Alfredo Luna-García's thesis
by
Alfredo Luna-García, MSE
University of Texas at Austin, 2002
Co- Supervisors: A. Daniel Hill and
Kamy Sepehrnoori
When naturally fractured carbonates formations is treated with Hydrochloric Acid at pressures below the
fracturing pressure, the acidizing process will likely be different form either a
matrix treatment or an acid fracturing treatment. In a matrix treatment of a relatively
homogeneous, un-fractured carbonate, the acid initially penetrates into the pore structure
and usually creates wormholes. in acid fracturing treatment, the acid flows through the
relatively wide fracture, resulting in more uniform etching of the fracture walls. In
naturally fractured formations, acid could either mainly generate wormholes, as in matrix
acidizing of carbonates without natural fractures, or etch the fracture surfaces, as in
acid fracturing. Determining the etching pattern is essential for modeling in carbonates
with natural fractures.
This study shows the influence of natural fractures horizontally or vertically oriented
on the acid-etching pattern. The natural fracture is represented by two halves of a
cylindrical core sample, two inches in diameter and two foot long that are assembled
with a gap to represent the fracture with a certain width. The gap between the samples is
in the range of average natural fracture widths indicated in the literature.
Based on the results of this study, the following conclusions are drawn:
In this study, acidizing creates three different acid etching patterns: Wormhole,
Meanderbelt Channel and Channel. Wormhole is created, when the ratio of width to depth
is in the range of 0.5 to 2.43. Meanderbelt Channel is created, when the ratio of width
to depth is in the range of 2.88 to 10.00. Channel is created, when the ratio of width to
depth is in the range of 11.8 to 40.00.
The orientation of the fracture and the fracture width determine the acid etching pattern.
When the fracture is vertically oriented and the fracture width is in the range of 4.96 x 10-3
cm to 4.12 x 10-2 cm, acidizing creates only Wormholes. When the fracture is vertically oriented,
acid etching patterns are independent of the roughness. For the same roughness, when the
fracture width is increased the pressure drop decreases.
When the fracture is horizontally oriented, acidizing creates three different acid etching
patterns: Wormhole, Meanderbelt Channel and Channel, which are determined by the fracture
width. In these experiments, when the fracture width is in the range of 7.22 x 10-3 cm to
4.12 x 10-3 cm, acidizing creates Wormholes. When the fracture width is in the range of 1.00
x 10-2 cm to 1.19 x 10-2 cm, acidizing creates Meanderbelt Channel. Finally when the fracture
width is greater than 1.83x10-2 cm, acidizing creates Channels.
When the fracture is horizontally oriented, for the Roughness No.1 acidizing creates Wormholes,
for the Roughness No.2 acidizing creates Meanderbelt Channel, for the Roughness No.3 acidizing
creates Channels.
The orientation of the fracture define the type of etching pattern, so the gravity effect, is important.
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