Wellbore Cement Research Articles

Polymer-Cement Composites with Self-Healing Ability for Geothermal and Fossil Energy Applications

Sealing of wellbores in geothermal and tight oil/gas reservoirs by filling the annulus with cement is a well-established practice. Failure of the cement as a result of physical and/or chemical stress is a common problem with serious environmental and financial consequences. Numerous alternative cement blends have been proposed for the oil and gas industry. Most of these possess poor mechanical properties, or are not designed to work in high temperature environments. This work reports on a novel polymer-cement composite with remarkable self-healing ability that maintains the required properties of typical wellbore cements and may be stable at most geothermal temperatures. We combine for the first time experimental analysis of physical and chemical properties with density functional theory simulations to evaluate cement performance. The thermal stability and mechanical strength are attributed to the formation of a number of chemical interactions between the polymer and cement matrix including covalent bonds...

Drilling Technical Difficulties and Solutions in Development of Hot Dry Rock Geothermal Energy

The exploration and development of hot dry rock resources, first of all, needs to address the drilling issues in deep, hot, hard and unstable formations. By studying geological features and storage conditions of hot dry rocks, the key technical difficulties of hot dry rock drilling are presented. The high-temperature resistance performance index of oil and gas drilling technologies at home and abroad are investigated. The applicability of high effective rock breaking tools, MWD instruments, drilling fluid systems, well cementing and completion technologies are analyzed, and feasibility analyses have been conducted on gas drilling, dry wellbore cementing and foam pressurized drilling techniques. On the basis of the above analyses, the developing directions and issues urgently to be addressed about domestic hot dry rock drilling technology are discussed so as to provide references for drilling program optimization and drilling technology research in the development of hot dry rock geothermal energy.

Incorporating reaction-rate dependence in reaction-front models of wellbore-cement/carbonated-brine systems

Contact between wellbore cement and carbonated brine produces reaction zones that alter the cement's chemical composition and its mechanical properties. The reaction zones have profound implications on the ability of wellbore cement to serve as a seal to prevent the flow of carbonated brine. Under certain circumstances, the reactions may cause resealing of leakage pathways within the cement or at cement-interfaces; either due to fracture closure in response to mechanical weakening or due to the precipitation of calcium carbonate within the fracture.In prior work, we showed how mechanical sealing can be simulated using a diffusion-controlled reaction-front model that links the growth of the cement reaction zones to the mechanical response of the fracture. Here, we describe how such models may be extended to account for the effects of the calcite reaction-rate. We discuss how the relative rates of reaction and diffusion within the cement affect the precipitation of calcium carbonate within narrow leakage pathways, and how such behavior relates to the formation of characteristic reaction modes in the direction of flow. In addition, we compare the relative impact of precipitation and mechanical deformation on fracture sealing for a range of flow conditions and fracture apertures. We conclude by considering how the prior leaching of calcium from cement may influence the sealing behavior of fractures, and the implication of prior leaching on the ability of laboratory tests to predict long-term sealing.

Investigations on the surface well cement integrity induced by thermal cycles considering an improved overall transfer coefficient

Wellbore cement is used as a mechanical and hydraulic barrier in order to support casing and to prevent vertical and horizontal fluid migration, making wellbore cementing jobs one of the most important operations in thermal recovery projects. Drilling high quality wells requires the use of high-performance drilling muds that can support and protect the wellbore during the drilling process. Drilling mud will create an internal and external filter cake to protect the well during drilling. Although the external filter cake is usually removed before or during cementing jobs, the internal filter cake remains in the near-wellbore formation throughout the life of the well. This directly influences the wellbore overall heat transfer coefficient. As thermal well design relies on maximizing the heat delivered to the reservoir, minimizing heat losses to top formations while also protecting casing and cement layers is mandatory.This paper presents a unique experimental investigation of thermal cycle behavior of cement with focus on surface casing cementing. A more accurate assessment of overall heat transfer coefficients, directing efforts towards increasing the efficiency and understanding of thermal well design has been also performed in order to better understand the induced thermal loads on cements under complex scenarios. Thermal mixing laws have been used to determine the approximate impact of drilling fluid invasion and filter cake formation within rocks’ pores on overall heat transfer coefficients. In an experimental approach it was noticed that applying temperature cycles to fresh cements (curing time less than 21 days) will lead to mechanical strengthening of the casing-cement-system, enhancing well integrity, whereas old cemented wells will lose strength.

