Improving the Sulfate Resistance Performance of API cement Class A upon Appropriate Slurry Design

Abstract

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).

Investigation of Performance of Nano Silica Cement Additive on Sulphate Attack In Geothermal Wells

“…Specimens with 0.3% Nano silica had the lowest value followed by 0.6% and 1.2% respectively. The solubility of ettringite increased with temperature until it became completely soluble at around 65℃ (Morales et al, 2003). Hence the decreasing trend of specimen elongation with a further increase in temperature above 65℃.…”

“…This process is completed within the first 24 hours of hydration (Morales et al, 2003). As the amount of gypsum decreased, part of the ettringite was transformed into calcium monosulfo aluminate hydrate following the equation below:…”

“…Ettringite, a crystalline mineral is expansive in nature and its formation was accompanied by growth in weight (Batilov, 2016;Morales et al, 2003). As the ettringite continued to fill up the pores of cement, internal pressure built up (Scrivener et al, 1999).…”

“…As the ettringite continued to fill up the pores of cement, internal pressure built up (Scrivener et al, 1999). This generated expansive stresses (Morales et al, 2003;Skalny et al, 2003) that initiated the cracking of cement and loss of strength (Scrivener et al, 1999) in a phenomenon called sulphate attack (Morales et al, 2003).…”

Cementing Solutions for Corrosive Well Environments

“…For temperatures below 140 °F, the use of high sulfate resistant "HSR" cements (C 3 A content below 3 wt% for API cements) and the elimination of portlandite in the set cement are essential. Morales et al (2003) discovered that decreasing the permeability is an even more effecient way to avoid the sulfate attack. The use of pore blocking additives (such as latex, sodium silicate or microsilica) or a lower water to cement ratio will improve the sulfate resistance of an API class A cement.…”