Improving the Understanding of Matrix Acidizing and Acidizing Design

This report delves into the critical aspects of matrix acidizing in petroleum engineering, focusing on enhancing hydrocarbon production through improved well stimulation techniques. It begins with an introduction to hydrocarbon extraction and the challenges of declining production rates due to formation damage. The study outlines the objectives of understanding matrix acidizing and identifying suitable acid formulations to enhance permeability. A comprehensive literature review covers well stimulation methods, matrix acidizing processes, sandstone acidizing practices, and experimental studies involving various acids like mud acid, fluoroboric acid, chelating agents, and organic acids. The report details the design of a matrix acidizing setup, including system requirements, components, and experimental parameters, followed by a methodology section explaining core flooding procedures and laboratory measurements. Ultimately, the study aims to provide insights into optimizing acidizing treatments for maximum recovery in carbonate and sandstone reservoirs, addressing the limitations and constraints encountered during research.

PETROLEUM ENGINEERING
[Author Name(s), First M. Last, Omit Titles and Degrees]
[Institutional Affiliation(s)]

Contents
INTRODUCTION...........................................................................................................................................3
1.0 Background........................................................................................................................................3
1.1 Problem Statement............................................................................................................................5
1.2 Objectives and Scope of Study...........................................................................................................6
1.3 Research Structure............................................................................................................................7
LITERATURE REVIEW....................................................................................................................................9
2.1 Introduction.......................................................................................................................................9
2.2 Well Stimulation................................................................................................................................9
2.3 Matrix acidizing................................................................................................................................11
2.4 Sandstone Matrix acidizing..............................................................................................................12
2.4. 1 Mineralogy of Sandstone.........................................................................................................12
2.5 Sandstone Matrix Acidizing Practice................................................................................................14
2.6 Treatment Design of Matrix Acidizing..............................................................................................15
2.7 Additives in Matrix Acidizing............................................................................................................16
2.8 Wormholing or channelling of sandstone........................................................................................16
2.9 Literature Reviews on Experimental Studies Conducted.................................................................19
2.9.1 Mud acid (hydrofluoric-hydrochloric acids)..............................................................................19
2.9.2 Fluoroboric acid........................................................................................................................22
2.9.3 Chelating agents.......................................................................................................................24
2.9.4 Organic Acids and Retarded Acids............................................................................................28
2.10 Review Summary...........................................................................................................................32
DESIGN OF A MATRIX ACIDIZING SETUP....................................................................................................34
System requirements............................................................................................................................35
Components of system..........................................................................................................................35
Experiment Parameters.........................................................................................................................39
METHODOLOGY.........................................................................................................................................41
Setup of Core flooding...........................................................................................................................41
Core flood experiment...........................................................................................................................42
Laboratory Procedures/Measurements................................................................................................42
Results.......................................................................................................................................................49
Conclusion.................................................................................................................................................56
References.................................................................................................................................................57

IMPROVING THE UNDERSTANDING OF MATRIX ACIDIZING AND ACIDIZING
DESIGN IN A PETROLEUM WELL
INTRODUCTION
1.0 Background
Hydrocarbons which are mainly oil and gas are extracted from numerous miles beneath the
ground. They are collected within a rock formation which has proper permeability and porosity.
They flow via the channels in between various pores from the rock pores all the way to the
production tubing and end up at the surface level finally. In the ancient production years, the
rates of flow of such hydrocarbons generated were promising owing to the naturally high
permeability and porosity of reservoirs (Akanni & Nasr-El-Din, 2015). Another reason for the
promising production during the early years was attributed to the high pressure difference
between the pressure of reservoir and the pressure of bottom hole.
Nonetheless, there has been a reduction in the production over time owing to numerous causes
including formation damage near the wellbore, declines in the pressure of reservoir as well as
swelling or migration of clays which then lead to clotting of pore spaces.
Acidizing treatment of one of most widely used and recommended techniques in the
enhancement of production of hydrocarbon after some time of production and has been use for
numerous years. A graph of Nigerian oil well upon treatment of mud acid and hydrofluoric acid
is shown in figure 1 (Akanni & Nasr-El-Din, 2016). The figure demonstrates that there are not
some much increments with mud acid with regard to the rate of production even though on other
hand, hydrofluoric acid treatment offers significant recovery in the field. This fact offers proof

that the appropriate acid combination for a given field would enhance production of
hydrocarbons.
Figure 1: Production improvement after treatment with hydrofluoric acid
Various types of acidizing treatments have been developed including matrix acidizing, acid
washing and fracture acidizing among other treatment types. The purpose in acid washing is
basically tubular and wellbore cleaning. There is no consideration to the treatment of formation
that is generated. Acid washing is mostly performed in cleaning out scale among other materials
which inhibit flow within the well.
For the case of fracture acidizing, an acid is introduced into a fracture that has been created by
acid itself or through a viscous fluid that is used in the generation of fracture. The acid reacts
with the fracture walls as it progresses leading to dissolution etching (Ali & Nasr-El-Din, 2018).
Matrix acidizing which is the main focus of this dissertation refers to a process where an acid is
introduced into formation at pressures that are lower than the fracturing pressure of formation
thereby there is no formation of fractures. The injected fluid undergoes a reaction with the