Bingham\u2019s model in the oil and gas industry

Yield stress fluid flows occur in a great many operations and unit processes within the oil and gas industry. This paper reviews this usage within reservoir flows of heavy oil, drilling fluids and operations, wellbore cementing, hydraulic fracturing and some open-hole completions, sealing/remedial operations, e.g., squeeze cementing, lost circulation, and waxy crude oils and flow assurance, both wax deposition and restart issues. We outline both rheological aspects and relevant fluid mechanics issues, focusing primarily on yield stress fluids and related phenomena.

Wellbore Cement Porosity Evolution in Response to Mineral Alteration during CO2 Flooding.

Mineral reactions during CO2 sequestration will change the pore-size distribution and pore surface characteristics, complicating permeability and storage security predictions. In this paper, we report a small/wide angle scattering study of wellbore cement that has been exposed to carbon dioxide for three decades. We have constructed detailed contour maps that describe local porosity distributions and the mineralogy of the sample and relate these quantities to the carbon dioxide reaction front on the cement. We find that the initial bimodal distribution of pores in the cement, 1-2 and 10-20 nm, is affected differently during the course of carbonation reactions. Initial dissolution of cement phases occurs in the 10-20 nm pores and leads to the development of new pore spaces that are eventually sealed by CaCO3 precipitation, leading to a loss of gel and capillary nanopores, smoother pore surfaces, and reduced porosity. This suggests that during extensive carbonation of wellbore cement, the cement becomes less permeable because of carbonate mineral precipitation within the pore space. Additionally, the loss of gel and capillary nanoporosities will reduce the reactivity of cement with CO2 due to reactive surface area loss. This work demonstrates the importance of understanding not only changes in total porosity but also how the distribution of porosity evolves with reaction that affects permeability.

Alteration processes of cement induced by CO2-saturated water and its effect on physical properties: Experimental and geochemical modeling study

The objective of this study is to understand cement alteration processes with the evolution of porosity and hardness under geologic CO2 storage conditions. For this study, the cylindrical cement cores (class G) were reacted with CO2–saturated water in a vessel (40°C and 8MPa) for 10 and 100 days. After the experiment, the CO2 concentration and Vickers hardness were measured in the hydrated cement core to estimate the carbonation depth and to identify the change in hardness, respectively. Diffusive-reactive transport modeling was also performed to trace the alteration processes and subsequent porosity changes. The results show that cement alteration mainly results from carbonation. With alteration processes, four different reaction zones are developed: degradation zone, carbonation zone, portlandite depletion zone, and unreacted zone. In the degradation zone, the re-dissolution of calcite formed in the carbonation zone leads to the increase of porosity. In contrast, the carbonation zone is characterized by calcite formation resulting mainly from the dissolution of portlandite. The carbonation zone acts as a barrier to CO2 intrusion by consuming dissolved CO2. Especially in this zone, although the porosity decreases, the Vickers hardness increases. Our results show that cement alteration processes can affect the physical and hydrological properties of the hydrated cement under CO2-saturated conditions. Further long-term observation is required to confirm our results under in-situ fluid chemistry of a CO2 storage reservoir. Nonetheless, this study would be helpful to understand alteration processes of wellbore cements under CO2 storage conditions.

