Matrix acidizing is a means to stimulate wells; in order to increase the productivity or injectivity. Amongst the wide acidizing fluids, emulsified acids are the most commonly used. Emulsified acids have a reputation to retard the reaction with formation rock, hence they cause deeper penetration. Apart from that, they need minimum additives, since acid is encapsulated by oil phase. That being said, only the oil phase is in immediate contact with the tubing. Thus, granting less corrosion issues. All of these perks of emulsified acids are based on the mere condition that they are stable during their journey in the well to reach the formation rock. Therefore, a stable emulsion (emulsified acid) is extremely critical to the acidizing job, and the degree to which it is stable dictates how successful the job will be. This work aims to experimentally study the factors which contribute to create a stable emulsion by conducting thermal stability and rheology tests. These factors include mixing means (magnetic stirrer & high-power mixer), emulsifier concentration (up to 1.5vol%), mixing speed (up to 1500 rpm), addition rate of acid phase to diesel one (separatory funnel & syringe pump). Moreover, the emulsified acid is prepared using 15wt% HCl and diesel. The volume ratio of acid-to-diesel is 70:30. And, all of the thermal stability tests are conducted at 120°C in a heating oven. Furthermore, the apparent viscosity is measured at different temperatures and supported by droplet size distribution. The results show that emulsifier concentration and mixing speed have significant effect when the emulsion is prepared using magnetic stirrer; however, this effect diminishes when the mixing means is a high-power mixer. Interestingly, the addition rate plays a key role on the stability; even a small difference as 0.5 mL/minute would result in changing the behavior of the emulsion at 120°C. Also, the results confirm that the smaller the droplet size of an emulsion, the more viscous it becomes. Furthermore, temperature increase results in emulsion degradation. This study offers a unique way to look at emulsified acids and addresses factors that have never been addressed before. The novelty of this study is the comprehensive knowledge it brings about the factors affecting the stability of HCl emulsion. It helps to better understand HCl emulsion and how it works. Hence, production engineers can design and conduct acidizing jobs seamlessly.
The main objective of matrix acidizing is to create deep channels(wormholes) that bypass the damaged zone around the wellbore, which eventually, will increase the productivity or injectivity of the well. HCl-diesel emulsified acid is used to achieve that goal as well as other objectives such as prevention of tubing corrosion. The cost of HCl-emulsified acid could be lower by reducing the cost of either the surfactant (emulsifier) or the continuous phase (diesel). Hence, in this study, a low-cost, more efficient emulsified acid is proposed, with cheaper solid stabilizer (nanoparticle) instead of surfactant, and less expensive continuous phase; waste oil instead of diesel. Rheology, stability and coreflooding experiments were performed at high-pressure high-temperature (HPHT) of a typical reservoir conditions in the Middle East. This study addresses the feasibility of a novel waste oil emulsified acid system in acid stimulation treatment. In particular, rheology and stability of the emulsion and reactivity with limestone reservoir rock at HPHT were investigated. The reactivity was carried out through a series of core flow experiments at HPHT and compared with the conventional diesel emulsified acid. Stability experiments were conducted at 375 °F. Coreflood experiments were conducted using Indiana limestone 12″ cores and 1.5 in diameter. The initial permeability of the cores was in the range of 2-4 md. The pressure and temperature were 3000 psi and 275 °F, respectively. Four injection rates were used 0.5, 2, 5, and 10 ml/min. For better comprehension for resulted wormhole characteristic from both stimulation fluids under investigation, the CT-scan image for the Indiana limestone cores after injection of both waste oil and diesel emulsified acids were performed and analyzed through a high resolution imaging and visualization software. From the laboratory results, the novel waste oil emulsified acid system showed a good potential as a stimulation fluid. For instance, it achieved lower pore volume of acid to break through (PVBT) compared to both conventional diesel emulsified acid and plain HCl. Moreover, diesel emulsion presented a better performance at lower injection rates whereas waste oil emulsion performed superiorly at higher rates. Thisindicates that, diesel emulsion is still better in terms of stimulation efficiency, however, waste oil emulsified acid still could be recommended in acid stimulation since it has low cost and acceptable performance. Furthermore, CT scan analysis shows that waste oil emulsified acid was capable of achieving narrow, branch-free, and deep wormhole which are the desired characteristic for any potential stimulation fluid. The novelty of this work comes from the fact that waste oil emulsified acid performance for carbonate acid stimulation has not been addressed before and hence this work to fill in the gap.