formation and some of present materials dissolved and thus removal of formation damage and/or
enhancement of permeability in the near-wellbore area. The acidizing process result in maximum
recovery of carbonate as well as sandstone reservoirs alongside enhancing economic reserves.
1.1 Problem Statement
There is a decrease in the production of hydrocarbons over time. This scenario may take place
owing to the few reasons among them presence of dissolved minerals in the pore spaces,
formation damage near well bore and enhanced skin factor among other possible explanations.
All the stated reasons would reduce the initial permeability and porosity of formation which
would in turn lead to a decline in the production of formation.
Matrix acidizing stand out to the first method that has been recommended in fixing the challenge.
Yet the acidizing process is unique for every formation and may not be generalized. Among the
factors influence the condition include formation history i.e. how it has been shaped, acidizing
history i.e. if there has been an acidizing treatment that has been used into formation as well as
location of formation n i.e. dome, anticline among others.
More damage would occur to the formation should the acid used be incompatible with each of
them or with the formation fluids of formation. By examining the sample of core extracted from
the wellbore (evaluation of features including solubility, formation lithography, mineralogy
among others) is then only way that a method may be chose for optimization of production.
This project is associated with a few constraints that are met in the course of conducting research
(Babaei & Sedighi, 2018). Equipment and material unavailability in the lab tend to be a limiting
factor as it confines the experiment to be similar to the ones that have so far been carried out in
the industry. a few of such constraints including the sample not being extracted from real

formation, unavailability of gas for SEM machine to be used in the identification of arious cores
as well as unavailability of equipment to be used in the injection of acids. Nonetheless, the
experiment would still proceed since the main objective is to establish the most appropriate and
effective acid formation that would result in an increase in the rock properties especially with
regard to permeability as well as porosity.
1.2 Objectives and Scope of Study
The main objective of this study is to gain a better understanding of matrix acidizing and come
up with the most suitable acid formulation which enhance the mineral properties the most,
specifically permeability. An increase in the permeability results in an increase in the ease of
flow via the connected pores ad thereby an increase in the rate of production in real formation
through the production tubing up to surface level. A few prior concerns that have been taken into
consideration include:
 Finding out the most appropriate acid formulation which would enhance the performance
of core the most, specifically permeability
 Examine the initial features of core (Badri & Taherian, 2018)
 Performing an experiment to test the impacts of different acid formulations on production
of formation
The scope of study is inclusive of:
 Carrying out research, calculations, experiments as well simulation with regard to matrix
acidizing
 Use of conventional acid systems including HCO, HF or even a combination of two
alongside boric acid only

 The cores are saturated using various acid formulations as there is not equipment in place
for the injection of acid in laboratory
1.3 Research Structure
This dissertation has six chapters as discussed below
Chapter 1: Introduction-The research topic is introduced and a brief background given. The aim
and objectives, research questions as well as significance of study are outlines. The main purpose
if this chapter is to introduce and brief the reader of dissertation on what is contained in other
chapters
Chapter 2: Literature review-Previous journals and works of other authors regarding
understanding of matrix acidizing and acidizing design in a petroleum well. The main purpose of
this chapter is to equip the author and reader with a deep understanding of topic of research.
Chapter 3: Methodology-The chapter discusses approach adopted in data collection as well as
various data collected techniques adopted
Chapter 4: Discussions-The information which was obtained from the literature review is
discussed in details with reference to research aims and objectives. In this chapter, the research
questions are answered.
Chapter 5: Conclusion-This chapter offers study summary of research conducted.
Recommendations and final remarks are included in this chapter
Chapter 6: References-All information sources of data used in the dissertation are noted in this
chapter and such include books, journals, and articles among other sources which contain
relevant information to the research topic.

