Mechanical Performance of a Gravel Pack Prototype Under High Stress Contrast
Authors: Villarroel Flavia Mara Guzman , Bloch Mauro , Vargas Euripedes 
Published: July 2010

Abstract
A gravel pack prototype has been installed in a large poliaxial frame for testing the two most common mesh size: 16/20 and 20/40, in order to check on the influence of the gravel size. This system was set on artificial cubic rock samples of 11.8 in side with a centered hole of 2.4 in diameter. The block was then compressed by hydraulic actuators (pressure capacity of 9 kpsi each) in two perpendicular orientations, manually controlled within ± 20 psi. In the first stage of the test, the sample was loaded and unloaded twice, in both orientations, for experimental adjustments. In the second stage, the pressure was increased at a specific rate in all the actuators, creating a final triaxial stress field on the block given by: S1 = 1,200 psi; S2 = 600 psi; and S3 = 0.0 psi.
Designing gravel pack for deepwater oilwells, when it is not possible to repair operational problems without very high costs, one should take into account the needs for a longer term operation, being more conservative in the gravel size selection. Visible screen erosion was observed when testing the gravel agent of 16/20 mesh, as compared to the 20/40 mesh, indicating that smaller particles size are better, regarding only equipment integrity (one should always expect permeability reduction with smaller particles in the pack). As a conclusion of the realized tests, it is recommended the utilization of 16/20 mesh, instead of the 20/40 mesh for gravel pack installation in deepwater oilwells, whenever under high in situ stress contrast, to improve equipment operational life.
Oil production from unconsolidated sandstones has been a constant challenge for the oil industry, leading to the development of several methods for sand control. The most applied technique in deepwater Brazilian oilfields is the well known gravel pack. In the mechanical point of view, gravel pack equipment is mainly affected by in situ stress magnitudes, which can permanently damage and compromise future well productivity.

Introduction
A major problem during hydrocarbon production from poorly consolidated sandstone is the influx of sand, or sand production. This problem is complex and can occur as a result of a large variety of reasons, like magnitude of the in situ stresses, formation strength, and wellbore-development operations. To avoid, or minimize this problem, which could result in operational problems, equipment collapse, and decrease in production rates, there are a lot techniques of sand control in the industry.
This study has been developed considering the massive use of the gravel-pack as sand control technique on Brazilian fields and the main objective was the investigation of the gravel-pack system behavior under high stress contrast. Laboratory tests help the observation of some phenomena in a reduced scale, but to turn it out on a more realistic physical model, some large-scale laboratory tests were carried out on a poliaxial cell.
Laboratory tests for reproducing well conditions with large samples have been made by a few authors (Kooijman, et al, 1992; Van Dam et al, 1999; Haimson, and Lee, 2004; Haimson, 2007;), and the main advantage of the large-scale laboratory test is the minimization of border effects, allowing a better representation of the analyzed phenomena.