Prediction of Scale Problems Due to Injection of Incompatible Waters

Abstract

Summary A newly developed model to predict chemical compatibilities in waterflood operations is described. The model calculates the coprecipitation of BaSO4, SrSO4, and CaSO4 at various locations in field operations as mixtures of injection and reservoir waters flow through injection wells, reservoir, and production wells into surface facilities. As its data base, the model uses comprehensive data of actually measured solubilities in fairly complex oilfield and geothermal brines at various temperatures and at saturation or atmospheric pressure. The solubilities at high pressures are calculated using thermodynamic parameters. The application of the model is illustrated by examples involving two reservoir and two injection waters. Introduction Two of the more difficult problems in designing a proper waterflood operation are (1) the predetermination of chemical incompatibilities of waters used in the flood and (2) the forecast of these incompatibility effects on future field operations. This forecast should cover the type, extent, and location of all future damages resulting from chemical incompatibility problems.No damage of any kind would occur if all reservoir materials were chemically compatible with the injected water. However, hardly any source water available in large enough quantities is fully compatible with all materials in the reservoir to be flooded.The water native to the reservoir to be flooded is in chemical equilibrium with the rock, hydrocarbons, and any other materials present in the reservoir (e.g., CO2, N2, H2S, etc). In contrast, the water considered for injection is in equilibrium with its own environment, which is normally quite different from that in the reservoir to be flooded. Any injection automatically leads to a readjustment of most chemical parameters as soon as the injection water enters the reservoir. The newly injected water must re-establish its own and new thermodynamic equilibrium with respect to all solids and fluids present in the reservoir to be flooded.In conventional reservoir engineering and waterflood design, the fluids and rock phases are considered chemically inert. That is, these liquid, gaseous, and solid phases have physical properties that can have large effects on the flow properties but are not considered to participate actively in any chemical reaction. In reality, this is not true. Any injected water having an origin different from the reservoir to be flooded will interact chemically with the fluids and solids in the flooded reservoir. These interactions, of course, will depend on the chemical compositions of all participants in these interactions (liquid, gaseous, and solid phases), the degree of mixing, the flow paths, and the temperatures and pressures at various locations within the flooded reservoir.To complicate the situation further, the reservoir water (i.e., the produced water) may be produced at thermodynamic conditions again different from those within the reservoir. For example, dissolved CO2 and H2S may break out of solution when the water is produced together with the hydrocarbons. This loss of reactive gases will change the composition and pH of the water, thus generating a possible compatibility problem when the produced water is reinjected. This means compatibility problems can occur, at least theoretically, even during reinjection of produced formation water originating in the reservoir to be flooded.Ignoring the chemical reactions between injected waters and reservoir materials can lead to the disasters often experienced in the field. The formation of scale in producing wells is the most obvious result of the frequently encountered compatibility problems. In this paper, we describe some preflood considerations necessary for proper flood design. JPT P. 273^