CaCO3 çökelmesinin çatlaklı ortamda akış davranışına etkilerinin incelenmesi
Investigation of CaCO3 precipitation's impact on flow behavior in fractured models
Yazar: EDA AY DİLSİZ
Danışman: DR. ÖĞR. ÜYESİ İBRAHİM METİN MIHÇAKAN
İstanbul Teknik Üniversitesi / Lisansüstü Eğitim Enstitüsü / Petrol ve Doğal Gaz Mühendisliği Ana Bilim Dalı / Petrol ve Doğal Gaz Mühendisliği Bilim Dalı
Derece: Doktora, 2023
Konu: Enerji = Energy ; Kimya = Chemistry ; Petrol ve Doğal Gaz Mühendisliği = Petroleum and Natural Gas Engineering

Özet:
Petrol, gaz ve jeotermal akışkan üretilen veya enjekte edilen yeraltı formasyonları içinde jeolojik geçmişten kalan ve/veya girişini halen sürdüren doğal sular bulunur.  Bu sular içerdikleri iyonlar, çözünmüş gazlar, askıda katı maddeler, bakteriler, hidro-karbonlar, radyoaktif maddeler ve hattâ zehirli maddeler gibi unsurlar yönünden oldukça zengin olabilirler.  Böylesine çeşitli içerik ile zengin rezervuar suları yalnızca rezervuar kayacının gözenekler ve/veya çatlaklardan oluşan akış yollarında değil, rezervuardan üretim yapan kuyularda da çeşitli işlemsel (operasyonel) sorunlara yol açabilmekte, yüzeye üretildiklerinde ise yüzey tesislerine, insan sağlığına ve çevreye zararlı olabilmektedirler.  Dolayısı ile, bu tür rezervuar sularının farklı sıcaklık ve basınç koşullarında sergiledikleri fiziksel, kimyasal ve termodinamik davranışlarının anlaşılması, bunların olumsuz etkilerini en düşük düzeye indirgemek ve (eğer olası ise) bu etkilerden kurtulmak için, yapılması gerekenler geçmişten beri bu konuda çalışan insanların ilgisini çekmiştir.
Üretim sırasında elde edilen veya üretimi artırmak için rezervuara enjekte edilen sulardan oluşan mineral çökelmesi, formasyon (kayaç) gözenekleri ve çatlakları, kuyu içi ve yüzey donanımları, yüzey toplama hatları gibi bu sulara temas eden her ortamda oluşabilmektedir.  Mineral çökelmesinin rezervuar kayacının gözenek ve/veya çatlak ağı içinde oluşması durumunda, bu birikimlere bağlı olarak kayacın gözenekliliği ve geçirgenliğinde azalma olabilmekte, sonuçta ise gözenekli ortamdan olan akış debisi ve basınç davranışları olumsuz etkilenebilmektedir.  Dolayısı ile rezervuardan yapılan üretim olumsuz yönde değişmektedir.  Bu yüzden, formasyon (kayaç) gözenekleri ve çatlakları içinde mineral çökelmesinin süreci ve akış davranışına etkisinin anlaşılması için, olayın mekanizmasını incelemek ve araştırmak bir gerekliliktir.
Termodinamik modeller kullanılarak kalsiyum karbonat (CaCO3) çökeliminin eğilimi, yani belirli koşullarda çökelmenin olup olmayacağı veya oluşabilecek maksimum çökelme miktarı belirlenebilmektedir.  Bunun yanında, bazı araştırmacılar tarafından "kinetik model" olarak anılan deneysel modellerin çökelme miktarını belirlemede daha iyi sonuçlar verebildiğini bazı çalışmalar göstermektedir.  Ne var ki, literatürde özellikle çatlak içi akışta CaCO3 çökelimine ilişkin yeteri kadar deneysel modele dayalı çalışma bulunmamaktadır.  Dolayısı ile, bu çalışmada çatlaklı ortamda mineral çökelim miktarını çatlağı içeren malzemenin yapısına bağlı etkileşimlerden arınmış bir şekilde kestirebilmek için, hem termodinamik modeller hem de deneysel modellerden yararlanılmıştır.  Öncelikle, kavanoz testleri ile statik koşullarda elde edilen CaCO3 çökelim miktarları, termodinamik modellerde elde edilen sonuçlarla karşılaştırılarak çözeltinin aşırı doymuşluğu belirlenmektedir.  Sonra, bu özelikteki çözeltiler farklı pürüzlülüğe, farklı açıklığa ve farklı uzunluğa sahip yapay çatlaklar içinde farklı sıcaklık ve farklı debilerde akıtılarak, çatlak içi CaCO3 mineral birikim miktarları elde edilmektedir.  Her deneyde, çökelmiş CaCO3 mineral miktarı çatlak çıkışında alınan çözeltilerin derişim analizi ile belirlenmektedir ve sonuçlar var olan deneysel (kinetik) ve/veya termodinamik korelasyon sonuçları ile karşılaştırılmaktadır.  Çatlak içinde süregelen CaCO3 çökelmesi ile birlikte çatlak geçirgenliği, akış rejimi, Reynolds sayısı ve sürtünme faktörü gibi değişkenlerdeki değişimler değerlendirilmektedir.  Bunlara ek olarak, daha önce bu konularda yapılmış çalışmalarda dikkate alınmamış olduğu görülen yataya göre "farklı akış doğrultusu" koşulları da yukarıda belirtilen deney sürecine dahil edilerek, aynı deney ve analiz yöntemi ile çatlak içinde oluşan CaCO3 mineral birikim miktarları saptanmaktadır.