Early-Age Stress and Pressure Developments in a Wellbore Cement Liner: Application to Eccentric Geometries

This paper introduces a predictive model for the stress and pressure evolutions in a wellbore cement liner at early ages. A pressure state equation is derived that observes the coupling of the elastic changes of the solid matrix, the eigenstress developments in the solid and porespaces, and the mass consumption of water in course of the reaction. Here, the transient constitution of the solid volume necessitates advancing the mechanical state of the poroelastic cement skeleton incrementally and at constant hydration degree. Next, analytic function theory is employed to assess the localization of stresses along the steel–cement (SC) and rock–cement (RC) interfaces by placing the casing eccentrically with respect to the wellbore hole. Though the energy release rate due to complete debonding of either interface is only marginally influenced by the eccentricity, the risk of evolving a microcrack along the thick portion of the sheath is substantially increased. Additionally, it is observed that the risk of microannulus formation is principally affected by the pressure rebound, which is engendered by the slowing reaction rate and amplified for rock boundaries with low permeability.

Numerical Simulation of Permeability Change in Wellbore Cement Fractures after Geomechanical Stress and Geochemical Reactions Using X-ray Computed Tomography Imaging.

X-ray microtomography (XMT) imaging combined with three-dimensional (3D) computational fluid dynamics (CFD) modeling technique was used to study the effect of geochemical and geomechanical processes on fracture permeability in composite Portland cement-basalt caprock core samples. The effect of fluid density and viscosity and two different pressure gradient conditions on fracture permeability was numerically studied by using fluids with varying density and viscosity and simulating two different pressure gradient conditions. After the application of geomechanical stress but before CO2-reaction, CFD revealed fluid flow increase, which resulted in increased fracture permeability. After CO2-reaction, XMT images displayed preferential precipitation of calcium carbonate within the fractures in the cement matrix and less precipitation in fractures located at the cement-basalt interface. CFD estimated changes in flow profile and differences in absolute values of flow velocity due to different pressure gradients. CFD was able to highlight the profound effect of fluid viscosity on velocity profile and fracture permeability. This study demonstrates the applicability of XMT imaging and CFD as powerful tools for characterizing the hydraulic properties of fractures in a number of applications like geologic carbon sequestration and storage, hydraulic fracturing for shale gas production, and enhanced geothermal systems.

Lattice Boltzmann simulation for steady displacement interface in cementing horizontal wells with eccentric annuli

When cementing horizontal wells, it is essential to keep the displacement interface steady, in order to achieve good cementing quality, especially in eccentric annuli. Considering the displacement behavior between shear-thinning fluids in a horizontal eccentric annulus, the Lattice-Boltzmann method (LBM) was used to investigate the displacement interface shape. The classical Hele-Shaw approach was used to simplify the model from 3D flow into a 2D model. In the simulations, the effects of eccentricity and density ratio on the displacement interface shape and on the interface length versus time were analyzed in detail, resulting in an understanding of the fingering mechanism. Comparison of LBM with the FLUENT solutions showed that the results with LBM are essentially in agreement. In general, a greater density ratio and a lower eccentricity will result in a shorter displacement interface i.e. better displacement in a horizontal eccentric annulus. For a given density ratio, there is an optimum eccentricity to achieve a steady interface. The study provides guidance to the design of wellbore cement jobs and demonstrates that LBM is a promising tool for modeling of the fluid displacement process.

Geochemical alteration of wellbore cement by CO2 or CO2 + H2S reaction during long‐term carbon storage

AbstractCement samples were reacted with CO2‐saturated synthetic groundwater, with or without added H2S (1 wt.%), at 50°C and 10 MPa for up to 13 months (CO2 only) or for up to 3.5 months (CO2 + H2S) under static conditions. After the reaction, X‐ray computed tomography (XCT) images revealed that calcium carbonate (CaCO3) precipitation occurred extensively within the fractures in the cement matrix, while micro‐fractures with aperture size <∼50 μm at the cement‐basalt interface were completely sealed by CaCO3 precipitation. Exposure of a fractured cement sample to CO2‐saturated groundwater (50°C and 10 MPa) over a period of 13 months demonstrated progressive healing of cement fractures by CaCO3 precipitation. After reaction with CO2 + H2S‐saturated groundwater, CaCO3 precipitation also occurred within the cement fracture as well as along the cement‐basalt caprock interfaces. X‐ray diffraction analysis showed that major cement carbonation products of the CO2 + H2S‐saturated groundwater were calcite, aragonite, and vaterite, all consistent with cement carbonation by CO2‐saturated groundwater. While pyrite is thermodynamically favored to form, due to the low H2S concentration it was not identified by XRD in this study. The cement alteration rate into neat Portland cement samples by CO2‐saturated groundwater was similar at ∼0.02 mm/d based on XCT images, regardless of the cement‐curing pressure and temperature (P‐T) conditions, or the presence of H2S in the groundwater. The experimental results imply that wellbore cement with micro‐fractures within the cement matrix or along the cement‐caprock interface (<∼200 μm aperture under current experimental conditions) is likely to be healed by CaCO3 precipitation during exposure to CO2‐ or CO2 + H2S‐saturated groundwater. Published 2016. This article is a U.S. Government work and is in the public domain in the USA

