When an expandable sand screen is installed, the gap between the outside of the screen and the rock can vary from zero upwards. The gap is influenced by the state of the hole after drilling, subsequent logging or completion operations, and the natures of the screen and expansion process. This paper describes experiments and modelling undertaken to help understand and clarify the impact of the gap on the short- and long-term integrity of the completion.

Experiments measuring hole closure showed that the presence of a screen always improved hole stability relative to that of the open hole; this benefit was reduced when there was a significant annular gap. This conclusion was supported by rock deformation modelling, which showed that for a screen with relatively low radial stiffness, a very small or zero gap was best, while a stiff screen could tolerate a slightly larger gap, up to a few millimeters. In both cases this is to allow mobilization of rock strength through yielding. Gaps larger than this allow the rock to break up, potentially leading to point-loading or erosion problems. Modelling of the transport of liquid and sand showed that a smaller gap reduces annulus liquid velocity, ultimately to the point where sand transport (and potential erosion) no longer occurs. It also identified conditions that could compromise the long-tem integrity of the screen, such as blocked screen sections or regions of enlarged gap, which generate persistent regions of concentrated flow through the screen wall, again potentially leading to erosion problems.

The experiments were carried out using a newly developed bistable sand screen1 . This has been designed to conform as closely as possible to the borehole wall, and have a very low deployment force, but the conclusions are equally applicable to other types of screen.

You can access this article if you purchase or spend a download.