Class A cement has been used for cementing surface casings and shallowwells. API Specification 10A classifies the well cements into three categories, ordinary, moderate and high sulfate resistant, depending primarily on theamount of tricalcium aluminate (C3A) present in the clinker. Cementclass A (ordinary) presents no C 3A restrictions. On the other hand, moderate and high sulfate resistant cements should contain less than 8 and 3 %C3A respectively. Under certain conditions, sulfate ions will chemically combine withtricalcium aluminate giving rise to an expansive reaction that can cause cementdistress. This process depends on cement permeability and environmenttemperature. Several research studies conducted in this field have proven thatthe degradation process decreases as temperature increases and can becontrolled lowering the cement permeability. The purpose of this study is to analyze the sulfate attack mechanism atseveral bottom hole conditions in order to ensure an appropriate well isolationdurability performance using cement class A. Specimens prepared with APIcements class A and G were exposed to solutions containing sulfatesconcentrations ranging from 0 to 30000 ppm and temperatures of 89.6 and 183.2°F (32 and 84 °C). The specimen's length and weight variation was evaluated periodically. The presence of expansive phases (ettringite) was detected bymeans of SEM and EDAX analysis. A model to predict the cement degradation process due to sulfates attack is proposed. The cement specimens (A and G) showed no significant expansion during thefirst 240 days of exposure, however growth of secondary ettringite in cementpores was detected by SEM analysis. The sulfate degradation process requires acertain incubation period before the cement pores get completely filled withettringite. The use of pore blocking additives increases the sulfate resistanceof cement class A, ensuring appropriate performance and durability. Introduction Sulfate attack is one of the degradation mechanisms of cementitiusmaterials(1). This phenomena has been and continues to be the subject of many investigations, as it may cause the premature failure of civiland highway concrete infrastructure(2–4). As a consequence, thefirst publications and standards addressing this topic have been orientated tothe building construction industry(5,6). Based on these findings, API Specification 10A(7) proposes a classification for oil wellcements that is similar to that established by ASTM and other European codes. According to API, oil well cements are divided into three groups, ordinary, moderate and high sulfate resistance, depending primarily on their chemicalcomposition. Table 1 presents the chemical requirements for cement class A andG as specified in API SP 10A. The Process of Sulfate Attack The hydrated calcium trisulfoaluminate(C3S.C3A.H32), commonly known as ettringite, is a crystalline meta-stable hydration product of Portland cement. During thefirst 24 hours of hydration, the tricalcium aluminate(3CaO.Al2O3, also referred as C3A in thecement nomenclature) reacts with gypsum (CaS04.2H2O), added to the clinker to regulate the C3A rate of hydration, to formwhat is known as primary ettringite (Eq. 1).Equation 1 As cement hydration takes place the amount of gypsum decreases and part ofthe ettringite is transformed into calcium monosulfo-aluminate hydrate asindicated in equation 2.Equation 2 This reaction also depends on several factors as the ratio between S04=/C3 A, the pore solution alkalinity, thewater to cement ratio (w/c) and the environmenttemperature(1,3,8–10). When hydrated cement is placed in anenvironment containing high sulfate concentrations, as could be the case ofmany formations, the calcium monosulfo-aluminate hydrate and the calciumhydroxide present in the cement paste will react with the sulfate ions comingfrom the environment to form what is known as secondary or delayedettringite (Eq. 3).
TX 75083-3836 U.S.A., fax 01-972-952-9435. AbstractDuring the well service life the cement isolation is exposed to extreme conditions that can cause its premature failure. Certain well completion operation as perforating and hydraulic fracturing, the changes in temperature and pressure during secondary recovery or the mechanical stress originated by formation displacements may cause severe damage to the cement isolation. The mechanical properties (compressive and tensile strength, toughness, Young modulus and Poisson ratio) of different cements were evaluated in order to establish their best service performance. Typical slurry designs were tested focusing the attention on the effect of certain additives such as latex, fibers and other polymers used as fluid loss control or dispersants. The cement mechanical properties were determined according to ASTM and API standard test methods. The cement toughness was evaluated following the API RP 43 standard for testing well perforators. The experimental results show that the mechanical properties of cement are strongly dependant on the particular additives used when preparing the slurry. Even when cement with no admixtures presents high compressive strength it also shows a fragile behavior with limited strain and low toughness. The use of latex improves the cement elastic behavior although it does not better its impact resistance. On the other hand, the addition of polymer fibers improves the cement toughness and its elastic behavior. As it was demonstrated by well testing profiles run before and after perforating, the addition of fibers improves the cement performance when exposed to the different events that may damage the isolation during the well service life.
TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractIn the last decade drilling fluids have undergone a continuous evolution towards the challenges of new drilling technologies. This process forced cementing companies to improve the mudcake cleaning systems efficiency in order to achieve successful cementing jobs. A basic procedure to evaluate drilling fluids compatibility with spacers, preflushes and cement slurries is provided in API recommended practice 10 B. However, no quantitative methods to evaluate preflushes mud removal efficiency are proposed. On the other hand, several laboratory methods have been developed to analyze this issue, although most of them are based on experiments performed at unrealistic down-hole conditions. This paper presents a new laboratory method to dynamically evaluate preflushes performance. The drilling fluid is circulated through a cell at a similar hydrodynamic flow regime expected in the well annulus. The mud cake is generated on four different filtering materials, applying differential pressure at the bottom hole circulating temperature. The mud and filter-cake removal is evaluated by two different techniques, static gravimetric and dynamic impedance. This last technique provides a continuous indication of the cake thickness variation during the test. Therefore, the preflushes chemical, viscous and abrasive action can be evaluated while the test is conducted. Three case studies, a low solids KCl, a PHPA and a plain water base drilling fluid, are reported were the performance of different preflushes systems are evaluated. The results of these studies were used to modify the preflushes pumping sequence and to adjust their volumes based on the information obtained from the impedance trends. The cleaning efficiency of viscous spacers used in-between chemical washes and the effect of mud filter cake leek off are discussed.
TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractIn the last decade drilling fluids have undergone a continuous evolution towards the challenges of new drilling technologies. This process forced cementing companies to improve the mudcake cleaning systems efficiency in order to achieve successful cementing jobs. A basic procedure to evaluate drilling fluids compatibility with spacers, preflushes and cement slurries is provided in API recommended practice 10 B. However, no quantitative methods to evaluate preflushes mud removal efficiency are proposed. On the other hand, several laboratory methods have been developed to analyze this issue, although most of them are based on experiments performed at unrealistic down-hole conditions. This paper presents a new laboratory method to dynamically evaluate preflushes performance. The drilling fluid is circulated through a cell at a similar hydrodynamic flow regime expected in the well annulus. The mud cake is generated on four different filtering materials, applying differential pressure at the bottom hole circulating temperature. The mud and filter-cake removal is evaluated by two different techniques, static gravimetric and dynamic impedance. This last technique provides a continuous indication of the cake thickness variation during the test. Therefore, the preflushes chemical, viscous and abrasive action can be evaluated while the test is conducted. Three case studies, a low solids KCl, a PHPA and a plain water base drilling fluid, are reported were the performance of different preflushes systems are evaluated. The results of these studies were used to modify the preflushes pumping sequence and to adjust their volumes based on the information obtained from the impedance trends. The cleaning efficiency of viscous spacers used in-between chemical washes and the effect of mud filter cake leek off are discussed.
TX 75083-3836 U.S.A., fax 01-972-952-9435. AbstractDuring the well service life the cement isolation is exposed to extreme conditions that can cause its premature failure. Certain well completion operation as perforating and hydraulic fracturing, the changes in temperature and pressure during secondary recovery or the mechanical stress originated by formation displacements may cause severe damage to the cement isolation. The mechanical properties (compressive and tensile strength, toughness, Young modulus and Poisson ratio) of different cements were evaluated in order to establish their best service performance. Typical slurry designs were tested focusing the attention on the effect of certain additives such as latex, fibers and other polymers used as fluid loss control or dispersants. The cement mechanical properties were determined according to ASTM and API standard test methods. The cement toughness was evaluated following the API RP 43 standard for testing well perforators. The experimental results show that the mechanical properties of cement are strongly dependant on the particular additives used when preparing the slurry. Even when cement with no admixtures presents high compressive strength it also shows a fragile behavior with limited strain and low toughness. The use of latex improves the cement elastic behavior although it does not better its impact resistance. On the other hand, the addition of polymer fibers improves the cement toughness and its elastic behavior. As it was demonstrated by well testing profiles run before and after perforating, the addition of fibers improves the cement performance when exposed to the different events that may damage the isolation during the well service life.
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