Estimating diffusion coefficient for a reservoir stimulation fluid

Abstract

The subject disclosure relates to matrix acidizing. More specifically, systems and methods are described for estimating a diffusion coefficient for an acid fluid used to stimulate a subterranean reservoir wherein a spent acid is formulated that includes one or more by-products of the reaction between the fluid and rock. A rock sample, such as in the form of a rotating disk is exposed to spent acid under elevated pressure and temperature conditions while the fluid is sampled and analyzed. A diffusion coefficient for the spend acid is estimated.

What is claimed is: 
     
         1 . A system for estimating a diffusion coefficient for a stimulation fluid used to stimulate a subterranean reservoir rock by at least partially dissolving the subterranean reservoir rock, the system comprising:
 a sealed reaction vessel configured to expose a partially spent stimulation fluid to a sample of reactant material under elevated pressure conditions such that the reactant material reacts with the partially spent stimulation fluid, the partially spent stimulation fluid being pre-formulated to include one or more reaction by-products being of a type produced during reaction of the stimulation fluid with the reservoir rock; and   a data acquisition and processing system configured to measure properties of the partially spent stimulation fluid after at least some of the reactant material has reacted with the partially spent stimulation fluid and estimate therefrom a diffusion coefficient for the stimulation fluid under partially spent downhole conditions.   
     
     
         2 . A system according to  claim 1 , wherein the reservoir rock is a carbonate reservoir rock and the reaction by-products include dissolved CO 2 . 
     
     
         3 . A system according to  claim 1 , wherein the sample of reactant material is a solid rotating disk having a material found in the subterranean reservoir rock. 
     
     
         4 . A system according to  claim 1 , further comprising a second vessel configured to generate the partially spent stimulation fluid by reacting a fresh acid with a material of a type found in the subterranean reservoir rock. 
     
     
         5 . A system according to  claim 4 , wherein the reservoir rock is a carbonate reservoir rock, the fresh acid is HCl and the material used in the second vessel is CaCO 3    
     
     
         6 . A system according to  claim 1 , wherein the stimulation fluid includes one or more of the following selected from a group consisting of: hydrochloric acid; hydrofluoric acid; organic acids; chelating agents; emulsified acid; and gelled acid.