Novel Application of Emulsified Acids to Matrix Stimulation of Heterogeneous Formations

Abstract

Summary Historically, emulsified acids have primarily been used in fracture acidizing. 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 acidizing. This paper discusses the use of emulsified acid as a stimulation fluid for matrix treatments in heterogeneous carbonate formations. Emulsified acid is diffusion retarded which makes it an effective wormholing fluid at low injection rates that occur, for example, in low permeabiliy or damaged formations. At these low rates, plain HCl acid will mostly spend on the formation face and is unable to create wormholes that penetrate deep into the formation. In this paper, results of flow tests are presented that 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. Introduction The goal of matrix stimulation of carbonate formations is to decrease skin by creating wormholes and to increase the effective wellbore radius while bypassing damaged areas. In naturally fractured carbonates, the wormholes will connect the existing fractures, thereby creating long and deeply penetrating flow channels. In formations without natural fractures, the wormhole length will be less, but it may still be several feet. The permeability of the near-wellbore area containing the wormholes is usually several orders of magnitude larger than the original permeability of the rock and skin values of -2 to -4 after an acid treatment are possible.1–4 The two main challenges that have to be addressed when designing a matrix treatment in a carbonate are acid placement and; acid penetration and optimum wormhole growth. Proper acid placement and fluid diversion is becoming increasingly important with the advent of longer (horizontal) completions in carbonate formations. Especially in heterogeneous formations in which the permeability contrast is large, injected fluids have the tendency to enter the sections that have the highest permeability and/or lowest skin. The low permeability or high skin sections will accept only little fluid. An effective diversion method is essential to help ensure complete zonal coverage and to remove damage from the entire producing interval. One method to improve zonal coverage is to use fluid systems with a viscosity significantly higher than the viscosity of the reservoir fluid.5,6 Acid-in-oil emulsions can have viscosities in the wellbore of 50 cp or more and they improve zonal coverage considerably, compared to plain acid. The problem of acid penetration and optimum wormhole growth is directly coupled to the acid placement problem. As discussed above, the low permeability or high skin sections in a heterogeneous formation accept only little acid. As a result, the velocity of the injected acid in these sections may actually be too low for wormholes to form and all of the acid will spend on the wellbore wall with little or no live acid penetrating deeper into the formation. This "compact dissolution" does not result in significant skin reduction and must be avoided if possible. In this situation, a retarded acid system such as viscosified acid or emulsified acid will improve the wormholing efficiency because it will provide deeper acid penetration. Traditionally, HCl has been the workhorse in carbonate acidizing. HCl is cheap, it has high rock-dissolving power, it does not generate insoluble reaction products, and it reacts very quickly with carbonates. A high surface reaction rate is an essential property for an acid because it leads to an overall acid spending rate that is limited by convection and diffusion processes, which is a requirement for wormholing to occur.7,8 However, from the point of view of acid placement and acid penetration in low permeability or high skin sections, plain HCl is not always a good choice. The low viscosity of plain HCl acid (about equal to water) often results in poor wellbore coverage unless highly effective diversion methods are used to ensure treatment of the entire pay zone. Furthermore, the high reaction rate of HCl may prevent the formation of deeply penetrating wormholes and lead to compact dissolution, especially in zones in which the injection rate is low. Acids viscosified with polymers are often used to overcome the problems encountered with plain HCl systems. The higher viscosity improves wellbore coverage9,10 and at the same time it retards the acid spending rate. However, the fluid maintains much of its viscosity while it is in the formation and the injection rate must often be reduced to avoid exceeding the fracturing pressure. Lowering the injection rates has a negative effect on acid penetration and can result in compact dissolution. Another potential problem with polymer gelling agents is filtercake formation which retards the surface reaction rate and inhibits wormhole growth. This paper presents the use of acid-in-oil emulsions as an effective fluid system for heterogeneous formations. Emulsified acids have been shown to be effective fluid systems in fracture acidizing but they have only recently been found to have an application in matrix acidizing.11,12 The combination of a high surface reaction rate (when HCl is the internal phase) and a low diffusion rate allows efficient wormholing in low-permeability zones (at low rates). Emulsified acid systems can be optimized to combine a number of useful properties: relatively high viscosity in wellbore, leading to fluid diversion and improved zonal coverage; emulsion break and viscosity decrease when acid spends; no residual formation damage; retarded acid spending rate, resulting in efficient wormholing at low rates and at high temperatures; high dissolving power, 50 to 80% of plain HCl; low corrosion rates, due to the external hydrocarbon phase; and can be modified on the fly.

Acid Stimulation of Extended Reach Wells: Lessons Learnt From N'Kossa Field

“…As for the viscosity, although it can reach 50 to 100 centipoise at 510 sec -1 , this does not refrain the fluid from being squeezable even in carbonates with permeabilities as low as 1mD, the pressure increase from the squeeze of the DEA emulsion is much smaller than would be expected from the viscosity; this feature was also observed on the coreflood tests described by Buisje and Van Domelen in ref 16 . DEA actually penetrates deeper than straight 1 centipoise HCl, due to its ability to wormhole at these high temperatures.…”

“…However at such high bottom hole temperature as in N'Kossa (150°C) which increases the reactivity of acid and due to the rather low permeabilities (10-50 mD) encountered on many of the intervals which reduces the rate at which we can squeeze in the formation, a straight HCl treatment would likely lead to immediate carbonate dissolution in facial dissolution mode rather than in wormoling mode and would not generate an efficient stimulation 11,12,13,14,15 . Therefore it was decided to treat the formation with a Diesel-Emulsified-Acid (DEA).The benefits of using this diffusion retarded system in high temperature carbonate acidizing have been well dicussed 16,17,18,19 in particular the ability to wormhole even at low injection rates. One striking feature presented by Bazin et.…”

“…These stages were expected to both speed up the pumping rate (by flushing the viscous emulsion by linear HCl in the matrix) and also to widen the wormholes already created by DEA (emulsified acids tend to create narrower wormholes than straight HCl 16,21 ). Thus the pumping sequence consisted in an alternation of linear HCl acid and DEA.…”

Effect of Presence of Crude Oil on the Performance of Emulsified Acids

“…Besides its slow reaction rate with carbonate rocks, it has a relatively high viscosity. As a result, it has a better sweep efficiency that will improve acid distribution in heterogeneous reservoirs (Buijse et al 2000). Also, the live acid does not come in contact with well tubulars; therefore, the corrosion level is low.…”

“…Coreflood results showed that the emulsified acid formed deep wormholes in tight carbonate cores, and the size and distribution of wormholes was dependent on the acid injection rate, acid volume and initial core permeability. Buijse et al (2000) studied the application of emulsified acid in stimulation of heterogonous carbonate reservoirs. Buijse et al (2000) found that acid-in-oil emulsions are effective stimulation fluids in large intervals, where streaks of high-permeability can act as thief zones.…”