Summary
In an acid-fracturing treatment, fracture conductivity is created by
differential etching of the fracture surface by the acid; without nonuniform
dissolution along the fracture face, the fracture will close after pumping
ceases, and little lasting conductivity will be created. Despite this critical
role of differential etching in the creation of fracture conductivity, little
is known about the texture of the fracture surface created during acid
fracturing or about the dependence of this texture on the acidizing conditions.
To study this important aspect of the acid-fracturing process, we developed a
new surface profilometer to measure the surface profile of a rock sample
accurately and rapidly and used the instrument to characterize fracture
surfaces after acidizing.
The profilometer measures the distance to the rock surface with a laser
device that measures distance with an accuracy of 0.001 in. The rock sample is
mounted on a servo-table that automatically moves the sample in selectable
increments (typically, 0.025 in.). With this device, the surface of a standard
API fracture-conductivity sample can be scanned in a few hours, and a digitized
profile image can be obtained. This digital image is used to characterize the
etched surface topography quantitatively.
We have measured the etched-fracture-surface profile for a wide range of
acidizing conditions. The etched-surface characteristics depend strongly on the
acidizing conditions, including acid type and strength, velocity in the
fracture, leakoff rate, and rock type. Results for typical acid-fracturing
fluids and conditions are presented along with recommendations for fluid
systems that create the smallest-scale differential etching.
Introduction
Acid fracturing is a well-stimulation process in which acid dissolution
along the face of the hydraulically induced fracture is expected to create
lasting conductivity after fracture closure. However, conductivity after
fracture closure is created by acid only if the fracture face is nonuniformly
etched by the acid, so that parts of the fracture face that have not been
etched deeply serve as pillars to maintain open flow pathways when the fracture
closes. At the scale at which acid-fracture conductivity is measured in the
laboratory, the texture of the fracture face should have a dominant influence
on the resulting fracture conductivity, at least at low closure stresses--if
the fracture faces are smooth, only a narrow slit will remain when the fracture
closes and conductivity will be low, while if the fracture surfaces are
particularly rough, large pathways throughout the fracture will be propped open
by the large surface asperities and conductivity will be high. As the closure
stress is increased, surface features along the fracture faces may be crushed,
and eventually, at high closure stress, the lasting fracture conductivity may
depend more on rock strength than on the initial etching pattern.
In this paper, we present an experimental methodology to characterize
acid-etched rock surfaces carefully and then relate the fracture-surface
features to the measured fracture conductivity. The preliminary results
presented here show how statistical properties of the surface-roughness
distribution are related to the fracture conductivity.
© 2008. Society of Petroleum Engineers
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History
- Original manuscript received:
28 June 2006
- Meeting paper published:
24 September 2006
- Revised manuscript received:
24 March 2008
- Manuscript approved:
4 April 2008
- Version of record:
15 August 2008
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