Abstract
A well-cemented casing is critical for the safety and performance of the well. Cement reduces risk of casing failure, provides hydraulic isolation, prevents unwanted production of formation fluids and protects ground water. Uncured cement and micro-annulus are two common conditions which are incorrectly interpretated as poor bond from conventional cement integrity logs. Use of an innovative shear and flexural technology makes it possible to confidently identify these two-condition preventing needless and costly remedial actions.
Conventional bond logging instruments generate compressional acoustic waves and show poor bonding, if recorded sooner than the time needed by cement downhole to achieve full compressive strength. Existence of a casing-cement gap, in a micro-annulus condition, also affects attenuation of compressional wave, making it difficult to differentiate from poor bond condition without a separate pressure pass. An Electro-Magnetic Acoustic Transducer uses shear waves responding to in-situ shear modulus of the cement coupled to casing. A flexural wave mode is also measured, with particle displacement normal to casing surface. Together, these measurements provide an unambiguous bond evaluation in both these conditions.
Cement slurries are designed and tested, in laboratory environment, to achieve a certain downhole performance on strength and curing time. However, the downhole conditions often vary from the assumptions made in the laboratory and a variety of conditions can develop behind the casing. The cement may require additional time to fully cure, cement properties such as density may be affected by downhole contamination, the cement sheath could be present but not bonded to the casing (micro-annulus). Long curing times are undesirable for the efficiency of completion programs. Pressure passes to close micro-annulus prior to recording bond log, is time-consuming and may not be even possible due to weak casing condition, open perforations, leaking plugs, pressure setup requirements, etc. Decrease in slurry density can affect attenuation of compressional wave resulting in incorrect estimation of cement strength and coverage. Through multiple field examples, of cement evaluation, using Electro-Magnetic Acoustic Transducer (EMAT) technology, we will demonstrate that a clear identification and cement bond evaluation can be done against green cements, altered or low-density cements, and micro-annulus conditions, avoiding need for extra pressure passes and enabling confident decision making on remedial cement repair or subsequent completion steps.
EMAT sensor technology and associated measurements are relatively new in cement evaluation. The technology is able to correctly evaluate cement bond over a wide range of slurry densities, offering completion engineers flexibility on curing times needed prior to recording cement evaluation logs. Squeeze intervals, if required, are picked with certainty avoiding unsuccessful multiples repair events with associated time and cost overruns.
