Development of a Deep-Penetrating Emulsified Acid and Its Application in a Carbonate Reservoir

Abstract

The goal of matrix stimulation of carbonate reservoirs is to improve near wellbore conductivity by generating wormholes and increasing the effective wellbore radius, bypassing damaged areas. In naturally fractured carbonates, the wormholes connect the existing fractures, creates longer flow channels. Most widely used stimulation method in carbonate reservoirs is matrix acidization using HCl with various concentrations. The rapid reaction of HCl with carbonate matrix does not allow the reactive fluid to penetrate deep inside the formation before the acid spends. Therefore, the efficiency of conventional acid is limited, where deep penetration is needed. Stable emulsified acids are found very effective, to combat the challenge. The basic advantages of using emulsified acid are: generates longer wormholes by deep penetration into the formation; better zonal coverage; preferential stimulation of oil zones and less corrosive to workstring and downhole/surface equipment. Based on the extensive laboratory studies, a stable deep-penetrating emulsified acid has been developed for carbonate reservoirs. The formulation is a mixture of HCl with compatible additives, diesel and some suitable emulsifiers. The ratio of the dispersed acid and external diesel phase is maintained as 70:30. Stability of the system is a major concern for adequate retardation and penetration deep inside the reservoir. Stability of emulsion largely depends on reservoir temperature and the HLB number of emulsifier. Two surfactants were identified for the system and the HLB number has been maintained in the range of 5.1–5.2 by optimizing the ratio of two surfactants for achieving the maximum stability. The developed acid system has been applied in some wells of an offshore field of India with encouraging results. The field comprises of naturally fractured limestone formation with number of hydrocarbon bearing sands. This paper describes the development of the Deep Penetrating Emulsified Acid and its application in the field along with treatment methodology and outcomes. Introduction Traditionally, HCl has been used for acidizing carbonate reservoirs as it has high rock dissolving power, does not generate insoluble reaction products, generic in nature and easily available.1, 3 Carbonate acidizing with HCl shows the formation of macroscopic channels, called wormholes. Formation of wormholes is the preferred dissolution process as it establishes good conductivity between reservoir and wellbore. Acid travel inside a pore can be represented by two perpendicular fluxes; a) axial transport by convection and b) transport to the pore walls by diffusion. During injection, the bigger pores accept more acid and it is used to increase the pore length and diameter, which promotes wormhole propagation.1 With low injection rate, all the acid gets consumed in the wellbore face itself, called compact dissolution. Compact dissolution leads to poor stimulation and should be avoided. Therefore, combination of high injection and low diffusion rate is favorable for the growth of denser and longer wormholes. However, the low viscosity and high reactivity of plain HCl often results in poor wellbore coverage and limits the propagation of wormholes. Hence, plain HCl is not always a good choice for stimulating heterogeneous carbonate reservoirs. A stable emulsified acid with controlled diffusion rate of acid towards the rock surface is favorable for the growth of desired wormhole pattern.

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.…”

“…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 .…”

“…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. ,, …”

“…Sarma 32 (2007), investigated the effect of emulsifier content and temperature on the stability of the emulsion by studying a coarse emulsion of water and oil consisting of 15 wt % hydrochloric acid and diesel oil. It was emphasized that emulsion’s stability depends on the quality of the surfactant used.…”