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Dissertation submitted in partial fulfilment of the requirements for the


Academic year: 2023

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Background of the Project

Problem Statement

Objective and Scope of Study

The Relevancy of the Project

Feasibility of the Project within the Scope and Time Frame


Literature Review

The science of acidification traces its origins more than 100 years ago, when Herman Frasch of Standard Oil patented the use of hydrochloric acid to stimulate carbonate formations in Lima, Ohio, at Solar Refinery in 1896 [1]. It was not until the 1960s that hydrofluoric acid treatments were again widely used in well remediation [2]. In sandstone, the primary objective is to remove any existing damage to the formation and restore the permeability of the reservoir (in the area near the wellbore) to its original condition or as close as possible [3].

A fluid routinely used for this purpose is mud acid, which is a mixture of hydrofluoric and hydrochloric acids (HF/HCl) that is able to dissolve most common harmful minerals. Side reactions that occur in almost all acid treatments of mud lead to the formation of precipitates. Formation damage or restriction of flow through natural flow paths in the formation can be categorized into three basic types.

Absolute permeability damage is damage from particulate material that occupies all or part of the formation's pore space. Increased fluid viscosity in the formation from emulsions, polymers, etc. can result in limited flow rates [7]. The purpose of carbonate acidification is to bypass near-drilling damage through the creation of new, highly conductive channels called wormholes.

Therefore, the longer and deeper the penetration of the wormholes, the greater the reduction of the formation skin. Once wormholes are started in the rock around the face of the perforation tunnel, it is desirable to extend them as far into the formation as possible. The final shape, pattern, and length of the wormholes depend on many parameters, including injection rate, type of fluid, and temperature [9].

The second stage is the main leaching and the last stage of sandstone acidification is the post leaching. The pre-wash is designed to displace the existing brine and dissolve the carbonates, while the main wash is designed to dissolve the clay, shale or any other material near the wellbore. Finally, the function of the backwash is to displace the main wash or also known as the mud acid stage [10].


The grain size to be selected and used to make core samples depends on the bulk of the sand retained in a sieve size. In addition, acetone is added to the mixture as it will strengthen the bond between the sand grains. After that, the mineralogy of the collected core sample will be determined using the thin section process.

A portion of the core samples will be cut and grounded in an optical plane, which will be placed under a microscope to determine the properties of the minerals in the formation. Then, before proceeding with the acid treatment, the initial porosity and permeability of the obtained core samples can be measured using the POROPERM instrument. Before the main rinse of the acid treatment was performed, all core samples will be pre-rinsed using an acid combination of 7.5% hydrochloric acid, HCl + 2.5% acetic acid, CH3COOOH.

By using a scanning electron microscope (SEM), which is an imaging tool, a clear microscopic image of the sample can be captured. From the result of acid treatment, the permeability of most samples shows a decrease in permeability. Thus, the use of the core filling equipment will prevent the reduction of the permeability of the samples.

Mineral research is really important and crucial to this study as it will give us an understanding of the reaction of the acid combination used to the mineral of the samples. Unfortunately, the mineralogy of the samples could not be identified in this project due to a lack of equipment. If the time allotted to complete this project is longer, the mineralogy of the samples can be identified.

One of the recommendations to reduce human error is to use a compression unit to compact the sand during sample preparation. Unfortunately, this proposed equipment cannot be used in this project, as the use of a high concentration of acid will corrode the equipment materials, which are mostly steel. We hope that the result of this project will be used by engineers and researchers in the future to understand more about matrix acidification in the sandstone formation as one of the well stimulation techniques to increase the production of hydrocarbons in the reservoir.

Figure 1: „Wormhole‟ created by matrix acidizing in Carbonate Formation  (Source: http://www.yennai.com/acidization.htm)
Figure 1: „Wormhole‟ created by matrix acidizing in Carbonate Formation (Source: http://www.yennai.com/acidization.htm)


Project Planning

Research Methodology

Using this sieving unit, different sizes of sand grains can be collected and if contaminants such as seashells and dried wood are present in the samples, they can be separated from the sand samples. In order for these sand grains to be formed into the core sample, the resin and hardener must be mixed together with the sand sample so that the formation structure can be strengthened. Mixing of the sand sample with resin and hardener as well as acetone was done using a mortar mixer.

Later, the mixture will be left in a steel former at a certain period of time. Next, the mixture was cut using a corer to obtain the desired size of the core sample which is approximately 1.5 inches. In the acidification process, the core samples were saturated using different combinations of acids and different acid strengths.

The purpose of the prewash is to remove unwanted minerals such as carbonate in the core sample before the actual acid saturation is performed. Pre-rinsing is performed using an acid combination of 7.5% hydrochloric acid, HCl + 2.5% acetic acid, CH3COOOH. Two combinations of acids were used during the main wash, as shown in Table 1.

Both the pre-rinse and main-rinse steps must be performed in a fume hood to avoid fumes evaporating, which will be dangerous. Using desiccators with a pump can speed up the process, as this will cause the core sample to become fully saturated with acid. It should be noted here that using acid will corrode the apparatus or tools when performing this experiment.

After the acidification process of core samples, determination of final permeability and porosity was measured using POROPERM instrument.

Figure 6: Sieving unit
Figure 6: Sieving unit

Activities/ Gantt chart and Milestone

List of Materials

List of Apparatus and Equipments

The goal of this project is to find the best combination of acids in sandstone formation for Pangkor sand. All the objectives of this project will be achieved by using experimental results from the laboratory work. In addition, the use of resin and hardener during the preparation of samples to bind the sand grains may not be compacted well and will result in a lot of pore space inside the samples.

The observation from Figure 13 below shows a clear image of the sample that the sand grains do not pack well in the samples. This reduction in permeability is mainly due to the formation of precipitate in the sample after acid saturation. Acid impregnation was carried out using vacuum pump driers which will bring the samples into a vacuum state.

Although the precipitate is formed, it can still be washed out of the samples by the dynamic flow. In the experiment, the saturation time for acid saturation may have been too long, especially for high acid concentrations, such as hydrochloric acid and fluoroboric acid. This conclusion may be different if further research in this project takes place in the future.

Mineralogy of the sample can help us understand which mineral component from sand samples is dissolved by the particular acid used. The second recommendation to improve this project is the use of appropriate equipment for acid treatment. Furthermore, it is important to include post-rinsing in the acidification treatment to remove unwanted sediment in the samples.

Conventional backflush such as combination of 3% ethylene glycol monobutyl ether (EGMBE) with diesel will wash the deposits out of the samples and will lead to an increase in permeability. If the duration to complete the project is longer, backwashing can be introduced in the acid treatment. In the future, in the author's opinion, studies should be conducted on the correct combination for post-rinsing in order to identify the best post-rinsing fluid to remove the residual deposits that formed in the samples.

All the procedures used in the experiment for this project are a combination of research and ideas of the laboratory technologist. The Importance of Secondary and Tertiary Reactions', paper SPE 54728, presented at the 1999 SPE European Formation Damage Conference, held in The Hague, Netherlands, 31 May - 1 June.

Table 4: Acid saturation for mainflush result
Table 4: Acid saturation for mainflush result




Figure 1: „Wormhole‟ created by matrix acidizing in Carbonate Formation  (Source: http://www.yennai.com/acidization.htm)
Figure 2: Acid dissolution created in formation
Figure 3: Project Activities Flow
Figure 4: Project Flow Chart



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