A Novel Procedure to Increase Well Response to Matrix Acidising Treatments

Abstract

ABSTRACT Matrix acid treatment design in clastic formations has traditionally been based on field experience or core-flushing tests. The choice of stimulation fluid compositions has frequently been limited to full-strength (12% HCl/3% HF) or half-strength (6% HCl 1.5% HF) mud acid. These practical rules of thumb have given good results but have not always ensured an optimum acid/rock reaction, i.e. the maximum net dissolution of the rock minerals and impairment coupled with a minimum of secondary precipitate formation. This paper shows how mathematical modelling of the chemical equilibria using a thermodynamic simulator can identify the ideal acid formulation, even when cores are not available. The input for the simulator is the mineralogical composition of the formation (from cuttings, sidewall samples or cores), the nature of the inorganic damage and the rock's porosity-permeability relationship. Chemical equilibria are calculated for different acid compositions taking into account the dynamic nature of the acidising process: the drop in acid strength during reaction with the damaged formation, and the changing rock composition due to attack by the acid. The results are translated via the porosity-permeability relationships into a theoretical well-productivity improvement factor. The acid composition which gives the highest productivity improvement is the optimum or Specially Formulated Mud Acid (SFMA) composition. It is important to note that the SFMA concept is not limited to HCl or HCl/HF acid formulations, but can also deal with other types such as Buffer Regulated HydroFluoric Acid (BRHFA). It will be shown that different rock compositions lead to very different SFMA formulations. SFMA recipes have been tested in core-flushing experiments, and have been found to give better performance than standard mud-acid recipes. The procedure was further validated by analysing the spent fluids regained by back production after field treatment. Field case histories in which improved production results were achieved using the SFMA approach are described.