To exploit a reservoir more efficiently, horizontal wells with long laterals are being drilled in many oilfields in the world. Uniform acidizing by bullheading is a challenge in these wells because the volume of acid that enters each segment tends to be different: larger at the heel while smaller at the toe. This further causes a non-uniform production profile which advances breakthrough of water.

In this paper, a method of using a temperature-sensitive viscous acid to achieve uniform stimulation profile is introduced. The acid is delicately designed so that the viscosity of it changes with temperature and the changing rate can be quantitatively controlled. As the acid flows in wellbore, the diffusion coefficient of H+ and therefore the acid-rock reaction rate gradually increases while it is being warmed by wellbore. A model describing the change of viscosity and H+ mass transfer coefficient is established and coupled with a wellbore heat transfer model.

The basic of this acid is hydrochloric acid but it is mixed with a special gelatinizer. The initial viscosity of the acid ranges from 60 to 1400 mPa.s at surface, depending on the concentration of gelatinizer. Its viscosity gradually decreases to 200 mPa.s with temperature increasing to 100 ℃.The leaking off velocity decreases from 2.0×10−6 m/s to 2.0×10−7 m/s accordingly, depending on the injecting time. And the acid concentration at the wall of wellbore increase by three times when the injecting rate doubles, also depending on the wellbore temperature and pressure. The acid-rock reaction rate is faster at toe than at heel, which just compensates the differences of acid volume allocation between toe and heel due to wellbore flow friction. A model describing the correlations between the acid viscosity, diffusion coefficient of H+, the concentrations of HCl, the injection rate and the temperature is generated. This model is coupled with an acid flow and reaction model and a wellbore heat transfer model and in the end a profile of H+ consumption along the lateral is predicted. With use of this model, by adjustment of acid formula and injection rate, a uniform stimulation intensity profile along the long lateral can be achieved. A well case is introduced to show the application of this model.

The dynamic temperature profile, the acid viscosity profile, the allocation of hydrogen ions as well as the equivalent wormhole lengths along the wellbore are all given in the paper. This paper introduces a chemical way to achieve uniform acidizing intensity in horizontal wells with long laterals without running in mechanical tools, reducing cost and leaving a good wellbore environment for future well interventions after treatment.

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