The cement job is one of the final operations during completion and has a major importance on the performance of the well productivity. A poorinterpretation of the cement job quality due to lack of information can lead to wrong decisions, such as inappropriate squeeze jobs and at worse poor reservoir fluid evaluation, usually these operations are expensive and result in highlosses.
The use of high vertical and horizontal resolution quantified acoustic impedance images from ultrasonic measurements brings a new perspective incement evaluation for oil/water wells and combined with traditional CBL-VDL log provides a complex understanding and definition of cement quality in oil/gas reservoirs helping the completion efficiency. Examples are presented and the interpretation is discussed showing the superiority of the measurements and results.
The lack of cement integrity can compromise the final well production and cause unwanted communication between different reservoir zones, which in some cases can result even in ecological damage when, for example, a hydrocarbonzone communicates to an aquifer, situation which can happen where this was carried on. Cement evaluation logs are being studied more and more in order to achieve a full understanding of the cement quality and performance of the cement job.
The usual interpretation of cement evaluation logs is based on sonic and ultrasonic principles, each one has its applications and limitations. Lately, with the introduction of the new wireline logging tool for cement evaluation and corrosion detection by ultrasonics, the USI* (Ultrasonic Imager), the cement quality analysis has had a great improvement due to its high vertical and horizontal resolution of the quantified acoustic impedance image display and together with the traditional CBL-VDL log, a conclusive interpretation of the cement quality can be achieved.
Some problems like gas contaminated cement and channelling have always been causing doubts on the interpretation of cement evaluation logs and in several cases increasing costs due to cement correction with squeeze jobs.
The technique proposed here is based on the fact that the information of the CBL-VDL log and the full radial cement acoustic impedance image, provided by the USI, complement each other, greatly eliminating interpretation doubts and giving a conclusive definition on the decision to run a squeeze job or not.
The detailed principles of the CBL-VDL (Fig. 1) and the USI (Fig. 2) log is well covered in the literature and can be found in several technical documents.
The CBL-VDL tool has a monopolar sonic transmitter which works at 20 KHz and the signal propagates in all directions. The log response, in simple cases, canbe interpreted and provides good information about the cement quality. Nevertheless, due to its characteristics a radial information is not provided and in cases where cement channels occurs, several doubts can arise, resulting in worthless expensive squeeze jobs or at worse the exclusion of a needed squeeze jobs. Besides, the CBL measurement is affected by microannulus and in gas contaminated cements the CBL-VDL log gives the wrong impression that the cement is good misleading the interpretation. In fact, one should be aware that this gas contaminated cement has a reduced compressive strength and due to that, even if today it is selling the annular space, may be in the future the cement can break depending on the differential pressures acting on it.