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.
When an expandable sand screen is installed in a reservoir section, the gap between the outside of the screen and the rock can vary from zero to a significant fraction of the well diameter. The gap is influenced by the state of the hole after drilling (diameter, rugosity, tortuosity etc.,), the nature of the screen, and the nature of any expansion process used (for example, expansion of a plastically-deforming pipe with a fixed size of mandrel is likely to leave a gap in boreholes larger than the design size, while a more compliant expansion system would be expected to push to pipe into closer conformity with the borehole wall). The gap is likely to vary with position along the well, and with time. The presence of a gap could influence the short- and long-term integrity of the completion in a number of ways, through its effect on yielding and failure of the formation around the screen, and by offering an annular route along the well for fluids or solids:
Yield and failure of the formation could result in large loads being applied to the screen, as yielded material or fragments are pushed against it. This loading could be uniformly distributed around the screen, or not. If it exceeded the rating of the screen in the appropriate configuration, the screen would collapse, reducing production, blocking access to the upstream part of the well, and potentially rupturing the sand exclusion material;
Yield and failure could also release matrix grains or fines from the rock. If these mix and settle around the screen, they may plug the screen and cause a reduction in well productivity;
If loose grains or fines can move along the annulus, they may be accelerated in the flow there to such an extent that they erode the screen or other components;
Yielded rock, if it stays in place, may have a lower permeability than intact rock, leading to higher skin and reduced productivity.
Even if there is no mechanical damage to the formation, a gap allows fluid transport along the well. This means that that proper assessment of inflow is difficult - for example, it may be impossible to determine the points of water influx - and subsequent workover treatments would also cause trouble.