Mechanisms of Downhole Sand Cavity Re-Stabilisation in Weakly Consolidated Sandstones

Abstract

Abstract This paper presents a theoretical and experimental investigation into the possibility of cavity re-stabilisation after initial sand failure by formation of stable wormholes (‘pipes’) behind the casing / liner. Piping has been observed in dikes in The Netherlands. A model has been developed to compute the degree of cavity re-stabilisation and associated sand production rates. The theoretical results show that formation of wormholes can result in re-stabilisation and associated reduction in sand production. The theoretical work has been verified by laboratory sand production experiments. Experiments on friable sandstones show that formation of wormholes can take place as part of a re-stabilisation process. Experiments in homogeneous unconsolidated sandstones, on the other hand, show that under elevated stresses, it is impossible to form stable wormholes in this case. Experience with dikes however show that wormholes in these sands can occur when they are interbedded with shales/clays. The results above in homogeneous sandstone are further supported by tests with horizontal pre-drilled, uncemented liners in weakly consolidated artificial sandstones. Formation of stable sand arches over the liner holes takes place at low values of far-field stress. However, elevated values of far-field stress lead to a continuous, high-rate (‘massive’) sand production.