In sand-producing formations, a cased hole gravel pack is usually the preferred completion method. There are various reasons why the alternatives, an open hole gravel pack or a sand consolidation treatment, may not be suitable. However, the productivity of the cased hole gravel pack is much poorer than the alternatives because of high mechanical skins.
Despite the importance of this mechanical skin, the existing pressure drop prediction models are incomplete. Many papers deal with the region of linear flow in the casing/cement tunnels and the flow through the gravel between casing and screen. What happens just outside the casing is rarely mentioned, even though the pressure drops beyond the casing may be significant.
One complication is that perforation tunnels in the formation might either remain intact or they could collapse before gravel packing, creating a void behind the casing.
This paper presents new methods for predicting the pressure drops outside the casing and so allows the construction of a comprehensive model for cased hole gravel pack productivity. A newly-developed finite difference computer simulator was used to produce a general method for intact perforations packed with gravel. For collapsed perforations, an analytical method was developed which is based on the amount of gravel placed behind the casing.