Effect of CO2-induced reactions on the mechanical behaviour of fractured wellbore cement

Geomechanical damage, such as fracturing of wellbore cement, can severely impact well integrity in CO2 storage fields. Chemical reactions between the cement and CO2-bearing fluids may subsequently alter the cement’s mechanical properties, either enhancing or inhibiting damage accumulation during ongoing changes in wellbore temperature and stress-state. To evaluate the potential for such effects, we performed triaxial compression tests on Class G Portland cement, conducted at down-hole temperature (80 °C) and effective confining pressures ranging from 1 to 25 MPa. After deformation, samples displaying failure on localised shear fractures were reacted with CO2–H2O, and then subjected to a second triaxial test to assess changes in mechanical properties. Using results from the first phase of deformation, baseline yield and failure criteria were constructed for virgin cement. These delineate stress conditions where unreacted cement is most prone to dilatational (permeability-enhancing) failure. Once shear-fractures formed, later reaction with CO2 did not produce further geomechanical weakening. Instead, after six weeks of batch reaction, we observed up to 83% recovery of peak-strength and increased frictional strength (15%–40%) in the post-failure regime, due to carbonate precipitation in the fractures. As such, our results suggest more or less complete mechanical healing on timescales of the order of months.

Wellbore cement degradation in contact zone with formation rock

In the work the risk of CO2 migration in deep wells, caused by integrity loss on cement–rock interface and thow wellbore integrity correlated with the formation rock lithology were determined. 19 composed samples of rock and wellbore cement were exposed to CO2-saturated brine, in the autoclave reactor, under the formation conditions (50 °C and 10 MPa). Mineralogical and textural changes in the cement–rock interface in the case of selected rocks (sandstones, shale, limestones, dolomites and anhydrites) were characterised. The performed examination indicates that both cement and formation rocks react with CO2 saturated brine under the experimental conditions. The cement alteration is characterised by carbonation process in the outer rim, but it is enhanced on the interface with formation rock. It was stated that the performance of the cement–rock interface is essentially dependent on the rock lithology, including mineral composition and rock structure. Some minerals are very easily dissolving, e.g., anhydrite, gypsum, calcite and feldspars, what is contributing to an increase in the porosity and permeability in cement–rock contact zone. Primary dissolution of certain minerals in the first stages of the experiment results in the secondary precipitation after the last stage of reaction contributing to a secondary reduction of pore space.

Investigation of possible wellbore cement failures during hydraulic fracturing operations

Abstract We model and assess the possibility of shear failure along the vertical well by using the Mohr–Coulomb failure model and employing a rigorous coupled flow-geomechanic analysis. To this end, we take various values of cohesion between the well casing and the surrounding cement to represent different quality levels of cementing operation (low cohesion corresponds to low-quality cement and/or incomplete cementing). The simulation results show that there is very little fracturing when the cement is of high quality. Conversely, incomplete cementing and/or weak cement can cause significant shear failure and evolution of long fractures/cracks along the vertical well. Specifically, low cohesion between the well and cemented areas can cause significant shear failure along the well, while high cohesion does not cause shear failure. The Biot and thermal dilation coefficients strongly affect shear failure along the well casing, and low Young's modulus causes fast failure propagation. Still, for the high quality of the cementing job, failure propagates very little. When the hydraulic fracturing pressure is high or when permeability increases significantly, low cohesion of the cement can cause fast propagation of shear failure and of the resulting fracture/crack, but a high-quality cement with no weak zones exhibits limited shear failure that is only concentrated near the bottom of the vertical part of the well. Thus, high-quality cement and complete cementing along the vertical well appears to be the strongest protection against shear failure of the wellbore cement and, consequently, against contamination hazards to drinking water aquifers during hydraulic fracturing operations.

