ARMA 19–1981 Well integrity of high temperature wells: Effect of mineralogical changes on mechanical properties of well cement TerHeege, J.H. and Wollenweber, J. TNO Applied Geosciences, Utrecht, the Netherlands Marcel Naumann Downloaded from http://onepetro.org/ARMAUSRMS/proceedings-pdf/ARMA19/All-ARMA19/ARMA-2019-1981/1133466/arma-2019-1981.pdf/1 by guest on 02 October 2021 Equinor ASA, Sandsli, Norway Pipilikaki, P. TNO Structural Reliability, Delft, the Netherlands Vercauteren, F. TNO Material Solutions, Eindhoven, the Netherlands This paper w as prepared for presentation at the 53rd US Rock Mechanics/Geomechanics Symposium held in New Y ork, NY , USA , 23–26 June 2019. This paper w as selected for presentation at the symposium by an ARMA Technical Program Committee based on a technical and critical review of the paper by a minimum of tw o technical reviewers. The material, as presented, does not necessarily reflect any position of ARMA, its of ficers, or members. ABSTRACT: Wells used for steam-assisted gravity drainage (SAGD), for cyclic steam stimulation (CSS), for hydrocarbon production in areas with anomalous high geothermal gradient, or for geothermal energy extraction all are operated in high temperature environments where maintaining long term wellbore integrity is one of the key challenges. Changes in cement mineralogy and associated mechanical properties may critically affect the integrity of high temperature wells. In this study, the relation between changes in mineralogy and mechanical properties of API class G cement with 40% silica flour was investigated by exposing samples for 1-4 weeks to temperatures of 60-420°C. The effect of mineralogical changes on mechanical properties was investigated using a combination of chemical and microstructural analysis and triaxial strength tests at confining pressures of 2-15 MPa. The combined effect of changing porosity, microstructure and strength of mineral phases determines changes in mechanical behavior, in particular the formation of stronger mineral phases with higher density affect Young’s modulus, failure strength and residual strength. Deformation of the cement is more brittle at low confining pressures and more ductile at high confining pressures with a considerable residual strength after failure and highest reduction in cement stiffness and increase in strength at temperatures of ~250°C. high pressure (HP) wells (Enform, 2012; Chartier et al., 2015). For standard curing conditions, hydration of 1. INTRODUCTION ordinary Portland cement (OPC, e.g., class G well Wells that are drilled or operated in high temperature cement) with a calcium oxide to silicon dioxide (C/S) environments can be subject to higher risks of losing ratio close to 3.1 produces the amorphous C-S-H and the integrity and zonal isolation. Examples of such high crystalline Ca(OH)2. The product is a mixture of ordered temperature wells include wells used for steam-assisted and disordered phases with C-S-H present as a highly gravity drainage (SAGD) or for cyclic steam stimulation disordered version of the crystalline tobermorite and (CSS), wells drilled in areas with anomalous high jennite phases (Taylor, 1990). Tobermorite preserves high geothermal gradient (HPHT wells), or geothermal wells compressive strength and low permeability. (Stiles, 2006; Chartier et al., 2015; Frioleiffson et al., Mineralogical and mechanical changes are expected to 2015). Maintaining long term wellbore integrity in these occur in OPC above ~110°C (Thorvaldson et al., 1938; situations is one of the key challenges determining the Patchen, 1960; Taylor, 1990; Kyritsis et al., 2009). technical and commercial success of projects that require Reactions result in different hydration products, i.e. wells to operate at high temperatures. crystallization of alpha dicalcium silicate hydrate (α- C SH or Ca SiO (OH) ) is favored. The lime-rich alpha- Changes in cement mineralogy and associated mechanical 2 2 3 2 dicalcium silicate hydrate (α-C SH) is denser than the properties critically affect the integrity of high 2 temperature wells. Standard cement formulations have usual calcium silicate hydrate (C-S-H gel) product. Bulk been reported to reach their mechanical and chemical volume decrease, and cement shrinkage or porosity increase is associated with this reaction, which results in (stability) limits due to the harsh conditions and extreme a decrease in cement strength and increase in permeability cyclic temperature loads in high temperature (HT) and (Patchen, 1960; Pernites and Santra, 2016). All types of findings aid in assessing risks of well integrity loss and OPC and other types of well cements with simila r mitigation of these risks by designing new cement composition show lower strengths when cured above formulations. The main challenge remains to optimize 110°C, compared to curing at 93°C. cement properties for prolonging the integrity of cement at high temperatures while maintaining low viscosity of Elevated curing temperatures are commonly reached