Method and apparatus for continuously checking casing cement quality

A method for time-lapse monitoring of casing cement of a cased well, the method comprising:

• passively detecting acoustic waves using a distributed acoustic sensing system, wherein the distributed acoustic sensing system comprises:
• an acoustic source permanently installed at the cased well that generates acoustic waves over a production lifetime of the cased well;
• and
• a distributed acoustic sensor permanently installed at the cased well and comprising:
• an optical fiber having inhomogeneities that reflect and backscatter light;
• a pulsed laser that couples to the optical fiber and transmits pulsed laser light along the optical fiber over the production lifetime of the cased well, wherein a portion of the pulsed laser light is reflected and backscattered;
• a detector that detects, over the production lifetime of the cased well, the reflected and backscattered laser light;
• and
• a processor that, over the production lifetime of the cased well, controls the pulsed laser, receives signals from the detector, and converts the signals into acoustic information;
• and
• analyzing time-lapsed acquisition of the acoustic information from the processor to identify, in real time, a cement fracture progression of loss of well integrity in the cased well.