Remediating Sustained Casing Pressure by Forming a Downhole Annular Seal with Low-Melt-Point Eutectic Metal

Proposal

Described are proof-of-concept developments to form a seal for mitigating sustained casing pressure caused by annular pressure buildup. Annular pressure can result from numerous sources, including tubing leaks, loss of isolation potential within the cement column because of poor mud displacement, free water-induced channels, stress fractures, and failure of the cement to cover all potential sources of annular pressure. In most cases, annular pressure is not observed at the wellhead until the well is placed on production, making it difficult to identify, access, or remediate the pressure source. A new and novel approach to remediation has been tested in which a low-melt-point alloy metal is dropped down the backside of the casing where annular pressure has been observed. The metal is allowed to accumulate at the top of cement or other physical barrier, melted with an induction-heating tool, and allowed to cool and solidify. This process forms an annular seal to stop fluid communication between the formation and wellhead.

This method was demonstrated within a full-scale, simulated well section. An electromagnetic induction tool provided sufficient localized heating to completely melt solder-type alloy metal placed between concentric casings. Subsequent pressure-testing verified that a complete melt, sufficient to provide an effective seal against fluid pressure, was achieved in both water- and synthetic-based drilling fluids. Shear-bond test results of various alloys were equal or superior to cement, and the solid-liquid phase transitions (set points) occurred at precise temperature levels. All metals tested contained bismuth because of its unique characteristic of expanding upon solidification to provide enhanced pressure-containment performance. Full-scale testing was conducted using 17-ft long concentric annular models constructed of 8-in. and 5-in. diameter steel pipes. Subsequent field-testing is currently being planned.

Introduction

This paper summarizes experimental efforts to form an annular seal for the purpose of mitigating sustained casing pressure, or annular gas pressure buildup. Annular pressure can result from numerous sources such as tubing leaks, loss of isolation potential within the cement column caused by poor mud displacement, free water- induced channels, stress fractures, or failure of the cement to cover all potential sources of annular pressure. In most cases, annular pressure is not observed at the wellhead until after the well is placed on production, making it difficult to resolve the matter, i.e., isolate the zone from which formation fluid communication is taking place. There is seldom a feasible means of physically reaching any of the key points of fluid communication after the occurrence has been observed. Past efforts to remediate have been compromised by failure to reach deeply into the annulus and by chemical contamination of the remedial sealant.

The proposed method drops a low-melt-point alloy metal down the backside of the casing where annular pressure has been observed. The metal is then melted by an innovative heating process and allowed to cool and solidify. The intent is to form an annular seal to stop fluid communication between the rock formation and the wellhead as deeply within the annulus as physically possible. This concept was tested using a commercial tool developed for use in artificial lift that produces heat at select locations by electromagnetic induction. The tests described are also intended to determine whether this tool has application for the purpose described.

Background

A major purpose of primary cementing is to form a permanent seal between the borehole wall and the casing run into it. Total success in this effort implies that all nonproduced formation fluids remain in their respective formations for the entire productive and post-abandonment life of the well. Sustained casing pressure (SCP) detected on the backside of the casing can be an indication of fluid or gas movement within the annulus. This movement can result from failed or insufficient cement coverage, communication through tubular connections and seals, or thermal expansion of fluids in a confined space during production operations. This discussion will focus on remediation of fluid movement or communication.