Gravel-Packing Studies in a Full-Scale Deviated Model Wellbore
Authors: Shryock Steven G. 
Published: December 2007

Summary
Full-scale visual studies in a deviated model wellbore have addressed several gravel-pack completion design factors. Visual observations of the placement process have shown ways to improve deviated-well completion designs and field gravel-packing practices. Factors controlling gravel transpose and deposition were viscosity. flowrate and leakoff of carrier fluid, wellbore geometry, and gravity settling.

Introduction
Successful sand control in deviated wells has become more important with the continuing increase in offshore drilling. Large oil and gas fields are being produced by means of directional wells from central platforms in deeper waters. Many of these wells penetrate formations at angles greater than 60 degrees (1.05 rad). Gravel packing, the most common and effective well completion for sand control, is not commonplace or routinely successful in these highly deviated wells. Gravel packing of deviated wells is more difficult than packing near-vertical wells simply because of gravity. In high-angle holes, gravel tends to dune in the upper end of the well-bore and bridge off the annulus, causing a premature sandout. The unpacked interval below the dune can lead to failure of the well completion. This study used a full-scale model wellbore to examine visually the gravel-placement process in highly deviated wells, The experiments are a continuation of studies begun with a vertical model. Although many of the same characteristics of vertical well packing occur in deviated wells, the change in wellbore angle has a significant effect on the gravel-packing process.

Background
Several other investigators have studied this problem with smaller models. Maly et al. described the duning phenomenon in detail. In their experiments with a water carrier fluid, gravel formed a dune and packed from the top of the well downward when the deviation was greater than 45 degrees (0.79 rad), but in more-vertical wells. the gravel packed from the bottom up. For successful top-down packing, the carrier fluid had to flow over the dune continuously and with adequate velocity. If enough fluid escaped through the gravel and into the liner sufficiently to reduce the velocity over the dune, the annulus would bridge, resulting in an incomplete pack. Maly et al. also found that the final dune length was affected by variables such as liner slot size, gravel concentration, and flow rate. They anticipated that liner slot plugging, carrier fluid viscosity, and flowrate also would affect dune length. Packing efficiency of deviated wells was improved by placing a number of deformable baffles at regular intervals along the tailpipe. These baffles helped maintain flow in the channel over the dune by restricting flow in the liner-tailpipe annulus. However, factors such as the pressure drop needed to deform the baffles and baffle spacing were critical. Gruesbeck et al. also described the deviated-well gravel-packing process from observations of scaled model experiments. They reported that in wells deviated more than 45 degrees (0.79 md), incomplete packing would result without special considerations. Gravel, carried by either low- or high-viscosity fluids, will bridge unless the design variables are selected properly. Packing efficiency increased with (1) decreasing gravel concentration, (2) decreasing particle diameter. (3) decreasing particle density, (4) increasing fluid density, (5) increasing flowrate, and (6) increasing resistance to flow in the liner-tailpipe annulus.
JPT
P. 603^