The Pre-salt reservoirs, located in deep water offshore Brazil, boast vast oil and gas reserves and are characterized by very long and thick evaporite formations, mainly halite and anhydrites with thin layers of carnallite. The geo-mechanical properties of these layers impose several obstacles to the well construction activities, being one of the most critical to the well annulus barrier placement and the assessment of its integrity by direct verification using wireline logs. This study highlights the substantial effects encountered in acoustic measurements caused by salts slowness and delves into the complexities of evaluating thick casings. Understanding and mitigating these challenges with proper logging tools are imperative for successful cement evaluation and ensuring the integrity of wellbore cement barriers in the Pre-salt environment.
Sonic logging is widely used to assess cement quality by measuring the amplitude of first casing reflection and analyzing its attenuation associated with the acoustic coupling between casing and cement, however, due to the similarity between casing and salt slowness, most of the time, a constructive interference on the first casing reflection can be observed, changing the peak amplitude, leading to inaccurate interpretation of cement bond. The analysis of the raw waveforms has been used to detect such phenomena and subside the impact attribution of the physical principle to be used as the main one for the barrier validation process.
Additionally, ultrasonic tools, designed to provide higher resolution evaluation, and cement azimuthal distribution around the casing by traditional measure of acoustic impedance, are also susceptible to a similar interference, further complicating the assessment of cement bonding and properties. In this specific case, a special analysis of the raw waveform in the processing of the measurement windows and the addition of a new measure named flexural wave attenuation, mitigating the interference, and providing a reliable cement bond evaluation.
Another peculiarity of saline formations is its mobility, obligating the operator to adopt the usage of extra thick casings capable of withstanding the tensions associated with this property. The presence of thick casings (over 0.8-in. thick) adds to the complexity of cement evaluation as they attenuate the ultrasonic signals and may result in wrong measurements. To overcome this obstacle, the use of a more powerful ultrasonic transducer was adopted, allowing a better signal to noise ratio and a more accurate measurement.
In conclusion, successful cementing analysis in the Pre-salt environment demands a comprehensive understanding of the influence of salt slowness on sonic and ultrasonic measurements, as well as the complications arising from thick casings. Innovative approaches, including advanced signal processing, extra measures, and improved tool designs, are vital for enhancing the accuracy and reliability of cement evaluation in this challenging geological and well construction setting. Addressing these challenges is crucial for ensuring wellbore integrity and optimizing production efficiency in the Pre-salt reservoirs.
