Carbonate Stimulation with Biodegradable Chelating Agent Having Broad Unique Spectrum (pH, Temperature, Concentration) Activity

摘要

The use of chelating agents to stimulate carbonate reservoirs has continued to increase since their first inception. A distinct chelant is presented in this paper. The new chelant is fully biodegradable and can be used in fluid formulations that specifically target the formation characteristics. The chelating agent can be used in concentrations from 0.2 to 0.6 M, with a pH of 1, 2, or 4, and is effective from 125 to 350°F. The specific fluid composition is a function of the mineralogy and rock lithology as well as temperature. Core flow testing at less than 200°F shows optimum dissolution with low-concentration, low-pH fluid; while, at temperatures >200°F, higher concentrations of chelant is optimum. The representative pore volume (PV) breakthrough curves provide an indication of the distinct reactivity of this chelant to other known chelants. Analytical characterization of the spent fluid (elemental analysis, pH) and of the core (CT imaging) are used to show that the activity in which the fluid displays on select limestone (calcite or dolomite) cores can be exploited to enhance wormhole structure generation. Core permeabilities ranged from 0.4 to 10 md, and core dimensions (radius × length) corresponded to 1- × 4-in., 1.5- × 6-in, and 2- × 6- or 10-in., respectively. The chelating fluid maintains in solution dissolved calcium, a property that is magnified because of the ease with which the acidic fluid spends as it is transported through the limestone matrix. The concentration of the chelant can be optimized to a specific pH to maximize its dissolution capacity. Overall, the new chelating agent affords superior complexation capacity from low pH to near neutral pH values while dissolving the minimum amount of mineral and effectively increasing porosity. The chelant expands the range of available options in the area of treating fluids for carbonate stimulation.