LITERATURE REVIEW
2.1 Introduction
The demand for energy around the globe has continued to be on increase in the recent past. As
per the prediction, it is anticipated that the demand of energy will be 40% more that the present
by year 2020 and thus the continuous expansion of limit and boundary in the oil and gas sector
with regard to technology and commercial aspects. The development of innovative as well as
feasible technology in stimulating and enhanced oil recovery has turned out to be one of main
focuses in the oil field.
2.2 Well Stimulation
The gas and oil operations including work over, drilling, completion, production alongside long-
time operations led to the deposition of minerals close to the wellbore over time. As a result, this
leads to the depletion in the production owing to the formation of damage which takes place
about the wellbore. Hence, the field engineers have to establish the solution to carry out well
treatment to hence the productivity of wells to ascertain economic returns. One such technique is
labelled well stimulation (Bastami & Pourafshary, 2016).
Well stimulation is a method that is used in the enhancement of production of gas or oil from a
reservoir to the wellbore. It has played an integral role in the oil and gas wells development
ascertaining good economic returns. Numerous creative and innovative approaches are currently
being used in the treatment of wells. Hydraulic fracturing is being used in improving the
production of oil and gas through the creation of fracture in the reservoir via injection of
hydraulic fluid pressure. Hydraulic fracturing still occupies the greatest interest in well
stimulation in the industry.

Nonetheless, acidizing as well plays a significant role in various case studies with the use of
main acid including hydrochloric acid and other acids among them acetic acid, formic acid as
well as hydrofluoric acid is equally important. fracture acidizing, matrix acidizing and hydraulic
fracturing the most commonly used stimulation techniques. Each of methods comes along with
limitations as well as advantages in well stimulation.
For the formations of sandstone, matrix acidizing often have limited depth penetration. Ideally,
matrix acidizing has a shorter depth of penetration of about 0.3M as compared to fracture
acidizing and hydraulic fracturing. It is usually not used for formations having low permeability
since it needed long penetration depth in order to be stimulated successfully. Nonetheless, matrix
acidizing is an effective and viable technique especially in cases where the well is fractured
naturally and is often used in the removal of formation damage close to the well that prevents
flow into well (Bhatnagar & Alam, 2017). Thus, the acid may dissolve the plugging minerals in
production flow path. The various methods for well stimulation have various suitability and
practicality for various formations as shown in the figure below
Figure 1: Various methods for well stimulation

2.3 Matrix acidizing
Matrix acidizing is a widely developed method in well stimulation. Acids are injected to the
bottom hole well using a pressure that is less than the pressure of formation of fracture during
acidizing process. Since the 1960s, sandstone matrix acidizing has remained to gain an avalanche
of applications as well as remaining to play a vital role in the petroleum industry. Various acids
had been invented for use in the stimulation of depleted reservoirs of sandstone after numerous
years of production of oil and gas. Acids have a major role to play when it comes to the
improvement of permeability as well as porosity of a reservoir formation. The acids as well serve
to lower the formation damage besides improving the productivity of well (Duthie, Saeed,
Shaheen, Saiood, Moore & Krueger, 2017).
Mud acid is the most commonly used acid in the treatment of sandstone stimulation. The mud
acid is formed by mixing hydrofluoric acid and hydrochloric acid. This is done due to the fact
that hydrofluoric acid is able to adequately dissolve the minerals that are found in a sandstone
matrix while hydrochloric acid has properties that enable the control of precipitation. However,
high temperature well acidizing tend to be of immense importance. Reservoirs having great
temperature and depth have turned out to be the epicentre of attention in the recent past to oil and
gas exploration reserves.
Wells are generating from deep hot reservoir using a temperature that is greater than 200⁰F and
such reservoirs normally have temperatures that range between 100⁰F and may go to higher than
500⁰F. The use of mud acid has been limited recently in the evolution of matrix acidizing with
focus being on wells having high temperatures. Hence, all the components of acid simulation
including corrosion rate, acid efficiency as well as stability have to be enhanced to ascertain
successful treatment of well.

There are a few significant challenges of adoption of mud acid at temperatures that are above
200⁰F. Mud acid may result in high reaction rates with the contents of minerals in the sandstone
matrix at conditions of high temperature. The resultant is a reduction I the effect of acidizing
process and could even completely fail in the worst case scenario owing to early and rapid
consumption of acid (Frick, 2018). Still, the use of hydrofluoric acid during situation of well was
found to be resulting in significant reduction in the compressive strength formation especially in
regions that are having high content of clay.
As a result, there will be disintegration of formation leading to reduction in permeability and
porosity. Besides, mud acid is associated with highly corrosive features that make it a life hazard
acid which is challenging for safety control as well as health. Hence, the acidizing treatment is
rendered of lower efficiency (Garrouch & Jennings 2017). It can be noticed that the high
temperature sandstone acidizing method is experiencing an upsurge in demand hence has turned
out to be a focus of attention among current researchers.
2.4 Sandstone Matrix acidizing
2.4. 1 Mineralogy of Sandstone
Sandstone, sometimes called arenite is a clastic sedimentary rock that is composed of silica and
numerous silicate minerals. Sandstone matrix is mainly composed of feldspar, quartz and various
forms of clay with the presence of zeolite at times even though in vary low concentrations. The
concentrations of various minerals present in an ideal Berea sandstone core sample that is used in
the industry for the purposes of core flooding test is shown in the table below.