Hydrochloric acid (HCl) as a stimulation fluid of deep oil and gas carbonate reservoirs is very common practice in the industry. Using HCl acid during matrix acid treatment in carbonate acidizing has the limitations of; rapid tubulars corrosion, formation face dissolution due to fast uncontrolled reaction rate especially at high temperature reservoirs and low injection rates, and induced formation damage due to sludge formation in presence of crude oil with high asphaltene content. There is also a difficulty of using HCl in stimulating multilateral and horizontal wells due to its fast reaction with the reservoir rock. With the intensive care given to the environment in the 21 century, chelating agents were introduced as alternatives to HCl to alleviate the problems associated with HCl. GLDA (Glutamic acid diacetic acid) chelating agent was used previously to stimulate carbonate reservoirs at high pressure and high temperature (HPHT) conditions. GLDA was prepared in fresh water to stimulate these reservoirs.

In this paper, the effect of dilution using seawater on the reaction kinetics of low pH GLDA (3.8 pH) with different carbonate rocks under HPHT conditions was investigated using the rotating disk apparatus (RDA). The reaction experiments of GLDA solution with carbonate rocks in both fresh (GLDA/DI) and seawater (GLDA/SW) were carried out at the same conditions. Indiana limestone and Austin chalk carbonate rock samples were used to investigate the effect of rock facie on the reaction.

The reaction regime of GLDA chelating agent with calcite is identified to be mass transfer limited in both seawater and fresh water. Also the overall reaction rate and calculated diffusion coefficient showed a high dependency in the temperature. At 200°F and 1000 psi the diffusion coefficient calculated for the reaction of GLDA/SW with Austin chalk is an order of magnitude higher than the reaction of the same fluid with Indian limestone.

The reported diffusion coefficients can be used to simulate for the optimum injection rate required for stimulating high temperature carbonate formation. Highlighting the effect of porosity facies in the acid reaction with carbonate rock will lead to better understanding of the overall reaction of stimulation fluids with carbonate rocks of the same lithology but different porosity facies.

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