Fundamental Wormhole Characteristics in Acid Stimulation of Perforated Carbonates
Authors: Seagraves A. N. , Smart M. E. , Ziauddin M. E. 
Published: January 2018

Abstract
We present results of a systematic study aimed at the identification of fundamental characteristics of wormhole formation in perforated core experiments and the determination of their underlying dependence on perforation properties. We performed a set of single-phase laboratory experiments in which medium- permeability (10–20 mD) and low-permeability (&lt; 3 mD) Indiana limestone cores were perforated by conventional shaped charges to produce worst-case damage conditions. Then, those cores were stimulated with 15% HCl until wormhole breakthrough. We observed strong characteristic differences in the evolution of pressure drop during acidizing, required acid volume, and resulting wormhole patterns with changes in initial core permeability. Notably, all tests in medium-permeability cores showed a "transverse wormhole" mechanism in which the dominant wormhole nucleates behind the tunnel tip, propagates perpendicular to the tunnel, and then turns and propagates along the core axis upon approaching the no- flow boundary. In contrast, tests in low-permeability cores showed dominant wormholes nucleating directly at the tunnel tip. We found that the acid volume required for breakthrough scales linearly with the axial distance between the dominant wormhole nucleation site and the back face of the core.
Mechanisms underlying transverse wormhole formation in the acidizing experiments were then identified and analyzed with a novel characterization and simulation workflow, which attempts to connect the initial state of the tunnel to the observed acidizing outcome. Image analysis of whole-core computed tomography (CT) scans extracted the initial tunnel geometry and identified the presence of debris and perforation- induced fractures prior to stimulation. CT analysis suggests that transverse wormhole nucleation sites coincide with the presence of secondary radial fractures at the tunnel wall that are impregnated with liner debris and protrude partially into the crushed zone. Thin section analysis was then employed to quantify the spatial distribution of crushed zone and virgin rock permeability. The tunnel geometry and thin section characterization data were combined to estimate the leakoff velocity profile along the tunnel using an approximate analytical solution for the flow field in the crushed zone. The leakoff velocity shows strong axial variation along the tunnel, with local maxima in the vicinity of dominant transverse wormhole nucleation sites. Local peaks in the leakoff velocity are shown to coincide with locations along the tunnel that have elevated levels of crushed zone permeability stemming from a lesser extent of macropore compaction in the near-tunnel zone.
Finally, we present a continuum acidizing model, which extends the two-scale continuum (TSC) approach of Panga et al. (2005) to explicitly model nonuniform perforation and crushed zone geometries. Numerical simulations of the prestimulation flow field and of wormhole formation in perforated cores suggest that the shift from transverse to tip wormhole nucleation with change of initial rock permeability is fundamentally related to the increasing influence of the no-flow lateral boundary as the crushed-zone to virgin-rock permeability ratio increases by an order of magnitude from medium-permeability to low- permeability cores.