Reaction Rate and Fluid Loss: The Keys to Wormhole Initiation and Propagation in Carbonate Acidizing

Abstract

The efficiency of the matrix acidizing process in carbonates depends strongly on the wormholing phenomenon - if worm holes are formed, the effects of near wellbore damage can be overcome with relatively small volumes of acid. Numerous previous studies have shown that worm hole patterns can be placed in these general categories:compact dissolution in which most of the acid is spent near the rock face;the wormholing pattern; anduniform dissolution in which many pores are enlarged, as typically occurs in sandstone acidizing We have developed a theory of the wormholing process which predicts when the wormholing pattern is most efficiently created as a function of the acid flux and other treatment variables. By testing this theory with several independent sets of laboratory data, we can now demonstrate the important roles that surface reaction rate and fluid loss play in the wormholing process. This theory accurately predicts the optimal flux (that which leads to dominant wormholes with a minimum of branching and hence a minimum acid volume) for experiments with HCl in limestone and dolomite at several temperatures and with acetic acid in limestone. Surface reaction rate differs by several orders of magnitude in these experiments and is the only process variable that differs greatly among them. Paradoxically, though worm holes are formed because the overall reaction rate is controlled by mass transfer in the wormholes, diffusion rates play only a minor role in the wormholing process. Fluid loss through the walls of the wormholes ultimately limits the distance to which worm holes can propagate. Because of this effect, laboratory linear core floods will give optimistic predictions of worm hole penetration distances. We developed a cylindrical flow model to represent the flow field around a worm hole propagating from a wellbore which illustrates how to translate laboratory results to field conditions. We have used these theories to predict optimal acid formulations and injection rates for field conditions. In general, the lower the reaction rate (such as at low temperatures in dolomites or with weak acids in limestones), the lower the injection rate required, making it easier to propagate dominant wormholes under matrix treating conditions in the field. Introduction Numerous studies of the wormholing process in carbonate acidizing have shown that the dissolution pattern created can be characterized as being one of three types:compact dissolution in which most of the acid is spent near the rock face;the wormholing pattern; anduniform dissolution in which many pores are enlarged, as typically occurs in sandstone acidizing. These studies have also shown that the acidizing process is most efficient (defined as the process that will enhance near-wellbore permeability to the greatest depth with the smallest volume of acid) when the wormholing pattern develops. A third observation common to these studies is that the pattern created depends on acid flux, with the compact pattern created at relatively low acid flux, the worm hole pattern developing at intermediate flux, and the uniform pattern at high flux. Of course, there is not an abrupt transition from one pattern to another. As acid flux is increased, the compact pattern will change to one in which large diameter worm holes are created; further increases in flux yield narrower wormholes which propagate farther for a given volume of acid injection; and finally, as acid flux is increased more, the worm holes become more and more branched until ultimately the uniform pattern is observed. P. 775^

Influence of transport and reaction on wormhole formation in porous media

“…All the fluids exhibit an optimum injection rate at which the number of pore volumes required to break through is minimized. This optimum is consistent with results obtained previously with HCl (Hoefner andFogler, 1988: Wang et al, 1993;Mostofizadeh and Economides, 1994;Frick et al, 1994;Bazin et al, 1995;Huang et al, 1997). Notice that as the diffusion coefficient was decreased with fluid type from HCl (3.6 x l o p s cm2/s) to HAc (1.1 x lo-' cm2/s) to the chelating agents [EDTA (6X lo-' cm2/s), CDTA (4.5 X lo-' cm2/s), DTPA (4X cm*/s)], the optimum injection rate decreased.…”

“…Bazin et al (1995) reported the optimum injection rate for the HCl/limestone system to occur at the transition between convection and mass-transfer limited regimes. In contrast, Wang et al (1993) and Huang et al (1997) investigated HCl/carbonate systems and proposcd that the optimum injection rate occurred at a transition between reaction rate and fluid-loss limited regimes. Despite mass transfer having a major influence on wormhole formation, diffusion plays only a minor role in their theory.…”

Core-scale description of porous media dissolution during acid injection - Part I: theoretical development

“…Two different approaches able to fit the wormhole propagation models at the core-scale can be distinguished in the literature. The first one used by Daccord et al [7] or Huang et al [19] consists to fit the injection flow rate with a surface area scaling method in order to keep the same injection velocity into the porous matrix. Nevertheless, this approach needs that the wormhole density is the same whatever the observed scale.…”

Pore-Network Modeling of Carbonate Acidization

“…Walsh et al (1984) extended this work in porous media using a 1D reactive model, successfully predicting the distribution roll-front uranium deposits. Huang et al (1997) extended this capillary model to field conditions using Darcy's law in radial flow, concluding that fluid loss from wormholes in field treatments exceeds that found in linear corefloods, thus overpredicting wormhole formation in field conditions.…”