Radial fracture in a three-phase composite: Application to wellbore cement liners at early ages

Little understanding exists between the early-age stress developments in a wellbore cement sheath and its risk of impairment. During hydration, the cement morphology and pore-pressure changes induce eigenstresses in the solid and pore volumes. Utilizing these stresses as the driving mechanism of fracture, this paper formalizes the inspection of a radial crack in an elastic cement sheath constrained by an inner steel casing and an outer rock formation. The solution is constructed in the framework of analytic function theory and seeks the Green’s function for an edge dislocation in the intermediate cement phase. A dislocation pile-up along the line of fracture constructs a singular integral equation for the crack opening displacement derivative, from which the energy release rate is readily deduced.Under the uniform development of eigenstresses, the stiffness ratios of steel-to-cement and rock-to-cement generally predict the crack to initiate along the steel-cement interface. Here, the impacts of (i) a rigid bond and (ii) a sliding interface with no shear are assessed. This leads to the primary result of the paper: the potential for radial fracture is substantially mitigated by ensuring the shear connection between the steel casing and the cement sheath.

Defining the Brittle Failure Envelopes of Individual Reaction Zones Observed in CO2-Exposed Wellbore Cement.

To predict the behavior of the cement sheath after CO2 injection and the potential for leakage pathways, it is key to understand how the mechanical properties of the cement evolves with CO2 exposure time. We performed scratch-hardness tests on hardened samples of class G cement before and after CO2 exposure. The cement was exposed to CO2-rich fluid for one to six months at 65 °C and 8 MPa Ptotal. Detailed SEM-EDX analyses showed reaction zones similar to those previously reported in the literature: (1) an outer-reacted, porous silica-rich zone; (2) a dense, carbonated zone; and (3) a more porous, Ca-depleted inner zone. The quantitative mechanical data (brittle compressive strength and friction coefficient) obtained for each of the zones suggest that the heterogeneity of reacted cement leads to a wide range of brittle strength values in any of the reaction zones, with only a rough dependence on exposure time. However, the data can be used to guide numerical modeling efforts needed to assess the impact of reaction-induced mechanical failure of wellbore cement by coupling sensitivity analysis and mechanical predictions.

Experimental Investigation of the Impact of Compression on the Petro-Physical and Micromechanical Properties of Wellbore Cement Containing Salt

In this study, we investigated the effect of compression on the micromechanical and the petro- physical properties of salted wellbore cement systems. The experiments were conducted using a customized bench scale model, which utilized an expandable tubulars simulating the compression of a previously cemented casing under field-like conditions. The “mini-wellbore model” sample consisted of a pipe inside pipe assembly with a cemented annulus. The cement samples were cured in a water bath for 28 days prior to the compression experiments to allow adequate hydration. The impact of compression on the cement’s petro-physical and mechanical properties was quantified by measuring the porosity, permeability and hardness of salt cement cores drilled parallel to the orientation of the pipe from the compacted cement sheath. Permeability (Core-flood) experiments were conducted at 21℃, 10,342 kPa confining pressure for a period of 120 minutes. During the core-flood experiments, conducted using Pulse-decay method, deionized water was flowed through cement cores to determine the permeability of the cores. The results obtained from these experiments confirmed that the compression of the cement positively impacted the cements ability to provide long term zonal isolation, shown by the effective reduction in porosity and permeability. Furthermore, the results confirm reduction in the detrimental effect of salt on the strength and stiffness in post-compression cement.

