Suneet Shukla's thesis
by
Suneet Shukla, MSE
University of Texas at Austin, 2002
Supervisor: A. Daniel Hill
Matrix acidizing is a commonly used well stimulation technique in which acid is injected
into the formation in order to dissolve a portion of the rock and hence, recover or
enhance the permeability in the near well-bore area. This thesis deals with the acidizing
of carbonates. In carbonates, when acid is injected selective dissolution of the rock takes
place and highly permeable flow channels, called wormholes, are created. The recovered
permeability depends on the wormhole density, wormhole radius and wormhole pattern which
in effect depends on the rock and acid properties and acid injection rates.
During acidizing, as wormholes are being created, not all of the acid reaches the wormhole tip.
A significant part of the acid is lost as fluid loss to adjacent pores. So, fluid loss
is a governing factor that controls the efficiency of wormhole formation. If this fluid
loss can be controlled then more of the acid can reach the wormhole tip and the reaction
and hence the penetration of wormhole can be faster. This thesis investigates the effect
of saturating the experimental cores with gas, and hence reducing the permeability for
acid, on wormholing efficiency. It was found during the course of the study that gas
injection prior to acid injection does significantly reduce the pore volumes of acid
required for breakthrough. This effect is observed both at room temperature as well as
at higher temperatures.
An analytical model was also developed to study the diversion effect of injecting gas
slugs into a heterogeneous formation with different permeability layers. The two-phase
acid diversion model also incorporates a wormhole propagation model and calculates skin
of the reservoir layers as acid volumes are being injected. It was noticed that gas does
have a small diversion effect, in which greater volumes of acid are diverted to the lower
permeability layers, however this effect is not too significant.
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