Abstract
Sand grains can be dislodged and mobilized when exposed to high fluid flow drag forces in oil recovery from poorly consolidated sandstone reservoirs. Sand production usually enhances the wellbore inflow performances; however the drawbacks such as erosion and clogging of production lines and premature water breakthrough in waterflooding outweigh the benefits. In this study, an existing coupled 3-D multiphase flow/geomechanical code is further developed and implemented to model the propagation of dilated zone resulting from sand production. The dilated zone grows when the forces from fluid flow exceed the confining effective forces. The constituent materials’ mechanical behavior is described by a nonlinear hyperbolic model in which the stress variation over the Mohr-Coulomb shear failure surface is honored by a force rebalancing scheme. The study results indicate that the dilated zone geometry is a strict function of the weak layer thickness. The small load bearing capacity of the failed region leads to formation of a low stressed strip at the cavity/sand interface which aids the sand production by pressure gradient. The results of the current study can help understanding the significance of different mechanisms contributing to sand production in order to mitigate the premature water breakthrough problem in oilfield waterflooding practices.
1. INTRODUCTION
Drag forces concomitant with fluid flow through loosely consolidated porous media might be strong enough to dislodge and mobilized the distorted sand grains towards the producer. The sand production phenomenon usually leads to a significant enhancement in well productivity [1]. Nonetheless, uncontrolled sand production may result in operational problems such as costly workovers in flow production eroded equipment and the environmental issues associated with disposal of produced sand [2, 3]. Sand production usually deteriorates the sweep efficiency causing premature water breakthrough in field waterflooding operations. The problem costs the oil and gas industry hundreds of millions dollars annually [4].
Various causes of sand removal from loose formations into production wells have been quoted by different authors [5, 6]. Vaziri et. al. [6] have divided sand production into early pressure induced and later production or water-cut induced stages. The former stage is triggered mainly by mutual acts of high drag forces and mechanically weak formation. However the latter is regarded to reduction in inter-granular capillary tension due to increase in water-cut. Dissolved gas liberation in multi-phase flow (i.e., foamy oil in heavy oil reservoir) is also a significant contributing factor in sand production [1, 4]. Morita and Boyd [7] have mentioned some sand production problems observed in the field due to reservoir pressure depletion in relatively strong formations, abnormally high tectonic force in relatively strong formations and sudden changes in flow rate.