Bu çalışmada, çatlak içinde akan akışkanın çatlak duvarları ile kimyasal ve/veya iyon alış verişi gibi etkileşimlere girmediği farklı akış koşullarında çatlak içinde CaCO3 çökelimini karşılaştırabilmek üzere, deneylerin tekrarlanabilmesi için 3B (üç boyutlu) dijital baskı yöntemi ile oluşturulmuş yapay çatlak modelleri kullanılmaktadır.  3B baskı yöntemi, çeşitli malzeme ve geometrik ölçeklendirme seçenekleriyle, kayaç veya benzeri herhangi bir çatlaklı yapıya yaklaşım sağlamakta, dolayısıyla yeni modelleme fırsatları sunabilmektedir.

Abstract:
A large amount of water is produced in oil and gas fields.  These waters can be rich in ions, dissolved gases, suspended solids, bacteria, hydrocarbons, radioactive, and even toxic substances.  Known as "connate water" or "residential water" in their respective locations, these waters are produced as by-products during oil and natural gas extraction.  Produced waters generated alongside petroleum and gas are generally considered waste streams due to their chemical and biological content.
A significant amount of hot water production is also carried out from the subsurface geothermal reservoirs.  While some of the produced hot water is directly utilized as hot water, the remaining substantial portion becomes "wastewater" after its heat is extracted, losing its high temperatures in central geothermal heating systems.  Injecting the wastewater back into the reservoir contributes to preserving the environment and reservoir pressure.  The methods used to utilize or dispose of these waters depend on their physical, chemical, and biological characteristics, as well as the associated costs. Standard methods for wastewater disposal include underground injection into suitable wells and transportation outside the field.  Produced water is used in water injection for oil displacement and pressure maintenance in oil fields. However, different precipitations can be formed when the injected water and formation water combine, which can cause severe problems in the reservoirs.
In the world, more than 60% of oil and more than 40% of gas reservoirs are fractured carbonate rocks, and the mineral precipitation observed in these formations is mainly calcium carbonate (CaCO3) due to temperature and incompatibility between the injected and formation water.  Few techniques for CaCO3 scaling removal are available due to their relative hardness and low solubility.  In most cases, the scaled-up wells are caused by carbonate formation.  In the well tests, it is seen that the flow from the matrix to the fracture is relatively low, and the flow in the fracture dominates the whole flow. Therefore, it is essential to properly understand the kinetics of CaCO3 scale formation and its detrimental effect on fracture.  Mineral deposition can occur wherever water comes into contacts, such as in the water obtained during production or injected into the reservoir to enhance production, formation of pores and fractures, wellbore and surface equipment, and collection lines.  When mineral deposition occurs in the formation and fractures, a decrease in reservoir porosity and permeability is expected, adversely affecting flow rate and pressure behavior.  Pressure changes and flow rate variations also negatively impact production.  Therefore, it is anticipated that the process of mineral deposition in fractures and formations and its impact on flow behavior should be addressed.
Mineral deposition in the oil, gas, and geothermal industry can lead to increased energy losses, equipment damage at the surface and downhole, increased pumping, cleaning, and maintenance costs, and even the loss of production or injection wells due to blockages.  These consequences can significantly increase costs.  Studies on mineral deposition primarily focus on deposits in the vicinity of wells, downhole environments, and pipelines.  The majority of these studies concentrate on preventive methods for deposit formation.  However, more information must be given regarding the extent of mineral accumulation within formations and fractures.