Microstructural characteristics of wellbore cement and formation rocks under sequestration conditions

Abstract One of the most important aspects of the interaction between well-bore cement and the formation rock are the porosimetric parameters of both materials. Porosity and pore-size distribution affect the behaviour of the porous solid material, including the fluid movement and flow. In order to analyse the porosity of rock and cement material in the presented investigation the following procedures were applied: mercury intrusion porosimetry (MIP), gas adsorption technique and X-ray microtomography (XMT). The investigation proved the usefulness of XMT method, which enables the microstructural characterisation of the composed (cement–rock) samples during the subsequent stages of the experiment. Imaging the cement degradation in the rock–cement contact zone is possible, thanks to the differentiated densities of the altered zones, resulting from the rock or cement dissolution, and new phases precipitation. Five rock samples of different lithology (quartzitic sandstone, eolian sandstone, shale, limestone and anhydrite) were selected for the examination. The prepared samples composed of well bore cement and rock were exposed to CO2-saturated brine, under static conditions. Basing on the shape of the gas adsorption isotherms the classification of the materials (cement and rocks) was performed. In the case of the rocks classified as macroporous, because of the porosity characteristics close to those of cement, the CO2 saturated brine flow is uniform in both materials (cement and rock), and the dissolution and precipitation processes are less intense. The obtained results seem to be very useful for predicting the integrity of wellbore, in case of different caprock and reservoir rock.

Embodied Energy and GHG Emissions from Material Use in Conventional and Unconventional Oil and Gas Operations.

Environmental impacts embodied in oilfield capital equipment have not been thoroughly studied. In this paper, we present the first open-source model which computes the embodied energy and greenhouse gas (GHG) emissions associated with materials consumed in constructing oil and gas wells and associated infrastructure. The model includes well casing, wellbore cement, drilling mud, processing equipment, gas compression, and transport infrastructure. Default case results show that consumption of materials in constructing oilfield equipment consumes ∼0.014 MJ of primary energy per MJ of oil produced, and results in ∼1.3 gCO2-eq GHG emissions per MJ (lower heating value) of crude oil produced, an increase of 15% relative to upstream emissions assessed in earlier OPGEE model versions, and an increase of 1-1.5% of full life cycle emissions. A case study of a hydraulically fractured well in the Bakken formation of North Dakota suggests lower energy intensity (0.011 MJ/MJ) and emissions intensity (1.03 gCO2-eq/MJ) due to the high productivity of hydraulically fractured wells. Results are sensitive to per-well productivity, the complexity of wellbore casing design, and the energy and emissions intensity per kg of material consumed.

Fe2O3 nanoparticles improve the physical properties of heavy-weight wellbore cements: A laboratory study

The application of nanomaterials to solve problems in wellbore cementing has been investigated in recent years by several research groups in the petroleum industry. This study includes the laboratory examination of the effect of Fe2O3 nanoparticles as a weighting agent on the physical properties of a heavy-weight wellbore cement. In the research process a candidate well is selected and the properties of the used cement slurry in a problematic section of the well are tested in the laboratory. Then Fe2O3 nanoparticles are added as a weighting agent to partially replace the Hidense and the improvements in the cement slurry and stone properties are studied. This article discusses the problems associated with the conventional heavy-weight wellbore cement used in the candidate well and gives the detail of the improvements in cement properties obtained by adding Fe2O3 nanoparticles to cement slurry formulation as a weighting agent. These properties include cement slurry rheological properties, free water, fluid loss, thickening time, cement stone elastic properties and compressive strength. The nano additive increases the yield point and plastic viscosity and decreases the free water and fluid loss of a cement slurry. In addition, cement stone compressive strength increases, however, there is an optimum concentration of nano additive at which the maximum compressive strength is reached. Moreover, elastic properties of the cement stone are improved and higher values for the Young's modulus and Poisson's ratio are achieved. The results of this study can be used to optimize the cement slurry design in any given set of conditions. The modified cement using this approach, is applicable in high pressure fluid bearing zones.