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Abstract
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.

Introduction
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 acid stage.