Abstract
Typical industry standard for evaluation of cement integrity at downhole conditions is through the use of acoustic measurements. These systems generate compressional acoustic waves which travel through wellbore fluid to impact the casing. Attenuation of these waves is used to derive cement compressive strength or acoustic impedance bonded to the casing.
Some of the long lasting fundamental limitations faced by the current technology are:
The wellbore must be filled with liquid for the compressional wave to be able to travel to the casing wall. Heavy muds and solids particles in the mud can cause further complications.
The in-situ compressive strength may not fully characterize cement mechanical properties.
Micro-annulus condition can be difficult to differentiate from poor bond condition without a separate pressure pass.
A new technology, which uses electro-magnetic acoustic transducers (EMAT), has set a new industry standard for cement evaluation. This new sensor technology is incorporated into an established and highly successful sectored pad-type tool design. This tool design provides multiple measurements azimuthally, making fully compensated measurements, allows good tool centralization in high wellbore deviations, and enables evaluation of multiples casing sizes in a single pass.
The primary mechanical forces responsible for the failure of the cement sheath are shear forces differences between the wellbore and surrounding formation. The new technology measures the horizontal shear wave which responds to the cement in-situ shear modulus coupled to the casing.
EMAT technology is wellbore fluid neutral and as such, eliminates the need of filling wellbore with fluid prior to logging. This presents a very important advantage as the cement condition can now be evaluated in downhole conditions similar to conditions when the well is on production. Additionally, fluid-filled micro-annulus can be better recognized without the need to pressurize the casing.
The wider dynamic range of the new measurement also yields a higher resolution evaluation, provides us with the ability to evaluate cement bond in a wide range of heavy cement slurry density down to ultra-light cements and in contaminated cements, as well as other slurries types such as resins.
This paper will discuss theoretical background, numerical modeling and actual recorded data demonstrating the above breakthrough advancements in cement sheath evaluation at in-situ conditions.
While preserving the advantages of current technology, the EMAT measurement now provides operators with much needed flexibility to analyze, with consistent high resolution, cement bond quality over a wide range of slurry densities. Operators are no longer required to isolate existing opened perforations and load wellbores with fluid in preparation for logging, resulting in significant time and cost savings.
