NMR as a characterization tool for wormholes: The complete picture

Acidizing is a common practice aspired to recover the initial skin factor or even decrease it further. Acids tend to create conductive channels (wormholes), through carbonate formations which connect the reservoir to the wellbore and bypass the damaged zones. Optimum wormholes are formed when certain conditions are met, such as optimum acid concentration and optimum injection rate. In order to thoroughly grasp wormhole creation, several characterization techniques should be performed. Computerized tomography (CT) scan and differential pressure data are two common practices to characterize wormholing in laboratories using core plug samples. Differential pressure data and CT scan can verify acid breakthrough occurrence, and qualitatively suggest the size and path of a wormhole. Nuclear magnetic resonance (NMR) was introduced in earlier studies as a new characterization tool for wormholes. It was able to detect the changes in micro- and macro-pores, due to acid injection. In addition, it was able to indicate the changes in interconnectivity between different pore systems and detect the materialization of formation damage. Nevertheless, NMR technique could not detect the new porosity corresponding to the generated wormhole due to inability to sustain saturation fluids inside the core plug samples. In this study, the NMR technique was further improved using a customized Teflon tube as a container for core plug samples. Since Teflon material does not interfere with NMR readings, this improvement allowed the core plug samples to maintain full saturation, which enabled the detection of the new wormhole porosity. As a result, NMR was able to indicate different characteristics of the generated porosity, including the size of wormhole, the changes in diffusion coupling, and in the distribution of each pore size. This technique can significantly elaborate new aspects of wormhole generation and characteristics in carbonate reservoirs.