In carbonate reservoirs, hydrochloric acid (HCl) is commonly used for stimulation purposes. The main disadvantage for using HCl is its high and rapid reactivity allowing for less control on acid reaction with formation. In addition, HCl may react with the tubing causing severe corrosion and damages. Several slow-reacting acid systems, though HCl-based, have been developed to retard the HCl reaction and control its reactivity. Therefore, the emulsified acid can flow deeper inside the formation before it breaks out and reacts with formation rock. However, the selection of the hydrocarbon phase of emulsified acid is critical to the creation and stability of the emulsified acid system. Diesel oil, crude oil and solvents such as xylene have been reported to be successful in the field. In this work, alternative and less expensive source of oil is utilized to replace the current practices, known as waste oil. Filtered waste oil is used to prepare the emulsified acid of this work. Extensive work has been carried out to study the chemical composition of the waste oil, namely Gas Chromatography (GC) to select the proper emulsifier. Emulsions are prepared using 15% HCl with emulsifier concentration ranging from 0.5 to 2.0 vol% and 70:30 of acid-to-oil ratio. Later, rheological analyses of the waste oil emulsion are compared to the commonly used diesel-based emulsified acid. Results showed the potential use of waste oil to prepare high temperature emulsified acid. The addition of cationic emulsifier helps to formulate a more stable emulsion. Apart from that, the optimum concentration of the emulsifier was at 1 vol%. The rheological data confirms the stability of the new emulsified formulation at higher temperatures. Moreover, to apprehend the behavior of this emulsion, droplet size distribution is addressed. This paper summarizes the findings of using waste oil emulsion and recommends it for field applications. The novelty of this work is the viscoelastic properties for waste oil emulsion have never been addressed before. Addressing them, will help to comprehensively apprehend this novel emulsified acid.
In matrix acidizing, the aim is to create long conduits (wormholes) inside the reservoir formation which ultimately results in productivity increase. Acid in oil emulsion is used in the industry for stimulating carbonates with diesel commonly as the oil phase and hydrochloric acid (HCl) as the acid phase. Emulsifying HCl has numerous benefits over regular HCl. Perhaps the main benefit is deep penetration near the wellbore. Also, less corrosion damages are caused since the external phase is hydrocarbon (diesel). Several studies showed the success of replacing diesel as an external phase with other hydrocarbon oil, such as crude oil and xylene. This work utilizes the extra hydrocarbon left unused –or sometimes dumped– from refineries, referred to as waste oil. The chemical composition of waste oil is studied. The HCl-waste-oil emulsion is prepared using 15 wt% HCl with a ratio of 70:30 of acid-to-oil. This paper reports results on the thermal stability and rheological properties of the HCl-waste-oil emulsion. All thermal stability experiments are conducted at a high temperature, 120 °C. The results show that the HCl-waste-oil emulsion is a shear-thinning fluid. Power-law model is applicable seamlessly to all of the apparent viscosity data for all measured temperatures. The optimum conditions are found to be; 0.5vol% and 600 rpm for emulsifier concentration and mixing speed, respectively. Overall, the lab results show a promising potential for the HCl-waste-oil emulsion to be used in the field. This work takes into account; reducing the budget of acidizing job along with environmental concerns. Apart from using low emulsifier concentration and low mixing speed, it makes use of the unwanted hydrocarbon from refineries.
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