Sanding is vitally a serious problem during heavy-oil production from unconsolidated reservoirs. In order to effective sand control and high well production, various sand exclusion technologies are used for horizontal well completion, especially, the open-hole gravel pack (OHGP) has been widely used. So it is critical to quantitatively evaluate permeability impairment because of sand particulates migration and plugging in gravel-packed layer, and it is also important to predict horizontal well productivity. In this paper, we have developed one numerical model of permeability impairment based on the previous experimental observation. This numerical model considers the liquid-solid fluid flow process, and sand particles depositing and plugging in gravel-packed layer. The numerical simulation results of permeability impairment at different sand volume fraction, fluid viscosity and sand radius are given, and the simulated results are in good agreement with the previous experimental results. Integrating the permeability impairment model a new model to predict productivity of gravel-packed horizontal well has been proposed. The proposed model couples reservoir inflow and seepage in gravel layer, and it can quantitatively evaluate the damage of gravel layer and predict well productivity performance. The numerical simulation of real-world scenarios investigates that it is more accurate than previous models, and it is very easy to use.

1 Introduction Recent years with reduction of conventional reservoir, the unconventional reservoirs (i.e. heavy-oil reservoir, low-permeable reservoirs, and fractural reservoirs) have become significant in oil and gas development, especially the heavy-oil reservoir has been main part of unconventional reservoir (Z. et al. 2004; Mohammadi et al. 2012). Because of high viscosity and unconsolidated condition, sanding is the main problem affecting the development of heavy-oil reservoir. Rock failure surrounding wellbore is prone to happen because of weak bonding strength, resulting in large mass of sanding. The sand production in well will not only lead to serious abrasion wear on down-hole and surface equipment, even well sandup or sand burial accident, but also decrease of recovery efficiency and oil well abandonment (Tian et al. 2005; Li et al. 2004; Shao et al. 2004; Li et al. 2011; Li et al. 2012). Sand production in oil-producing wells is usually related to the following two mechanisms: (1) Mechanical instabilities and localized failure; (2) Hydro-mechanical instabilities due (Veeken et al 1991). Many studies have been performed in the past on different aspects of sand production. However, they have mainly concentrated on finding effective methods to avoid sand production because of the high operating costs involved in handling and disposing of produced sand (Travis et al. 2004; Brígida et al. 2011). In order to ensure safe operation of facilities, reduce formation damage and enhance productivity, some of the most common sand exclusion techniques used for horizontal completions include: the open-hole gravel pack (OHGP), stand-alone screens (SAS) and expandable sand screen (ESS). Gravel packing sand control is widely used in oilfield for the existing advantages, such as good sand control effect and little impact on productivity (Dong et al. 2009, 2010). Sand migration and plug are important factors affecting the validity length of gravel packed well. Therefore, it is of great significance to evaluate the influence of formation sand particles migration on gravel layer permeability and recognize the plugging mechanism clearly, which is important to predict productivity accurately and improve the effectiveness of sand control.

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