Mineral scales are a common occurrence in hydrocarbon production systems. Oilfield brines contain varying amounts of dissolved solids that can precipitate during production because of changes in temperature, pressure, brine pH or brine composition.
In addition to deposition due to physical changes in the produced fluids, scale can form because of the interactions of the brine with treating chemicals used in the system. An example is found in the case of some hydrogen sulfide scavengers.
Hydrogen sulfide, H2S, can be efficiently removed from produced brine and gas using a variety of methods. In the oilfield, the most commonly applied H2S scavenging chemistries are the amine aldehyde condensates. While this type of chemistry is very effective in H2S removal, a significant increase in brine pH is an unfortunate side effect. In cases where the brine contains high calcium concentrations, large amounts of calcium-containing scale can form, restricting flowlines, valves, general hardware and valve soft goods.
This paper discusses steps taken to eliminate severe scale deposition problems resulting from a hydrogen sulfide scavenger treating program on production platforms in the Gulf of Mexico. Previous attempts to control scale caused by H2S scavengers have included brine pH reduction by the addition of acids and scale inhibition treatment programs. However, the treatment requirements specified for the locations precluded the use of traditional remedies and led to the development of a novel approach that gave positive results far exceeding expectations.
Removal of H2S from the produced fluids from the Viosca Knoll Carbonate system is necessary to meet the sales specification for the maximum allowable H2S concentration in the gas of four parts per million, (ppm). In order to treat the production, large quantities of H2S scavenger must be used to remove and maintain the required concentration limit of H2S. The potential for creating large amounts of calcium carbonate scale is extremely high because of the large quantity of scavenger used, the quantity of the produced water and its composition, and the high temperatures encountered. The high-pH scavengers that are injected dramatically raise the pH of the produced water resulting in an increase of the calcium carbonate scaling tendency. The unnatural and extreme scaling tendencies caused by the addition of high-pH scavengers leads to excessive consumption of scale inhibitor and unacceptable amounts of deposited scale.
Oilfield scales are the hard, adherent deposits formed when inorganic minerals dissolved in water precipitate during the production of oil and gas. If left untreated, scale can reduce the productivity of the well by blocking the flow paths of the oil and gas into the wellbore. Downhole and surface hardware can be damaged by the accumulation of scale, further reducing the ability to produce and transport hydrocarbons. The potential economic impact of scale formation is large, ranging from losses incurred by remedial treatments to lost production from permanent formation damage.
The Saturation Index, SI, is a measure of the degree of supersaturation of the scaling components dissolved in a brine. The severity of a scale problem can be calculated and ranked by using the brine's composition and the conditions from the field. A Saturation Index of 0.0 indicates that the brine is in equilibrium at the given conditions. A Saturation Index of 1.0 indicates that the brine is ten times supersaturated. Brines with SI values above 1.0 usually have or will have a scale deposition problem.
While useful for predicting the probability of scale, the Saturation Index does not give information on the quantity of scale that could form. A measure known as the PTB Value, Pounds per Thousand Barrels, has been developed to calculate the maximum mass of scale that could precipitate under the given conditions.