Using thermodynamic models, the tendency of CaCO3 precipitation can be determined, i.e., whether precipitation will occur under specific conditions or the maximum amount of precipitation that can occur.  It is known from studies that experimental models can give better results in determining precipitation behavior.  However, more empirical studies on the effects of flow on mineral precipitation in fractured media need to be done.  Therefore, the experimental and thermodynamic models are used in this study to predict the amount of mineral precipitation. The thermodynamic models used in this thesis study are the Langelier saturation index and the Oddo-Tomson saturation index.
This work investigates precipitation behavior in different temperature conditions, laminar or turbulent flow conditions, and fracture properties like high roughness, length, and aperture.  The amounts of precipitation in fracture under other conditions not determined in the literature are given, and the results are compared with existing kinetic and thermodynamic correlation results.  The precipitation conditions of CaCO3 within a solution depend on the changes in thermodynamic, hydrodynamic, and kinetic properties. Factors such as pH, temperature, pressure, flow rate, degree of supersaturation, and the presence of other salts or interfering ions in the solution can influence the formation of precipitates.  After flow experiments, the amount of CaCO3 precipitation and pressure differences were measured.  When the temperature is 80°C, the flow rate is 4.16x10-5 m³/s, and the roughness is high; precipitation is 47.5% higher than when the temperature is 50 °C. 
In addition to precipitation amounts, this article presents decreasing crack permeability due to precipitation. Precipitation of minerals can drastically influence the reservoir's permeability and ability to transmit mass and energy. With the pressure differences measured in the experiments, permeability decreases due to precipitation were calculated using Poiseuille's law (cubic law) for laminar flow and Forchheimer's equation for turbulent flow. The permeability reduction due to precipitation formation at a temperature of 50°C varies within a range of up to 32%, and at 80°C, it decreases by up to 59% at maximum.  This study's findings can help better understand that the permeability reduction of fracture is more significant in turbulent flow conditions compared to laminar flow. In addition, temperature and high roughness are very effective parameters in precipitation and permeability change. At high temperatures and high roughness, more minerals precipitate, and permeability values decrease considerably. In the later injection periods, the decrease in permeability caused by the precipitation can make the fracture impermeable and increase the pressure losses during the injection.
Another parameter that affects flow behavior and precipitation amount is determined by the direction of supersaturated solution flow and the slope angle. In an upward flow, the potential increases, leading to higher precipitation amounts in that direction. This is because, during injection, a portion of the energy is expended for potential change, resulting in a higher pressure difference. The increased pressure difference leads to a higher amount of precipitation.
Within the scope of this study, Chapter 2 provides a compilation of published information from the literature on calcium carbonate precipitation. Factors influencing precipitation and methods related to its kinetics are briefly introduced.
The results of static jar tests, which provide information on the amount of calcium carbonate precipitation, are presented in Chapter 3 to determine the characteristics of the water intended for use before injection experiments. Additionally, the required waiting times and test temperatures for preparing supersaturated CaCO3 solution are evaluated in this chapter.
In Chapter 4, the material used to produce artificial fracture core plugs is determined to facilitate flow with different surface roughness and fracture apertures. In this section, shape deformation problems, such as the displacement of glass beads due to the effect of flow and bending of the plug due to sealing pressure, are solved using three-dimensional (3D) models. The 3D printing method, with various material and geometric scaling options, enables digital approximation to a rock sample and offers new opportunities in earth science research.  Repeating the same fracture characteristics for different flow conditions in fracture flow tests is essential. By using 3D fracture models, the same fracture properties were obtained under other flow conditions. 
After determining the characteristics of synthetic water prepared at different saturations, in Chapter 5, flow and pressure behavior are examined by injecting these waters into cracks with varying areas of surface and apertures at various temperatures and flow rates. The deposition amounts are determined during this investigation.
In Chapter 6, a numerical simulation program called Petrasim, which utilizes the codes of the TOUGH, is used to compare results obtained from flow and deposition experiments in fractured media and for modeling the experiments. Petrasim can model multiphase flow, allowing for the analysis of flow and deposition behavior in fractured systems.
In statistical experimental design, a mathematical model is obtained using statistical techniques to analyze the data obtained from experiments and identify the factors that affect the output and can be controlled. In Chapter 7, the factorial design, an experimental design method, is applied to the deposition amounts obtained at different levels (experimental conditions) and the parameters influencing the results in injection experiments. A correlation is proposed based on the analysis of variance (ANOVA) to determine the amount of deposition within the fractures. This chapter defines the flow regime and friction coefficient in solution flow within cracks created at specific roughness and apertures.
Finally, Chapter 8 presents the results and recommendations obtained from this thesis study.