Wormhole Modeling in Carbonate Acidizing

by
Tianping Huang, Ph.D.

University of Texas at Austin, 2000
Supervisors: A. Daniel Hill

The main objective of this study is to understand the wormholing process during carbonate matrix acidizing treatments. The efficiency of the matrix acidizing process in carbonates depends strongly on the wormholing phenomenon. If wormholes are formed, the effects of near wellbore damage can be overcome with relatively small volumes of acid. A theory, which predicts when the wormholing pattern is most efficiently created as a function of the acid flux and other treatment variables, was confirmed with several independent sets of laboratory data. This study demonstrates the important roles that surface reaction rate and fluid loss play in the wormholing process.

Fluid loss through the walls of the wormholes ultimately limits the distance to which wormholes can propagate. Because of this effect, laboratory linear core floods will give optimistic predictions of wormhole penetration distances. A cylindrical flow model is developed to represent the flow field around a wormhole propagating from a wellbore, which illustrates how to translate laboratory results to field conditions. Two optimal acid flux design charts are generated to predict optimal acid formulations and injection rates for field conditions.

In a treatment design, the acid volume needed to bypass the near wellbore damage area or penetrate a distance away from the wellbore depends not only on wormhole propagation distance but also on how many wormholes are created. This study investigates the wormhole population density, which means how many wormholes per area of rock surface or per wellbore length will be created. When a wormhole is initiated at an arbitrary point along the wellbore, the local pressure distribution is changed, resulting in lower pressure gradients into the formation in the region surrounding the wormholes. In this region of reduced pressure gradient and, hence, reduced flux, initiation of other wormholes is suppressed. At a sufficient distance from the initial wormhole, the pressure field is undisturbed and other wormholes will develop.

Numerical simulations of the flow field around a wellbore with wormholes extending into the formation are used to quantitatively predict the wormhole population density along a wellbore as a function of the acidizing conditions. An example shows how the wormhole population density model is used to predict the required acid volume (gal/ft) to yield the desired productivity improvement.

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