Perm Journal of Petroleum and Mining Engineering (Dec 2019)
The influence of acid concentration and injection rate on the wormhole development in formation conditions, a laboratory study
Abstract
Carbonate rocks contain about 60 % of global oil and gas reserves. Stimulation of wells tapping into carbonate reservoirs is often performed using hydrochloric acid treatments based on the chemical reaction of hydrochloric acid with carbonate minerals. Carbonate reservoirs generally exhibit significant inhomogeneity, therefore the acid being injected into the formation causes uneven rock dissolution in the near-wellbore region, resulting in the emergence of highly conductive flow channels (wormholes) with complex geometry. They ensure a good hydrodynamic connection between the well and the formation. Each specific rock-acid compound system has an optimum injection rate which can help form long weakly ramified wormholes at minimum acid injection volume. The optimum injection rate value is influenced by many factors, such as pressure, temperature, acid concentration, solution composition, mineral composition of rock etc. As of today, laboratory research is the main method to determine the optimum parameters for treating the near-wellbore formation region with acid. The paper summarises the results of the analysis examining the influence of various factors on the optimum injection rate and the amount of injected pore volumes of an acid compound until a wormhole breakthrough. It is shown that the factors have a cumulative effect on the effectiveness of the acid treatment and have to be considered simultaneously in laboratory tests. The results of the performed analysis have been taken into account when planning further laboratory research. Within the design scope of hydrochloric acid treatments in wells of one of Iraq’s carbonate fields, laboratory tests have been conducted to assess the impact of acid concentration and injection rate on the effectiveness of acid stimulation in the conditions that were expected in case of the hydrochloric acid treatment. The determined injection parameters help obtain an optimum structure of wormholes at a minimum acid injection volume. The results of the performed research have been successfully used to design hydrochloric acid treatments in wells of the field under study.
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