Casing corrosion mitigation in brine production wells

Challenges with casing corrosion in brine production wells

• Production wells suffer from the hyper-saline working environment, with corrosion causing havoc on ferrous materials.
• Corrosion affects the downhole well completion and life of the producing well.
• Corrosion has an impact on both finances and well as environmental concerns, and is an issue that needs to be taken seriously as a project moves into production.
• Accurate and fast corrosion monitoring systems are needed to effectively manage well maintenance so that overall field production is not affected.

Casing corrosion a wide-spread problem

The natural tendency of ferrous material to corrode, is only accelerated in the brinefield, where salinity levels can reach 250 – 280kppm – as salty as it gets. In hyper saline environments, corrosion is a major factor as tools and equipment are devoured at an accelerated pace. Mitigating this loss of material is essential to well integrity, which affects production and sustaining capex of a mine.

The consequences of corrosion in casing are loss in wall thickness and strength, leading to a reduction in ductility, usually leading to casing deformation or total failure.

Un-monitored corrosion can cause unexpected casing failures, resulting in expensive and complicated repair jobs. If left unattended this can lead to aquifer cross contamination and loss of production.

Casing corrosion is often accelerated where there has been damage to the casing during installation, or wear from repeated running of drilling equipment or pumps inside the casing.

Monitoring of corrosion

The operator must continually monitor and inspect the infrastructure to gauge the integrity of down hole and surface piping equipment. A variety of corrosion monitoring techniques are available. Some corrosion measurement techniques use in line monitoring tools placed directly in the production system. These tools are exposed to the flowing production stream. Other techniques involve laboratory analysis.

The weight loss technique using coupons, a direct visual identification method, is a well-known and simple monitoring method. This technique exposes a specimen of material – the coupon – to the process environment for a given amount of time before a technician removes it from the system and analyses is it for its physical condition and the amount of weight loss. However, the coupon alone cannot be used to accurately pinpoint the time or location of a corrosion event such as a leak.

Apart from this, the coupon technique is useful where easy access is available for placing and extracting the coupon, making it essentially impossible for the well’s down-hole tubular and casing strings.

The remaining options are non-destructive measurement techniques that incorporate one or more of the various logging tools that are deployed down hole on wireline.

Advances in down-hole corrosion monitoring

Logging techniques for monitoring downhole corrosion include ultrasonic, electromagnetic and mechanical methods that yield detailed information about the location and extent of a corrosion event.

Ultrasonic monitoring employs an ultrasound source to perform measurements and generate images of the downhole environment.

Most ultrasonic tools work by the principle of pulse echo measurement. Measurements include cement evaluation, open hole imaging and corrosion imaging.

An ABI televiewer, transmits an ultrasonic signal at a frequency of 1.2 MHz is designed for cement evaluation and pipe inspection.

The quality of the cement bond is directly related to the degree of casing resonance: a good cement bond dampens the acoustic signal and causes a low amplitude secondary signal to be returned to the transducer; a poor cement job or free pipe allows the casing to ring and returns a higher amplitude echo.

Additionally, the ABI measurements include 2D internal radius imaging of the casing – derived from the wave line of the main echo from the internal surface – and the 2D casing thickness, derived from the frequency response.

The ABI tool records two echoes: the main echo from the internal surface of the casing and the smaller echo from the external surface. The radius and thickness of the casing are computed from the arrival times of the two echoes. The relative sizes, or amplitudes, of the two echoes are qualitative indicators of the casing condition.

Ultrasonic inspection provides several advantages as a corrosion measurement tool, including its sensitivity to both internal and external defence and instantaneous in-field notification when a defect is encountered.

In addition, the technique requires access to only one side of the material to gauge the condition of the entire object and obtain detailed exterior and interior images of the pipe.

However, inspection is difficult for materials that are irregular in profile, such as filters.

Operators may also employ another corrosion monitoring method: electromagnetic (EM)-based inspection. The basic principle of this technique involves measuring the changes to a magnetic field as it passes through a metal object; the changes are related to the condition of the material such as its thickness and its electromagnetic properties.

There exist two types of EM corrosion monitoring tools. The first, a flux leakage tool, magnetizes the metal object using an electromagnet. When the magnetic flux encounters a damaged section of the metal; coils on the tool’s sensors detect this leakage. While this method is useful for measuring abrupt changes in pipe thickness, such as pitting or holes in the inner string, and the location of these changes, it is less effective for monitoring the steady increase of corrosion or corrosion that varies gradually over a large section of pipe or concentric casing configurations.

The second EM-based monitoring technology-the remote field eddy current tool-measures the signal of not only the primary EM field but also the secondary field from the induced eddy currents in the surrounding pipe.

The EM casing inspection tool has been used to detect large holes, casing splits and corrosion-related metal loss from both the internal and the external surfaces of casing.

These measurements can be obtained without the operating company having to pull completion tubing out of the hole, saving rig time, intervention expenses and interruption to production.

An initial reconnaissance run is carried out while the EM casing inspection tool is lowered on wire-line, in order to flag areas of interest for later detailed diagnostic scans to be run when later logging up to the surface. The tool can record a continuous log of both the average inner casing diameter and the total metal thickness, providing corrosion estimates. The tool reads overall metal thickness, enabling detection of corrosion of the outer casing. Radius measurements if the inner casing can be carried out despite the presence of most types of scale.

Identification of horizons with greater corrosion can be identified when the tool is used across several boreholes, allowing for future mitigation measures to be taken at these levels.

In addition to the acoustic and electromagnetic monitoring techniques discussed, mechanical methods are also helpful.

Multifinger mechanical caliper tools rely on direct contact with the pipe wall to measure the deformations arising from the loss of metal due to corrosion or the build-up of scale, however, they do not give any information about the external condition of the pipe wall.

The multifinger caliper tool can be deployed with as many as 60 fingers, depending on internal casing diameters, providing a 3D mechanical image of internal pipe-wall variations.

Repeated measurements can track corrosion over time, allowing Operators to make decisions based on real, and not inferred, data, without the need for removal of the casing until the required time, optimizing down time.

Combining Measurements for Improved Corrosion Monitoring

The combination of various tools greatly improves the operator’s understanding of reasons for localized corrosion.

Improved Corrosion Mitigation Through Management

Greater understanding of tubing integrity can be obtained from the use of downhole corrosion monitoring tools allowing the operator to make more-informed and cost-effective mitigation and repair decisions.

As more and more steel pipes are placed in hyper-saline environments, corrosion monitoring will become and increasing problem, and an adequate monitoring system is essential to allow operators to make both profitable and environmentally responsible decisions.

For assistance in monitoring well condition, contact Zelandez, the pioneering brinefield specialists.

PipeSight

Our Insight product PipeSight offers detailed analysis of the well casing condition and completion, using the benefits of the EM, Multi-Finger and Acoustic Televiewer tools.