Understanding and Preventing Incomplete Shunted Open Hole Packs in Trinidad's Colibri Field

Abstract

Three out of four shunted gravel packs in Colibri suffered from severe losses near the end of the gravel pack and failed to achieve a final screenout even though as much as 50% excess gravel was placed. This study uses reservoir, completion, and bottom hole gauge data to understand why the final screenout was not achieved and how to ensure complete packing/final screen-outs in the future. The evaluation started with building representative models for each well in a gravel pack simulator to predict the pressures and packing mechanisms downhole. The data used in the models included the relevant completion and reservoir data. The model was then used in conjunction with the treatment records (surface pumping data and bottom hole gauge data) to evaluate the packing extent and sequence for different parts of the openhole. This evaluation pinpointed the time and location where losses occurred as well as the extent of openhole packing in each section. Various theories were formulated to describe the observations and one theory was selected as the most likely. The evaluations showed a consistent packing behavior across all the wells in which the screens were initially covered without the aid of shunts from toe to heel with much less gravel than required to fill the annulus void. After this initial packing stage, shunt packing began, and additional packing in all of the sections occurred. During this part of the packing process, losses began which resulted in all of the final slurry exiting the open hole into the reservoir. Based on the analysis, the most likely cause of the strange packing behavior was the combination of low screen inflow area, relatively low gravel permeability, higher fines content in the natural gravel sand and high gravel carrier fluid viscosity, which resulted in a high pressure drop across the pack/screen interface. The one well which achieved a final screenout had a lower pumping rate, resulting in lower pressure drop.