packing has been successfully applied in conventional wells for several decades, and increasingly, the technique is being applied in extended-reach open-hole horizontal wells.

Horizontal gravel packing is process intensive and requires special attention to drill-in fluid selection, well displacement and service tool operation to ensure successful gravel placement and well productivity.¹ Specialized downhole tools facilitate circulation of the gravel pack sand in place. The tools create a circulating path for the gravel slurry down the workstring, out into the annulus below a packer and down the annulus outside the screen. The gravel is retained by the screen and the carrier fluid flows into the screen, up the washpipe, out in the annulus above the packer and back to surface. Fig. 1 illustrates the basic gravel pack circulating paths.

The washpipe extending down inside the screen directs the point of fluid returns to the end of the screen. As well deviation increases, large washpipe becomes a critical factor in achieving complete gravel fill around the outside of the screen. Test data and field experience show that the washpipe OD to screen ID ratio needs to be approximately 0.8.^2,3 The large OD washpipe restricts the amount of carrier fluid that diverts into and flows down the screen/washpipe annulus.