Data Integrated Analysis of Flexural Attenuation and Sonic P&S Waves Defining the Future Drilling Strategy for an Exploratory Block in Pakistan
Authors: Lakhan Mukhtiar Ahmed , Bari Abdul , Sharif Duria , Fareed Abdul , S.Tariq Hasany , Khan Mohammad Rasheed , Ramzan Mohammad 
Published: March 2023

Reservoir or nonreservoir geomechanics is pivotal to drilling safe and fit-for-purpose boreholes that lead to sustainable operations in the well lifecycle. Accordingly, a 1D mechanical earth model (MEM) plays a crucial role in guiding the exploration to production lifecycle with applications ranging from mud-weight optimization to hydraulic fracture designing. While numerous other petrophysical logs are critical inputs to the MEM, sonic compressional (P) and shear (S) slowness play a direct role in model building, as several geomechanics parameters are derived using them. In cased holes, however, obtaining sonic waves data can be a challenge attributed to contaminated and poor cement behind the casing that creates a hindrance in processing capabilities. This work is a culmination of integrative data analysis based on ultrasonic flexural attenuation and sonic P&S logs that resulted in successful creation of 1D-MEM for an exploratory block in Pakistan.

Extensive data gathering through logging was planned in multiple sections at Well X-1, which was the exploratory well in the block, to characterize formations. Likewise, MEM was critical to the success of future wells pertaining to wellbore stability study and optimum mud-weight window selection. Nevertheless, the hole condition in a 12.25-in. section did not encourage the acquisition of an openhole log and, therefore, an alternate strategy for logging in a cased hole was planned. Accordingly, comprehensive cement evaluation was performed using the flexural attenuation-based ultrasonic technology that azimuthally deciphers the material behind casing through solid-liquid-gas mapping. This log is acquired in combination with omnidirectional sonic-based measurements (CBL-VDL). Next, acquisition of acoustic attenuation-based sonic measurements was conducted to obtain P&S slowness for 1D-MEM creation. Finally, using a methodical workflow, integration of the multiple cement evaluation measurements was done to delineate zones of processing for validity of P&S waves. Point-forward, MEM creation, and porosity estimation was performed with meaningful insights.

Analyzing sonic cement logs in silo caused doubts on the cement quality because of high CBL amplitude representative of low-quality cementing. Therefore, the traditional approach of using CBL amplitude led to the apparent low-quality flagging of P&S slowness across most of the interval. Integration of flexural-attenuation and sonic CBL allowed for systematic insights into the presence of wet microannulus, which otherwise would have been missed in the absence of ultrasonic measurements. This would have direct repercussions on the validity of P&S waves and ultimately on the 1D-MEM model. However, this integration enabled us to delineate the key intervals for P&S processing where cement placement was least affected. As a result, with minimum and maximum stress vectors, a 1D-MEM model was created that helped the operator to devise a drilling strategy for the next wells in the field. In addition, the P-waves computation also provided us with a reasonable estimate of porosity throughout the casing interval, which was initially not available because of poor hole condition.

The importance of data-driven integrative analysis is vital to avoid loss of valuable insights from the acquired data. Ultimately, this paved the way for designing efficient drilling and wellbore stability strategies in future appraisal/development wells in the area.