An Optimal Foam Quality for Diversion in Matrix-Acidizing Projects

Abstract

Abstract The proper placement of acid is a crucial factor in a matrix-acidizing project. Unfortunately, much of the acid flows through the less-damage zones, leaving the damage area untreated. Foams have been used to divert the acid to the damage areas. However, the complexity of foam behavior in a porous mediahas reduced the efficiency of the diversion. Behenna1 claimed that if foam is generated or injected into the less-permeable layer (damage area), or if insufficient foam is injected into the more-permeable layer, diversion is not obtained. More recently, Alvarez et al.2 concluded based in aproposed model, the fg* model, that foam injection in the low-quality regime (low foam quality, fg) is better for acid diversion. They claimed that in this regime, more foam is injected into the higher-permeability layer before switching from foam to acid. In this study experimental results confirming the implications of the fg* model are shown. Additionally, the experimental results suggest the existence of an optimum foam quality value for improving the efficiency of an acid-diversion work. Experiments were carried out in a composite core(parallel cores) having a permeability ratio of 15. The cores were packed withsand having different particle diameter to obtain the permeability contrast. An anionic surfactant was used in these experiments. Foam quality was evaluated from 95 percent to less than 40 percent. All the experiments were performed at600 psi outlet pressure and room temperature. The gas used was nitrogen. The existence of an optimum foam quality for acid diversion implies that matrix-acidizing projects should be designed carefully. Experimental work, such as the one performed in this study, and the fg* model may help tochoose in advance the foam quality condition for a particular matrix-acidizing project obtaining a more cost-effective job. Introduction Matrix acidizing is defined as a treatment used to remove damage near the wellbore3. This treatment intends to dissolve any damage and some particles of the porous medium increasing or reestablishing its permeability. The technique consists in injecting a reactive fluid (acid) into the porous medium at pressure bellow the fracturing pressure. The most common acids used in the petroleum industry are HCl and HF. Matrix acidizing has been used extensively in the oil industry. However, its success depends strongly in the proper placement of the acid into the more damage zones4,5. Unfortunately, much of the acid flows through the less-damage zones, leaving the damage area almost untreated. Therefore, diversion techniques are required in order to improve the efficiency of any matrix-acidizing job. In the last two decades, foams have been widely used as diverting agent in matrix-acidizingprocesses. Factors favoring the use of foam over solid diverters are the cost of the latter and the lack of confidence in the effectiveness by using solids for long intervals6. Bernard and Holm7 were the first to indicate that foam could selectively block high-permeability zones, or by analogy less-damage zones. Smith et al.8 proposed the use of foam as a diverter agent in acidizing jobs. They found, using two parallel core holders packed with sand with a permeability ratio of 20, that 99% of the fluid was diverted into the less-permeable layer. Burman and Hall9 concluded that diversion efficiency increases with permeability and decreases as foam quality is increased. They experiments were performed in two cores with porous medium of permeabilities varying from10 to 158 md. Thompson and Gdanski10 concluded that foam-acid diversion is limited by permeability differences. More specifically, they observed that good diversion is expected for permeability ratios less than 10:1. Their experiments were carried out with parallel limestone cores with permeabilities ranged from 1. 5 to 9 md. They also pointed out that a minimum foam quality of 60% is required to block the acid flow and that foams of 70% or 80% were the most effective.