Well Intergrity Interpretation
Well integrity, in regard to oil wells, is defined by NORSOK D-010 as the “Application of technical, operational and organizational solutions to reduce risk of uncontrolled release of formation fluids throughout the life cycle of a well”.
Well Integrity is defined in ISO/TS 16530-2 as: “containment and the prevention of the escape of fluids (i.e. liquids or gases) to subterranean formations or surface”
In API RP 65-2, well integrity is defined as: “a quality or condition of a well being structurally sound with competent pressure seals by application of technical, operational, and organizational solutions that reduce the risk of uncontrolled release of formation fluids throughout the well life cycle “.
Well Integrity Interpretation Services
Our well integrity interpretation services concern the following logging tools data:
The Cement Bond Log (CBL) gives a continous measurement of the amplitude of sound pulses from a transmitter to receiver. This amplitude is maximum in free pipe and minimum in well-cemented casing. The wave train can be displayed as a Variable Density Log (VDL) where the positive and negative cycles of the wave train are shaded black and white respecticvely.
While cement bonds that are either complete or totally absent can be clearly identified, signals showing partial bonds provide insufficient data to determine hydraulic isolation.
From VDL analysis, we can detect undesirable micro-annulus & channeling and 4 cases of Cement bonding index:
- No bonding ( no cement at all) : free pipe, free formation
- Partial bonding «cement/formation»: free pipe (or micro-annulus)
- Partial bonding «casing/cement»: free formation (channeling)
- Total bonding «casing/cement/formation»: Good cement bonding index
Radial Bond Tools
A Radial Bond Tool (RBT) overcomes this problem by using multiple receivers positioned circumferentially around the tool. Each provides bond data covering a sector of the casing so that a ‘map’ of the cement can be generated in addition to a conventional CBL log.
Ultrasonic Scanner Tools
Ultrasonic tools are superior to the acoustic CBLs, although they remain adversely affected by highly attenuating muds. They are used for 2 modes: cement evaluation and casing inspection.
The most recent tools have a single rotating transducer that incorporates both the source and receiver of ultrasonic energy. The tool has to be centered. The data for circumferential inspection of the casing, as described above, and for the evaluation of cement bonding are obtained on the same logging pass. Acoustic energy is reflected at interfaces that correspond to changes in acoustic impedance (the product of acoustic velocity and density). The first reflection is at the casing itself. The second reflection may be at the outside of the casing. If cement is bonded to the casing, there will be a strong reflection. If there is unset cement or water behind the casing, there will be a weak reflection. The received waveform is the sum of the reflected waveform from the original burst and the exponentially decaying waveform from the resonant energy that is trapped between the inner and outer edges of the casing. By analyzing the entire waveform, an acoustic-impedance map of the cement can be constructed. This map can indicate the presence of channels and their orientations.
The multi finger tool is used to detect very small changes to the internal surface condition of tubing or casing with a high degree of accuracy. The tool may run with extended length fingers to increase the measurement range. The multi finger tool is available in a range of diameters to suit varying casing/tubing sizes. The number of fingers increases with the diameter of the tool to maintain maximum surface coverage. The tools can be run in combination with other Well Integrity instruments and Ultrawire™ Production Logging tools. A continuous measurement of the internal pipe’s surface condition is made as the tool is logged up.
Multi-Finger Imager Processing Software, is the industry’s leading package for multi-finger caliper & Magnetic Thickness data processing and interpretation, and now includes a comprehensive reporting option for pipe thickness measurements. We provide detailed analysis and clear visualization that enhances the interpretation of caliper and pipe thickness log data. We maximize data value by efficiently identifying and diagnosing well integrity issues.
Magnetic Thickness (Flux-leakage tools / Electromagnetic phase-shift tools )
Flux leakage is a semiquantitative method that uses a strong magnetic field to identify and, to a certain extent, quantify localized corrosion on both the inner and the outer surfaces of the casing. A downhole electromagnet that fits snugly within the casing creates a low-frequency or a direct-current magnetic field. Magnetic flux is concentrated within the casing, which is close to magnetic saturation. The tool contains spring-loaded, coil-type, pad-mounted sensors that are pushed close to the casing during logging. Where casing corrosion is encountered, the lines of flux “bulge out” from the casing as though they were leaking from it. The primary sensors pass through this excluded flux and measure the induced voltage. The amplitude and spatial extent of the sensor response is related to the volume and shape of the corrosion metal loss, thereby allowing an estimate of the size of the defect.
The electromagnetic phase-shift technique provides an estimate of casing thickness across approximately 1 ft [300 mm] of casing length, so its spatial resolution is weaker than that of the first methods. Electromagnetic phase-shift tools make measurements that are averages around the circumference of the pipe. They lack the localized investigative capability of flux-leakage tools and are best used to investigate larger-scale corrosion. Essentially, a transmitter coil generates a low-frequency alternating magnetic field, which couples to a receiver coil. It also induces eddy currents in the surrounding casing and formation. These eddy currents generate their own magnetic field, which is phase-shifted by the presence of casing. The phase-shifted field is superimposed on the transmitted field. This total field is detected by a receiver coil. The phase shift between the transmitted and received signals is related to the thickness, electrical conductivity, and magnetic permeability of the casing. If the last two are known, the casing thickness can be determined. In practice, the electromagnetic properties of the casing can vary with composition, aging, and stress. To overcome this problem, modern tools comprise multiple sensor coils, which allow variations in the electromagnetic properties of the casing to be factored into the computation of casing thickness. Advantages are that the method is sensitive to large areas of corrosion and to gradual thinning of the casing. The sensors do not need to be in close proximity to the casing, so a single tool can examine a range of casing sizes. Disadvantages are the low spatial resolution and the lack of response to nonmagnetic scale. Moreover, the alternating-current magnet requires a relatively high power, which makes the tool difficult to deploy in memory mode.
Mini-PLT provides the customer with highly accurate data sets of downhole production information.The main PLT sensors used for Well Integrity are Temperature, Pressure & Flowmeter. The string can be run in realtime or, by simply changing the telemetry tool for a memory section, in memory mode on slick line or coiled tubing.
The noise-logging tool “listens” passively to downhole noise, for example, from gas bubbling up through liquid in the wellbore. Behind pipe, a channeling flow often passes through cramped spaces and constrictions. These “tight spots” cause high velocities, sudden pressure reductions, and significant flow turbulence. The noise tool listens to the noise associated with the turbulence.
Spectral noise logging (SNL) is an acoustic noise measuring technique used in oil and gas wells for well integrity analysis, identification of production and injection intervals and hydrodynamic characterisation of the reservoir. SNL records acoustic noise generated by fluid or gas flow through the reservoir or leaks in downhole well components.
Over many years, downhole noise logging tools proved effective in inflow and injectivity profiling of operating wells, leak detection, location of cross-flows behind casing and even in determining reservoir fluid compositions.