S.H. Al-Mutairi, SPE, M.A. Al-Obied, SPE, I.S. Al-Yami, SPE, A.M.
Shebatalhamd, SPE, D.A. Al-Shehri, Saudi Aramco
SPE International Symposium and Exhibition on Formation Damage Control,
15-17 February 2012,
Lafayette, Louisiana, USA
Heavy, viscous oil deposits and tar have low oAPI gravities and occur as part
of several oil formations. Unlike normal oil deposits, heavy oil and tar tend
to contain more inorganic impurities and to be more sulfurous and aromatic. And
as so, they tend to have different responses to acidizing fluids during matrix
acidizing treatments. One fundamentally interesting phenomenon is the
wormholing characteristics of acidizing tar formation. This paper discusses the
effect of acid and its wormholing characteristic on tar and on carbonate rock
that was saturated with crudes that have varying oAPI gravities.
Experiments included acid flooding of core plugs that were saturated with
different oAPI gravities. The extreme case included flooding the acid through
tar saturated plugs. The wormholes were characterized by CT Scanning.
Differential pressures, number and sizes of wormholes and breakthrough volumes
were all measured for each experiment. The tests involved regular hydrochloric
acid and emulsified acids.
This study showed that regular and emulsified acids produced comparable
wormhole penetration in tar. Tar formations were difficult to exhibit face
dissolution even at extremely low injection rates. In general, it was noticed
that penetration and, hence, benefit from emulsified acid is reduced when
higher oAPI oil saturated the rock. The wormhole breakthrough volume in a rock
saturated with intermediate oil was less than that of a rock saturated with
condensate oil. Condensate might have allowed better diffusion of acid droplets
to react with the rock.
This work provided a fundamental investigation that can lead to development in
producing these challenging prospects. In addition, these results are of
special interest when long horizontal injectors or producers are placed within
the tar zone of conventional oil reservoirs.
Stimulating carbonate rock with concentrated hydrochloric acid is a normal
practice for enhancing the permeability around the wellbore or bypassing the
formation damage. Hydrochloric acid is known to create wormholes that enable
the passage of the flow from the formation to the wellbore. The deeper these
wormholes are the better utilization of the needed volumes of acids. The target
of a normal stimulation job is to increase the depth of penetration of acids
with minimum volume of acids used, defined as the efficiency of wormholes. The
efficiency of the wormhole depends on several factors such as type of acid used
and its concentration, formation type and pumping parameters. The degree of oil
phase saturation and type of oil saturating the rock are important factors that
might affect the wormholing efficiency.
There are several theories about the mechanism of wormholes and models for
predicting their initiation and propagation. Most of these models relate the
final length and number of wormholes to the pumping parameters such as flow
rate and volume of the acids. However, the stimulation programs depend on
experience and lab studies to design acid recipes. Coreflooding experiments
tend to provide more representative data that can be scaled up to the formation
size and magnitude. The issue with lab experiments is that most of them are
conducted on clean rock sample because it is assumed that the preflush will
displace all fluids out from the target zone and precondition it for the main