Oilfield Scaling - Part I: Mathematical and Laboratory Modelling

Abstract

Abstract 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 reduction. The effect of porous media on the BaSO4 scaling kinetics is studied. A new methodology for determination of kinetic constant from coreflood tests consists of the sequence of diffusivity tests, of transient tests with chemical reaction, and of steady state tests on simultaneous injection of both injected and formation waters. The diffusion coefficient versus flow velocity is found from the diffusivity tests. Steady state tests allow determination of chemical reaction rate constant versus velocity. The transient test data as compared with the mathematical modelling data validates the results of steady state tests treatment. The test data are treated using semi-analytical and numerical models. The main result of the work is the proportionality between the chemical reaction constant and flow velocity in the range studied. Introduction 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 sulphates1–4. The chemical reaction rate 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 and, therefore, the chemical rate constants inside and outside porous media should be different5. Nevertheless, presently the chemical rate constants used in mathematical modelling are obtained in laboratory reactors without porous media6,7. 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 flow in porous media accompanied by chemical reactions and salt precipitation consist on mass balance equations with the reaction rate sink terms7–11. The rate terms should depend on porous media properties. In order to 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 steady state scaling tests have been presented in the literature3,12. Nevertheless, there were no attempts of rate constant determination from laboratory tests. In this paper, the laboratory and mathematical study of scaling formation in porous media is presented5. The sequence of coreflood tests allowing for determination of the chemical reaction rate constant from coreflood tests for various flow conditions have been proposed:displacement of water by traced water at the absence of reaction in order to determine diffusion coefficient;displacement of Ba-rich formation water by SO4-rich injection water (transient tests);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 core13. The steady state tests being performed at different velocities determine the chemical reaction rate constant versus flow velocity. The transient tests allow comparing experimental results with the data of numerical modelling based on the steady state test data treatment. Two series of steady state tests on simultaneous injection of formation and injection waters with barium sulphate formation have been performed for different velocities in two cores. It was shown that the chemical reaction rate constant is proportional to flow velocity, in the range studied.