Heavy, viscous oil deposits and tar have low API 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. As a result, they tend to have different responses to acidizing fluids during matrix acidizing treatments. One fundamentally interesting phenomenon is the wormholing characteristic of acidizing fluids on tar formations. This article discusses the effect of acid and its wormholing characteristic on tar and on carbonate rock saturated with various crude oils that have varying API gravities.Experiments involved the acid flooding of core plugs that were saturated with oils of different API gravities. The extreme case entailed flooding the acid through tar-saturated plugs. The wormholes were characterized by computed tomography (CT) scanning. Differential pressures, the number and sizes of wormholes, and breakthrough volumes were all measured for each experiment. The tests involved regular hydrochloric (HC1) acid and emulsified acid.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 therefore, benefits from emulsified acid are reduced when higher API 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 fundamental investigation can lead to further 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.
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