The effect of organics on feldspar dissolution and the development of secondary porosity

Abstract

The laboratory simulation of feldspar dissolution in organic-bearing aqueous solutions at reservoir temperatures ( 95° C 1 bar and 100° C 88 bar ) has revealed a number of important facts which have implications for secondary porosity formation in feldspar-bearing sandstones. The experiments were performed using the two most common carboxylic acids found in oilfield waters, oxalic and acetic, a polycarboxylic acid (EDTA) and a phenol (catechol). Oxalate in solution can significantly enhance feldspar dissolution rates. However, this effect is dramatically suppressed by the presence of Ca as well as Mg in the solution. The presence of acetate can effectively buffer the pH of a subsurface fluid, which in turn controls the dissolution rate of feldspar. The dissolution rate of K-feldspar increase in acetate-buffered solutions as the pH is lowered from neutral to acidic. However, results do not indicate that feldspar dissolution can be enhanced solely by the presence of acetate ions in solution if the pH of a solution is the same as that without acetate ions. The acetic acid indirectly affects feldspar dissolution by controlling the pH rather than by complexing Al and Si at reservoir conditions. Some carboxylates, such as EDTA, promote feldspar dissolution to a greater extent than oxalic acid because they are able to complex Al but do not form an insoluble calcium salt. Orthodiphenols, such as catechol, are much more effective than oxalate for the dissolution of feldspar. The presence of a very small amount of these organics can significantly enhance secondary porosity formation. The dissolution rate of An-rich feldspars is greater than alkali-feldspars under similar conditions. Therefore, improved porosity predictions can be made if more information about feldspar composition can be obtained.