ABSTRACT:

Whenever designing gravel pack for deepwater oil wells, where 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. A gravel pack prototype was built and installed in a large poliaxial frame for mechanically testing the two most common mesh sizes: 16/20 and 20/40, in order to check on the influence of the gravel size under high in situ stress contrast. The gravel pack 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 in two perpendicular orientations. Visible screen damage was observed when testing the gravel agent of 16/20 mesh, as compared to the 20/40 mesh, under the same stress field, indicating that smaller particles size are better, regarding equipment integrity (one should always expect permeability reduction with smaller particles in the pack). As a conclusion of the realized tests, in association to the usual requirements for sand control with gravel packs, it is recommended the utilization of 20/40 mesh, instead of 16/20 mesh, for gravel pack installation in deepwater oil wells, at depths with high in situ stress contrast, in order to improve equipment operational life.

1. INTRODUCTION

A major problem in hydrocarbon production from poorly consolidated sandstone is the influx of sand particles (sand production). This is a complex phenomenon, affected by the in situ stress contrast, mechanical properties like formation strength, rock cohesion and also wellbore standard operations. In order to avoid or minimize operational problems due to equipment erosion, and decrease in production rates, several techniques of sand control have been developed for the oil industry. This paper shows an analysis of the gravel pack system behavior under high stress contrast through laboratory tests, carried out in a big poliaxial frame. Laboratory tests for reproducing well condition with large samples have been made by a few authors [1, 2, 3, 4], with the main advantage of minimizing boundary effects, allowing thus a better representation of the analyzed phenomena. The true triaxial (poliaxial) testing with cubical samples was adopted for increasing the stress field contrast having S1 ? S2 ? S3. The difference among triaxial laboratory tests is shown on Figure 1. The utilized poliaxial frame had 6 pistons independently pressurized and manually controlled. The equipment is installed at the Petrobras Research and Development Center - CENPES, in Rio de Janeiro, Brazil [5], allowing a variety of tests on large-scale cubic samples. Figure 2 shows an overview of the equipment.

2. PHYSICAL MODELING

The goal of the work was to analyze the influence of the gravel diameter on the gravel pack system under high stress contrast. Two common gravel sizes were tested: mesh size 16/20 and 20/40. The gravel pack prototype was installed in the central hole of the cubic samples, horizontally placed in the frame for simulating horizontal wellbores.

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