Some Influences of Saturation and Fluid Flow on Sand Production: Laboratory and Discrete Element Model Investigations
Authors: Bruno M.S. , Bovberg C.A. , Meyer R.F. 
Published: December 2007

Abstract
A combined program of laboratory strength measurements on core samples, sanding cavity growth and visualization studies with a physical sand-pack model, and numerical modeling simulations using the discrete element method (DEM) is currently underway to investigate the influences of fluid saturation and fluid flow gradient on sand production.
The experiments to date have revealed some interesting and important sanding mechanisms. At a given global pressure gradient, sand cavity growth increases with higher water saturation. Sanding appears to occur in episodic stages. At a given flow rate and saturation condition, sand cavity growth progresses and then stabilizes. Additional cavity growth then requires either an increase in pressure gradient or an increase in water saturation. Discrete element modeling of sandpacks seems to capture the observed sanding behavior, including the influences of saturation and flow rate and the episodic progression and stabilization of sanding cavities.

Introduction
Field evidence indicates that sanding in wells tends to increase when the water content of produced fluids increases. Several mechanisms have been proposed to explain these observations. It is possible that the inherent rock strength or cementation properties of the material are degraded by high water saturation. It has also been speculated that a reduction in capillary pressure-related grain bonding, due to increasing saturation of the wetting phase fluid, contributes to increased sanding at high water saturations. Finally, it is possible that for gas wells increased water saturation produces an increase in the near wellbore pressure gradient, due to changes in pore fluid viscosity and relative permeability to gas.
To quantify some of these influences on sand production and to develop improved predictive tools, we are pursuing a combined experimental and modeling program. The program includes:laboratory strength measurements on consolidated core samples,sanding cavity growth and visualization studies with a physical sand-pack model, andnumerical modeling efforts.
This paper summarizes some of the preliminary results from the sandpack experiments and numerical modeling.

Sandpack Experimental Approach
To simulate and monitor sand cavity growth, a vessel 12 inches in diameter and 18 inches in length was constructed (see Figure 1). This apparatus is capable of simulating only a limited range of conditions: cavity growth on the order of inches in diameter and flow rates on the order of several psi per foot. In the field, sand cavities can range from about an inch to several feet in diameter and pressure gradients from zero to hundreds of psi/ft. Even with this limited range, however, we believe the physical model can provide useful insights into sanding mechanisms.
The following typical experimental steps are followed to investigate sand cavity growth:

1) Prepare a sandpack to 100% water saturation.
The vessel is initially filled with water. A fine grained, clean, silica sand is then added to the water and compacted in stages until the vessel is completely filled with 100% saturated sand. The volume of dry sand is about 17,000 cc and the initial volume of water is about 6,000 cc. This represents an average porosity of about 26%.

2) Displace water with air to achieve desired water saturation
The vessel is rotated 900 to its side (as shown in Figure 1).
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