A cement barrier in an oil well is intended to prevent any movement of fluid along the wellbore. If there is movement of fluid, acoustic energy will be produced and may be detected by a suitably sensitive sensor. This paper describes the controlled testing and real world application of a new passive acoustic tool and the benefits it brings to evaluation of even very small fluid migration issues.
Traditional methods of cement analysis rely on evaluating the placement and condition of a cement barrier, which may confirm the presence of cement behind the casing or liner. There are innumerable examples of cemented annuli that exhibit sustained annular pressure at surface – an indication of fluid migration from a charging source downhole – despite a good cement condition log having been acquired. In such cases there will be acoustic energy generated by the movement of even small amounts of fluid, the presence of which may be used to indicate a failure of performance of the cement.
A number of test cells were used to recreate realistic downhole scenarios of fluid flow, the cases including free-pipe, a single small channel, an exterior micro-annulus and "good" cement. The sealing performance of the annular barrier was tested against both gas and liquid over a range of flowrates and pressure differentials to best simulate the downhole environment. Reliable detection of annular flow at rates as low as 0.03 litres per minutes was recorded for liquid, while gas migration was detectable even at very low differential pressure. The latter case is of great interest in wells planned for permanent abandonment where even slow annular pressure build-ups are unacceptable. A logging campaign covering 8 wells was subsequently performed; rig-less and with production tubing still in place; to identify the source of B-annulus charging. The new technique was able to successfully identify areas of gas migration in each of the wells allowing detailed planning to be conducted prior to bringing in a rig.
The analysis of cement from the perspective of absolute performance as opposed to the traditional approach investigating the condition allows very detailed analysis of well barriers. This is especially useful in operations for permanent abandonment of wells were any leakage is unacceptable. The ability to map the fluid migration from source to surface is a powerful tool in the efficient design of remediation programs.