by cement slurry during well cementation. SAGD, CCS, HPHT oil and gas wells or geothermal wells in areas with moderate to high geothermal gradients. Changes in cement properties need to be accounted for in 2. EXPERIMENTAL APPROACH the design of cement formulations as well as cementation 2.1. Sample material procedures of these wells to ensure the plugging of loss Downloaded from http://onepetro.org/ARMAUSRMS/proceedings-pdf/ARMA19/All-ARMA19/ARMA-2019-1981/1133466/arma-2019-1981.pdf/1 by guest on 02 October 2021 zones during drilling, proper anchoring, corrosion Synthetic cement samples, Dyckerhoff HT Basic Blend of protection of casing strings, zonal isolation and well cement clinker (API class G cement) with 40 % silica integrity at elevated temperatures and pressures (HPHT flour was used as a starting material (Papaioannou, 2018). conditions). A common practice is to use Portland-type Specimens are prepared using the Dyckerhoff blend and cements with added silica (such as fly ash, silica fume or demineralized water with a water to solid (cement + silica quartz flour) to stabilize cement at higher temperature. flour ) ratio of 0.371 (m/m). For the mixing process, the EN 196-3 protocol was followed. After each step the Reactions of Ca(OH)2 with added silica at elevated temperatures result in a C-S-H with a lower C/S ratio. weight of the samples was checked for dehydration. After Addition of 35 to 40% by weight of cement is enough to mixing, the slurry was injected into cylindrical molds with 25 mm internal diameter and 60 mm length using a prevent the reaction forming α-C2SH. Instead, cement phases such as tobermorite or xonotlite are formed 10 ml syringe in order to achieve homogeneity and (Patchen, 1960). Moreover, for a water to solids ratio of minimize air entrapment. The molds were subsequently 0.4, a formulation with added silica results in 91% of the sealed to prevent water evaporation, and exposed for 72 water being chemically bound against only 39 % of the hours at 60°C and atmospheric pressure in a furnace. available water in the OPC. Addition of ~40% silica flour Samples exposed to 120, 250 and 420°C were thereby favors formation of high strength and low subsequently cooled down for 2 hours before demolding. permeability mineral phases. Pressure resistant glass bottles were used for exposure of pairs of samples under suspension of water at 120°C and Despite the importance for well integrity of high 2 bar in a furnace. Metal sample holders sealed by a cap temperature wells, data on mineralogical and mechanical with two different types of O-rings were used with a pre- changes during exposure of well cements to HPHT defined amount of water to obtain required exposure conditions is limited. Some relevant laboratory studies are conditions. Samples exposed to 60 and 120°C were available (Taylor 1990; Gabrovšek et al., 1993; Meller et cooled down in two hours, while for samples exposed to al., 2009; Pernites and Santra, 2016), but investigations of 250 and 420°C samples a 3 day gradual cooling protocol the relation between mineralogical and mechanical was applied. For mineralogical and microstructural changes at different exposure times and temperatures are analysis, samples were exposed for 4 weeks to 60, 120, sparse. In this study, the relation between changes in 250 and 420°C. For the triaxial experiments, samples mineralogy and mechanical properties of API class G were cut and manually polished to obtain samples with cement with 40% silica flour was investigated. Samples plane parallel surfaces, a final length between 49.6 and were exposed to temperatures of 60°C (curing only) up to 52.2 mm, and a final diameter between 24.6 and 25.0 mm. 420°C (curing and exposure) for exposure times of 3 days Samples were tested after curing (“cured only” samples), (curing time) to 4 weeks (HT exposure). The effect of or after exposure of 1 week to 120°C at 2 bars, 4 weeks to mineralogical changes on mechanical properties was 250°C at 40 bars, and 2 weeks to 420°C at 350 bars, investigated using a combination of (1) chemical analysis respectively. using X-ray diffraction (XRD), (2) microstructural 2.2. Mineralogical and microstructural analysis analysis using scanning electron microscopy (SEM) and optical microscopy, and (3) mechanical properties Analysis of mineral phases in the samples was performed (Young’s modulus, failure stress, residual stress) using using X-ray diffraction (XRD) and energy dispersive X- ray spectroscopy (EDX). Microstructural analysis was triaxial tests at confining pressures of 2-15 MPa. performed using optical microscopy and scanning Results show that the formation of high temperature electron microscopy (SEM). mineral phases in the cement such as tobermorite, xonotlite and wollastonite is associated with marked XRD analysis was performed on grinded aliquots to changes in elastic properties and failure behavior. Critica l access the crystalline compounds and deterioration conditions for the changes in mechanical behavior are products.