Geometric and Fractal Analysis of Complex Wormholes Resulting from Gypsum Core Flood Tests

ABSTRACT: Wormholes are long, finger-like channels that form due to the flow and dissolution heterogeneity in the soluble porous rock matrix. Wormholes are major flow pathways, which significantly increase the permeability of the rock formation. A fundamental understanding of the wormhole formation is crucial in civil, environmental and energy engineering research and practice. A series of gypsum core flood tests were conducted to study the effect of flow rate on the dissolution of the gypsum rock matrix and the formation of wormholes. High-resolution X-Ray computed tomography (CT) was used to determine the geometry of the wormholes resulting from different flow rates. Specifically, the study then focused on the geometry of the wormholes resulting from different flow rates using skeleton analysis and fractal analysis. These analyses showed that higher flow rates resulted in more complex wormholes regarding the wormhole length and fractal dimensions.

1. INTRODUCTION

In an underground rock-fluid system, wormhole formation is a process resulting from flow and dissolution heterogeneity. Wormhole formation is common during underground reactive transport processes such as gypsum karst, limestone karst, acid stimulation and CO₂ sequestration. The reactive transport processes in the underground rock-fluid system involve two coupled processes: reactions (for example, dissolution) and fluid flow (flow in the porous matrix and fractures) driving the evolution of the rock-fluid systems. The formation of wormholes is a favorable process in oil reservoir acid stimulation, as it increases the reservoir permeability and thus oil production. However, it could also be an undesired process from the civil engineering perspective when the wormholes further develop into larger caverns, sinkholes and ground subsidence (Li et al., 2019). It is therefore essential to have a better knowledge of the factors that influence the formation of wormholes.

There have been many experimental studies to investigate the effect of flow rate on wormhole formation (Daccord, 1987; Daccord and Lenormand, 1987; Hoefner and Fogler, 1988; Daccord et al.,1993a,b; Taylor et al., 2002; Noiriel et al., 2009; Gomaa et al., 2010; El-Maghraby and Blunt, 2012; Sayed et al., 2012; Smith et al., 2013; Hao et al., 2013; Mohamed et al., 2013; Smith et al., 2014; Reynolds et al., 2014; Ghommem et al., 2015; Menke et al., 2016; Wang et al., 2016; Noiriel and Daval, 2017; Smith et al., 2017; Cai et al., 2018; Lin et al., 2016; Al-Khulaifi et al., 2018; Menke et al., 2018). In these experiments, core flood tests have been used extensively because of their versatility in controlling and monitoring the confining stress, deviatoric stress, inlet pressure, outlet pressure and specimen deformation during the tests. Some researchers used X-ray computed tomography (CT) scans on the specimen before and after the test to study the change of pore space and the formation of the wormholes (Gouze et al., 2003; Noiriel et al., 2004; Noiriel, 2015; Deng et al., 2015, 2016, 2017). The X-ray CT scans showed the 3D geometry of the wormholes and provided a generally accepted conclusion that higher flow rates result in more complex wormholes. However, these CT analyses mainly focused on the statistical parameters of the pore space such as porosity, and pore size distribution. The descriptions of the wormholes so far were mostly qualitative, making it difficult to compare the effect of flow rate on the formation of wormholes. One therefore needs to characterize the wormholes with quantitative parameters to compare the wormholes resulting from different flow rates.