To meet rising global demands for energy, the oil and gas industry continuously strives to develop innovative oilfield technologies. With the development of new enhanced oil recovery techniques, sandstone acidizing has been significantly developed to contribute to the petroleum industry. Different acid combinations have been applied to the formation, which result in minimizing the near wellbore damage and improving the well productivity. A combination of hydrofluoric acid and hydrochloric acid (HF:HCl) known as mud acid has gained attractiveness in improving the porosity and permeability of the reservoir formation. However, high-temperature matrix acidizing is now growing since most of the wells nowadays become deeper and hotter temperature reservoirs, with a temperature higher than 200°F. As a result, mud acid becomes corrosive, forms precipitates and reacts rapidly, which causes early consumption of acid, hence becoming less efficient due to high pH value. However, different acids have been developed to combat these problems where studies on retarded mud acids, organic-HF acids, emulsified acids, chelating agents have shown their effectiveness at different conditions. These acids proved to be alternative to mud acid in sandstone acidizing, but the reaction mechanism and experimental analysis have not yet been investigated. The paper critically reviews the sandstone acidizing mechanism with different acids, problems occurred during the application of different acids and explores the reasons when matrix stimulation is successful over fracturing. This paper also explores the future developing requirement for matrix acidizing treatments and new experimental techniques that can be useful for further development, particularly in developing new acids and acidizing techniques, which would provide better results and information of topology, morphology and mineral dissolution and the challenges associated with implementing these ''new'' technologies.

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Properties of rock, such as effective porosity, permeability and pore size distribution (PSD), are generally referred to as petrophysical properties. These properties are among the most significant for reservoir evaluation. Acid stimulation treatments are usually used in sandstones to mitigate the impact of formation damage, with the aim of restoring or enhancing the natural matrix permeability and consequently boosting the well productivity. Hydrochloric acid (HCl) is commonly used in the preflush stage to remove calcium and other metal ions, preventing the development of calcium fluoride (CaF 2) and other silicate precipitates that could block the pore throats, while an acid mixture (HF-HCl combination) is usually preferred as the main stimulation fluid for the removal of quartz and remaining metal ions. However, sometimes the application of these acids can lead to other problems, including fast reactions, corrosion of pipes, environmental hazards, precipitation reactions and formation damage due to the incompatibility of HCl with clay minerals, so chelating agents have been proposed as an alternative for matrix stimulation fluids. In this study, three different chelating agents, ethylenediaminetetraacetic acid (EDTA), N-(2-hydroxyethyl) ethylenediamine-N,N′,N′-triacetic acid (HEDTA) and N-acetyl-l-glutamic acid (GLDA), have been used to stimulate Berea sandstone, Colton tight sandstone and Guelph dolomite samples. Core flood experiments were conducted on 1.5 × 3 (in 2) core plugs, at a temperature below 180 °F. A slow injection rate of (1-0.5 cc/min) was chosen for the treatment fluid, promoting the dissolution of ions by increasing the contact time between the fluid and the rock. Furthermore, nuclear magnetic resonance, wettability and micro-computed tomography (CT scan) analyses were employed to evaluate the effect of the acid treatment on formation properties such as porosity, PSD, pore topology, wettability and pore structure. After exposing the samples to HEDTA, large wormholes were detected in their pore network, demonstrating that HEDTA has the highest potential to create new pore spaces when compared to GLDA and EDTA when reacted with both types of samples.

The aim of this research is to find the best suitable acid to acidize undamaged low permeable sandstone formation Stimulation of sandstone formations is a challenging task, which involves several chemicals and physical interactions of the acid with the formation. Mud acid has been successfully used to stimulate sandstone reservoirs for a number of years. Matrix acidizing may also be used to increase formation permeability in undamaged wells. The change may be up to 50 to 100% with the mud acid. For any acidizing process, the selection of acid (Formulation and Concentration) and the design (Pre-flush, Main Acid, After-flush) is very important. Different researchers are using different combinations of acids with different concentrations to get the best results for acidization. Mainly the common practice is combination of Hydrochloric Acid-Hydrofluoric with Concentration (3% HF-12% HCl). This study presents the results of a laboratory investigation of Orthophosphoric acid instead of hydrochloric acid in one combination and the second combination is Fluoboric and formic acid and the third one is formic and hydrofluoric acid. The results are compared with the mud acid and the results analyzed are porosity, permeability, strength, color change and FESEM Analysis. All of these new combinations shows that these have the potential to be used as acidizing acids on sandstone formations.

As more technologies to enhance oil production recovery are being developed, well stimulation has been introduced and it acts as one of the important roles to fulfil the demand of world for energy. Well stimulation is a well intervention performed on an oil or gas well to increase production by improving the flow of hydrocarbons from the drainage area into the well bore. Well stimulation generally uses fluids which create or enlarge the flow channels of formation so that it can overcome low permeability of formation. A most common and one of the most successful well stimulation is matrix acidizing. In matrix acidizing, acids are injected at matrix pressure below formation fracturing pressure. The permeability of formation will be increased either by removing the pore-plugging materials in the formation or by creating new or enlarged flow paths in the rock depending on the formation and acid types. The right type of acid or combination of acids must be used to remove the damage. Thus, careful acid choice and treatment design are critical for a successful matrix acidizing. This paper presents the results of laboratory investigation of different acid combinations: HF/HCl, HF/HCOOH, HF/H3PO4, HBF4/HCl and HBF4/HCOOH. Hydrofluoric acid and fluoboric acid are used to dissolve clays and feldspar, while hydrochloric acid, phosphoric acid and formic acid are added to control the speed of reaction to allow the maximum acid to penetrate the core sample and also act as a buffer to maintain pH of the solution. Different tests have been performed on the core samples before and after the acidizing to do the comparative study. The results analyzed are permeability, porosity, color change, strength and mineralogy. Most of the new combinations used in this research showed better results than mud acid. But overall it is clear that in coming future lot of work can be done on sandstone acidizing prospects.