Calcium Carbonate Scale in Oilfield Operations (1987)

ABSTRACT Calcium Carbonate (CaCO3) is one of the most common scales in oil field operations. Various fluids (brine, oil and gas) can mix in the reservoir and/or wellbores under drastically varying sets of thermodynamic, kinetic and hydrodynamic conditions that will affect the carbonate scale forming tendencies. Many of these critical conditions are ignored in today's oil field operations as far as calcium carbonate scale is concerned. Presently, the mechanisms of CaCO3 scale formation, as conceived by the industry, considers only the liquid brine phases. The flashing of gases from both the oil and the brine phases is critical for the CaCO3 scale formation. Pertinent aspects of this flash process are generally ignored. In addition, the complete mechanism of the CaCO3 must also consider the partitioning of gases (particularly CO2) between the liquid oil and brine phases during the entire gas flash process. This means, the formation of CaCO3 can not be determined by considering only the brine phase as attempted by the industry. Instead, the entire three-phase PVT behavior (oil/brine/gas) and associated CO2 partitioning must be considered to determine the CaCO3 scale formation in an oil field. The existing and published models on the CaCO3 formation consider only the basic thermodynamics of the brine phase and totally ignore the critical effects of the oil phase behavior on this scale formation. This automatically means that the oil industry can not adopt any of the existing scale models without serious modifications to account for the unique effects of the oil phase. The main variables dictating the location and amount of calcium carbonate scale deposition in an oil field are as follows: Pressures and temperatures at any location within the entire system.The brine and oil compositions prior, during and after the reservoir fluids have been exposed to temperature and/or pressure changes.The bubble point and pertinent flash behavior of the three-phase oil/brine/gas system as a function of pressure and temperature.The distribution of CO2 between oil and brine phases and the drastic variations of this CO2 partitioning prior and during any production operation.The constant variation of the water(brine)/oil ratio ("WOR"), the gas/oil ratio ("GOR") and the gas/water(brine) ratio ("GWR") during any production operation. These reservoir and production variables at various locations in the production system may change constantly as a function of location and time during any type of production operation within a given field. These critical variables are not considered in their entirety in the existing and most frequently used CaCO3 scale prediction models. In the present paper, the problems of calcium carbonate scale formation in oil field operations (primary, secondary, and tertiary production modes) are critically discussed. The various effects of the gas distribution (especially CO2) between oil and brine phases under reservoir and various production conditions are delineated as far as CaCO3 scale is concerned. Some algorithms are given. Finally, the paper discusses a methodology which can be used to predict CaCO3 under all field conditions as a function of the water composition, pressure, temperature, ‘VOR", "GOR", "GWR", total CO2 in the system and CO2 partitioning between the various liquid phases. The methods and problems of obtaining all the required input information in order to apply this or similar models for site-specific oil field operations will be discussed in detail. PVT and CO2 partitioning data obtained through some recent work with three-phase fluid systems from California oil fields will be used to illustrate the complex CaCO3 scale behavior in an oil field.