Abstract

The formation damage in scaled-up production wells caused by incompatibility of injected and formation waters have long been known. Precipitation of salts results in permeability decline. Among the most onerous of all scaling species is that of sulphates, particularly barium and strontium sulphates.

We study effects of porous media on the BaSO4 scaling kinetics.

A new methodology for determination of reaction rate coefficient from coreflood tests consists of the sequence of diffusivity reaction free tests, of transient tests with chemical reaction, and of simultaneous injections of both injected and formation waters with quasi steady state concentration profiles for barium and sulphate ions. The diffusivity tests serve for dispersivity coefficient determination. Quasi steady state tests allow determination of the reaction rate coefficient versus velocity. The transient test data as compared with the mathematical modelling data validate the model and the data of steady state test data treatment.

An analytical model developed is used for treatment of quasi steady state test data. The transient tests are treated by a numerical model.

The main result of the work is proportionality between the reaction rate coefficient and flow velocity for the parameter range studied.

Introduction

The BaSO4 scaling is a chronicle disaster in waterflood projects with incompatible injected and formation waters. This is usually due to precipitation of barium sulphate from the mixture of both waters and consequent permeability reduction1–3.

The rate coefficient for the reaction between incompatible chemical species in injected and formation waters is the main parameter that determines the oilfield scaling intensity in cases where the aqueous solution is far from equilibrium. This rate is highly affected by flow velocity, diffusion/dispersion in porous media, pore space geometry and, therefore, the reaction rate coefficients inside and outside porous media should be different.

Nevertheless, presently the reaction rate coefficients used in mathematical modelling are obtained in laboratory reactors without porous media4–8. Usually the solid grains are used during water mixing in reactors in order to induce the precipitation centres, but the pore space structure and the relative fluid-rock flows are not represented.

The mathematical models for reactive flow in porous media consist on mass balance equations with the reaction rate sink terms8–10. The rate terms should depend on porous media properties.

In order to reliably predict well behaviour during the oilfield scaling, the effect of porous media flow on chemical reaction rate should be studied systematically.

The design and results of barium sulphate steady state scaling tests allowing for such study have been presented in the literature11–13. Nevertheless, there were no attempts to determine the reaction rate coefficients from laboratory coreflood tests.

The laboratory and mathematical study of scaling formation in porous media has been performed13,14. The sequence of coreflood tests allowing for determination of the reaction rate coefficient for various flow conditions has been proposed:

  1. displacement of water by traced water at the absence of reaction in order to determine diffusion coefficient;

  2. displacement of Ba-rich formation water by SO4-rich injection water (transient tests);

  3. simultaneous injection of Ba-rich formation water and SO4-rich injection water (steady state tests).

The diffusive tests allow determining the diffusion coefficient dependency of flow velocity for a given core15. The quasi steady state tests being performed at different velocities determine the reaction rate coefficient versus flow velocity. The transient tests allow comparing experimental results with the data of numerical modelling based on the steady state test data.

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