M.A. Buijse, and M.S. van Domelen, SPE, Halliburton B.V
Historically, emulsified acids have primarily been used in fracture acidising. By combining information from theoretical studies, experimental studies and field testing, a better understanding has recently been gained of the application of emulsified acids in matrix acidising. This paper discusses the use of emulsified acid as a stimulation fluid for matrix treatments in heterogeneous carbonate formations.
Theoretical and experimental studies published on the acid wormholing process in carbonate formations have shown the efficiency of systems that have low diffusion rates, for example acid-in-oil emulsion systems. Emulsified acid is especially effective when the injection rate is low, such as in low permeability formations or damaged formations. In these cases, plain HCl acid will spend on the formation face and will not create wormholes that penetrate deep into the formation. At comparable injection rates, emulsified acid is capable of forming deeply penetrating wormholes and efficiently stimulates the formation.
In this paper, results of flow tests are presented which compare the efficiency of emulsified acid with that of plain HCl acid. Several emulsified acid systems were tested. The effects of the injection rate, viscosity and acid/oil volume ratio were analyzed on core samples. Rheological properties and temperature stability (up to 250 F) of the emulsion systems were analyzed by means of Fann-50 tests.
Acid-in-oil emulsions are effective stimulation fluids in large intervals where streaks of high-permeability can act as thief zones. This is shown by means of example calculations of the fluid flow and distribution in a well.
Practical and economical aspects of mixing and pumping emulsified acid in the field will be discussed. Characteristics of candidate formations will be summarized and rules-of-thumb for preliminary treatment design will be presented.