Case History: Water Shut-off using Plastic Resin in a High Rate Gravel Pack Well
Authors: De Landro Wendell V.C. , Attong David 
Published: December 2007

Amoco Trinidad Oil Company has successfully field tested a non-rig isolation procedure to shut-off water encroachment in two of its gravel packed wells using a plastic resin plug. Amoco operates a gas production platform offshore southeast Trinidad with high volume gravel packed wells. High transmissibility of formation fluids in the completion near the wellbore usually makes it difficult to control unwanted water production. In the past. several techniques to control water have been attempted with varying degrees of success. These included setting plugs in the prepacked gravel screens or installing pre-designed hardware to plug off anticipated future water movement. These two techniques are usually unsuccessful because 1) the uncertainty in predicting reservoir performance and 2) the water bypasses any plugs in the screen assembly and flows through the high screen-casing annulus. Cement squeeze techniques work in medium to high pressure reservoirs but require rig assistance and potential damage to the completion may be sustained. The plastic resin technique uses dump bailers on slickline to place the resin across the desired interval which hardens and seals the flow area in the screens and effectively eliminates the permeability in the gravel thereby shutting off water flow from the reservoir. Because rig assistance is not required, this isolation procedure becomes more economical depending on the remaining gas reserves. Ultimate gas recovery is anticipated to be greater by having economical options available to pursue remaining reserves.

Introduction

Amoco's Cassia field held the major reserves for supplying high pressure to satisfy our contractual obligations, Figure 1 shows the location of the Cassia field offshore. In 1983, production began and ended with nine wells draining four reservoirs. The '22' sand which had a significant amount of reserves remaining showed early signs of water production. The primary concern was that the reservoir would water out before the remaining reserves could be produced. Deliverability from this reservoir was 70 mmcfd, being supplied by Cassia #8 and #10. A structure map of the '22' sand can be seen as Figure 2. A shortfall situation in the supply of high pressure gas on the island was looming on the horizon. The problem at hand was how could we prolong the life of the 22 sand and maintain deliverability. The implications were enormous, there would be power outages throughout the country, plants at the Pt. Lisas industrial estate for example methanol, steel, urea and other plants being starved for gas and the resultant law suits for breach of contracts with the various parties involved. The method adopted to solve this problem had to be clinical, there was no room for error deliverability had to be maintained. One of the overriding constraints was that there was no rig readily available to repair the wells. The solution entailed attempting a water shut-off in one of these wells using a safe technique. After a lot of research, a resin plug was selected as the optimum solution.