Stimulation of Water Disposal Wells Using Acid-In-Diesel Emulsion: Case Histories

Abstract

Two wastewater disposal wells in a carbonate field in Saudi Arabia suffered loss of injectivity due to severe formation damage. Lab tests conducted on reservoir cores indicated that regular 15 wt% HCl did not form deep wormholes and caused surface wash-out only. A thorough experimental study was conducted to evaluate using acid-in-diesel emulsions to stimulate these wells which had several tight zones. The emulsified acid consisted of 70 vol% of 15 wt% HCl, 30 vol% diesel and an emulsifier (a cationic surfactant). This is the first time emulsified acid has been used to stimulate disposal wells. Experimental results indicated that the acid-in-diesel emulsion behaved as shear-thinning fluid. The stability of the acid was found to be a function of emulsifier concentration. The reaction rate of the emulsified acid with reservoir rocks depended also on emulsifier concentration at the reservoir temperature (55 °C). Very low reaction rates were obtained at emulsifier concentrations greater than 20 gals/1000 gals were of acid. These results indicated that longer soaking times would be needed to stimulate disposal wells. This is to ensure complete acid spending. Coreflood results showed that the emulsified acid formed deep wormholes in tight carbonate cores (< 100 md), where the core permeability increased after the treatment. Permeability ratio (final/initial) of reservoir cores exponentially increased with the acid injection rate. The size and number of the wormholes depended on the acid volume, injection rate and initial core permeability. The acid-in-diesel emulsion (280 cp at 2 s−1 and room temperature) was applied in two wastewater disposal wells. The designed treatment included a gelled acid stage for acid diversion, and an emulsified acid stage to create deep wormholes in the reservoir. The treatment was very successful and the injectivity of both wells has significantly increased.

A Review on Use of Emulsified Acids for Matrix Acidizing in Carbonate Reservoirs

“…Water in oil emulsions provide better retardation due to the low diffusivity created by the oil phase around acid droplets, which allows for minimum acid volume contact with t h e r o c k − t u b u l a r s u r f a c e . T h u s , m o s t r esearch 18,26,[29][30][31][32][33][34][35][36][37]39,40,[42][43][44][45][46][47][48][49][50][51]53 has adapted W/O emulsions due to their desirable retardation effect and corrosion inhibition in comparison to O/W emulsions, 27,28 as depicted in Figure 5.…”

“…Beyond critical micelle concentration, droplet size does not decrease; however, in the case of pickering agents, this can lead to increased stability with a relatively lower amount than that used for surfactant. Moreover, macroemulsions are favored for matrix acidizing over microemulsions, which provide sufficient stability with ample acid volume. − The use of a pickering emulsion and a hybrid emulsifier (combination pickering particles and surfactants or polymers) greatly minimizes the emulsifier concentration while forming a strong structural network, which provides enhanced stability. ,, …”

“…Coarse emulsions have until now been considered more cost efficient in matrix acidizing operations due to having low amount of surfactants and the ability to hold large volume of acid in contrast to microemulsions, which require large volume of surfactants for stability. Hence, very few publications − created microemulsions, whereas research studies ,− created coarse emulsions, as illustrated in Figure .…”

“…To validate that the emulsion formed is water in oil, an electrical conductivity test is performed, which would show near-zero electrical conductivity if oil is the external phase. ,,, If there is slight conductivity, then the emulsion is stirred for an additional 10–20 min maximum to ensure proper homogenization of the phases. , This also highlights the resistance to instability by the colloid due to increase in oil percentage and viscosity. As the oil percentage increases, the emulsion viscosity increases, thereby enhancing the severity of emulsification.…”

Application of Novel Polymer Assisted Emulsified Acid System Improves the Efficiency of Carbonate Acidizing

“…Several field studies have been reported for the application of emulsified acid (Al-Anazi et al 1998;Nasr-El-Din et al 2000;Madyanova et al 2012). Application of matrix acid treatments in heterogeneous formations or large permeability contrast formations requires a diverting agent stage and then treatment of the well with the emulsified acid.…”