Using LWD Ultrasonic Images for Heterogenous Carbonate Reservoir Evaluation in Buzios Field, Presalt of the Santos Basin, Brazil

Challenging deep-water drilling operations in Brazil over the last 20 years have demanded the acquisition of a wide range of petrophysical measurements, often using logging-while-drilling (LWD) tools. Image logs provide a wealth of information for the evaluation of carbonate reservoirs, particularly for quantifying vuggy porosity, dissolved fractures, and caves. This extra-matrix porosity is crucial for constructing a porosity and permeability model that closely approximates the actual reservoir conditions. With the significant advancement of production in the pre-salt oil fields of the Santos Basin, numerous wells have been drilled exclusively using LWD logs for data acquisition, and understanding the acquisition of acoustic images has been essential to avoid impacting the quality of permeability-porosity interpretation in these wells. In early wells, the LWD suite often included micro-resistivity imaging sensors, but ultrasonic imaging sensors have been found to provide better information in many scenarios. An understanding of how these sensors respond to changing borehole and formation conditions is essential for proper interpretation. The ultrasonic tool utilized in the Buzios field has four ultrasonic transducers, ninety degrees apart, that fire simultaneously every 2 ms, resulting in 2000 firings per second. The high firing rate serves to provide high- resolution images without necessarily limiting the rate of penetration (ROP). The ultrasonic transducers operate in pulse-echo mode. The source pulse is generated by applying a voltage to the transducer and the pulse wave travels through the borehole fluid with part of the wave reflected from the formation surface. The transducer acts as both as transmitter and receiver for the ultrasonic signal. The borehole radius and shape can be derived by measuring the travel time of the pulse wave from transmission to the echo being received. The amplitude of the returning signal, while attenuated by the intervening borehole fluid, is also a function of the acoustic impedance of the formation and is influenced by small features in the borehole wall, such as fractures or vugs. Combining travel-time and amplitude images with other petrophysical measurements helps to provide a comprehensive understanding of the borehole and the surrounding formation. The recent drilling of an injection well in the Buzios field yielded excellent image quality, surpassing that of other wells drilled using the same tool and under similar conditions. The control of drilling and rotation rates, as well as favorable sea conditions, contributed to this superior quality and resulted in a more effective and accurate well evaluation. The well exhibited good permeability-porosity characteristics in all evaluated intervals, with numerous vuggy features and dissolved fractures interpreted, particularly in the coquinas of the Itapema Fm., exhibiting exceptionally high transmissibility and injectivity through a short falloff test. Distinguishing real formation features from artifacts created during the drilling process and understanding the impact of vibration were important for accurate interpretation. Knowledge of the field stress and regional structural and stratigraphic dip, allowed for a comprehensive interpretation.