NMR as a Characterization Tool for Wormholes

Abstract

Summary Acidizing is a common practice that aims to recover the initial skin factor or even decrease it further. Acids tend to create conductive channels (wormholes) through carbonate formations that 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. To thoroughly grasp wormhole creation, several characterization techniques should be performed. Computed-tomography (CT) scan and differential-pressure data are two practices commonly used for characterization of wormholing in laboratories by use of core-plug samples. Differential-pressure data and CT scan can verify the occurrence of acid breakthrough 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. NMR can detect the changes in micro- and macropores that are caused by acid injection, and it can indicate changes in interconnectivity between different pore systems and detect the materialization of formation damage induced by injected acids. However, the NMR technique cannot detect the new porosity corresponding to the generated wormhole because of an inability to sustain saturation fluids inside the core-plug samples. In this study, the NMR measuring method is further improved by use of a customized polytetrafluoroethylene (PTFE) tube as a container for core-plug samples. Because PTFE material does not interfere with NMR readings, this improvement allows the core-plug samples to maintain full saturation, which enables the detection of the new wormhole porosity. As a result, NMR can indicate different characteristics of the generated porosity, including the size of the wormhole and the changes in diffusion coupling and the distribution of each pore size. This technique can elaborate new aspects of wormhole generation and characteristics in carbonate reservoirs, in addition to assessing the damage caused by the stimulation fluid.