Wormhole Characterisation Using NMR
Authors: Al-Duailej Yaser K. , Kwak Hyung T. , Caliskan Sinan , Al-Yami Ibrahim S. 
Published: May 2016

In carbonate reservoirs, the mechanism of acidizing revolves around the development of wormholes. Therefore, understanding these wormholes is essential to increase their efficiency, and as a result, optimize the stimulation treatment. Characterization of wormholing efficiency using coreflooding data and computerized tomography (CT) scanning is widely known in the industry. In addition, computational fluid dynamics (CFD) have been used to model the flow in wormhole networks.

Using Nuclear Magnetic Resonance (NMR) to study and characterize a full breakthrough-wormhole is still a new initiative. NMR, in general, provides fluid saturation by imaging the nuclide concentration of a variety of nuclei and the information about the pore structure and the fluid-rock interaction by measuring the longitudinal and transverse relaxation time, and diffusion constant of the nuclei of interest, mostly proton (1H). This set of information is translated into porosity values and distribution, and can be further processed to generate accurate permeability values.

In this work, NMR analysis was successfully conducted on fully broken through core plugs. The results of NMR showed changes in the porosity and permeability. In addition, NMR detected a connectivity changes between the micro- and macropores. The acquired data from NMR was combined with CT and coreflooding data to further help to understand the properties of wormhole created.

Introduction

The objective of conducting well stimulation is to enhance the formation permeability or bypass damaged zones. Matrix acidizing and fracturing are the two main techniques of well stimulations. Choosing either of these options depends on the condition of the well, type of formation and completion, well testing results, etc. Matrix acidizing technique depends on the reservoir type. Acidizing sandstone formations occurs by dissolving clays and quartz to clear the way for the fluid flow. In limestone or dolomite (carbonate in general), acids create permeable channels usually referred to as wormholes. The efficiency of these wormholes is evaluated by its penetration depth, the required acid volume to reach that depth, stability and conductivity of the wormhole.

Wormholes are created by reacting acids with calcium carbonate at certain condition that favors the fluid going deeper (penetration) rather than sideways (face dissolution). The main factors that govern the creation of wormholes are:acid strength and concentration,rock composition,injection rate, andtemperature and pressure.

For an acid to create wormhole, these factors must be at certain conditions. For example, the reaction rate has to be slow. The reaction between strong acids, e.g. Hydrochloric acid (HCl), and calcite (calcium carbonate) is very fast and vigorous, while weak acids, such as organic acids or chelating agents, react rather slowly with calcite. Therefore, slow reacting acids are expected to create wormholes more than strong acids. Hydrochloric acid, as a strong acid, has been the industry's favorite choice when acidizing conventional wells in carbonate formations of low temperatures, because of its high dissolution power and relatively cheap cost. However, the problems of high reaction rate between acid and rock, and corrosion of tubulars are a concern. As a solution, emulsified acids, where the acid is mixed and coated with a hydrocarbon phase at a certain ratio, have been used as retarded versions of strong acids. One of the famous emulsified acids is HCl in diesel emulsified acid. The diesel phase protects tubular from corrosion and the formation from reacting vigorously with HCl. Several studies have been conducted to optimize the emulsified acid to get the best penetration and wormholing.