Cement Bond Logs Research Articles

Evaluation of Epoxy Resin Thermal Degradation and its Effect on Preventing Sustained Casing Pressure in Oil and Gas Wells

Epoxy resin applications in oil and gas wells have significantly increased for remediation and sustained casing pressure mitigation because of its solids-free nature and excellent thermomechanical/bonding properties when used either as a single component or as a resin/cement-enhanced composite. Therefore, it is imperative to assess the formation and degradation of structures in cured epoxy resin at downhole temperatures, particularly because hydrocarbon production requires long-term wellbore integrity. Differential scanning calorimetry was used to determine the glass transition temperature ( $$T_{\mathrm{g}}$$ ) of the proposed resin system, and thermogravimetric analysis (TGA) was used to characterize the thermal degradation response by monitoring the resin specimens’ mass loss over time under controlled temperature ranging from 300 to $$680\,{^{\circ }}\hbox {F}$$ at atmospheric pressure. The thermal kinetic response based on TGA was then modeled using the Arrhenius equation to predict the resin lifetime under expected wellbore conditions. This work also presents a case study in which an epoxy resin–cement composite is used as an annular barricade to help prevent and reduce sustained casing pressure. The resin–cement composite was placed in the annular section between the 18 5/8- and 13 3/8-in. casing sections as a chemical packer tailored to improve bonding to steel pipe, along with optimizing its mechanical response to cyclic downhole loads, which resulted in no up-to-date sustained casing pressure. For a resin system subjected to downhole temperatures of $$263\,{^{\circ }}\hbox {F}$$ , the model predicts that reaching 10% mass loss by thermal degradation can take more than 160 years, which is beyond the operational life of the wells where the system is evaluated. This indicates that the investigated resin system provides long-term dependability that ultimately results in reduction of intervention/remediation costs, along with production maximization. Additionally, the resin mechanical properties were evaluated at different temperatures to assess their response to expected thermal loading, which resulted in competent barriers that can withstand the cyclic loads generated by continuous wellbore operations. Furthermore, cement bond log results further support the optimum annular integrity attained when utilizing a cement–resin composite as chemical packer for enhanced isolation and annular pressure buildup mitigation.

Technology Trends in Evaluating Cement Jobs Using Logging Tools

This article, written by Special Publications Editor Adam Wilson, contains highlights of paper SPE 188274, “Technology Trends in Cement-Job Evaluation Using Logging Tools,” by Mahmoud Khalifeh, SPE, University of Stavanger/IRIS; Dave Gardner, SPE, IRIS; and Muhammad Yasser Haddad, University of Stavanger, prepared for the 2017 Abu Dhabi International Petroleum Exhibition and Conference, Abu Dhabi, 13–16 November. The paper has not been peer reviewed. Cement-job evaluation is a process that identifies whether the objectives of a cement-job operation have been achieved after cement placement. This paper reviews the technology trends in cement-job evaluation using logging tools and considers the main advantages and concerns associated with each technology. The technologies covered are acoustic tools, temperature logging, noise logging, resistivity logs, oxygen-activation logs, X-ray measurements, gamma/gamma density measurements, neutron/neutron logging, and fiber-optic measurements. Introduction After a cementing operation has been performed and the cement has set, cement evaluation is conducted either through hydraulic testing or by using various well-logging methods. Verification of casing-cement quality by logging tools has been a contentious subject for the past few decades. On one hand, some experts claim that current technologies are not sufficient to show the sealing capacity and quality of casing cement. On the other hand, others claim that data processing and misinterpretation of the obtained data from cement-job evaluations are the crucial challenges, not the logging technologies. However, another critical element must be considered as a limiting factor—downhole condition. The downhole condition, which includes well parameters and well design, has a major influence on technology development and data interpretation. The well parameters include downhole temperature, downhole pressure, wellbore-fluid properties, casing size and thickness, cement thickness and type, fast formation, slow formation, and unconsolidated formation. Fig. 1 shows the most-likely well design and the worst-case scenarios that need to be considered when developing technology with an assumption that there is no formation washout. As shown in Fig. 1, cement sheaths may be relatively thin but they are very long, especially in the case of surface and intermediate casings. This means that extensive vertical defects along the bore-hole axis would be necessary to create a leakage pathway. Cement defects that can cause leakage include low top of cement (TOC), mud channels, chimneys, and microannuli. Acoustic and ultrasonic logging tools are the standard tools used for indirect cement-quality measurement and for finding TOC. However, limitations exist concerning the measurement and interpretation accuracy that can be achieved using standard cement-bond logs. In addition, logs do not provide continuous, real-time, long-term monitoring of cement-barrier quality. Therefore, different technologies have been proposed that might have the potential to address the limitations of acoustic and ultrasonic logging tools.

Multi-frequency acoustic method and equipment for cement bond log of wells based on one-sided excitation and reception of elastic waves from the top of the well

Multi-frequency acoustic method and equipment for cement bond log of wells based on one-sided excitation and reception of elastic waves from the top of the well

Petrophysical Properties of Casing Cement While Curing

For geothermal as a well as hydrocarbon wells the casing is cemented to ensure a good contact between casing and the rock mass and, more importantly, for preventing the contamination of water resources. Cement shall thus have sufficient strength to prevent collapse of unstable formations and sufficient little permeability to prevent movement of undesired fluids along the well. The ultimate goal is the long-term integrity of the casing and the well. In a series of experiments, we conduct tests on various cements (with some bentonite content) for strength (uniaxial and triaxial), static elastic properties (Young's Modulus and Poisson's ratio), permeability, compressional wave velocities and dynamic elastic properties of the cement while curing. With this set of data the qualitative insight from cement bond logs may be enhanced by quantitative mechanical properties.

Comparative wellbore integrity evaluation across a complex of oil and gas fields within the Michigan Basin and implications for CO2storage

AbstractWellbore integrity is a fundamental aspect of site characterization and selection for storage in areas with existing or abandoned oil and gas wells. The condition of well cement, installed casing strings, and plugs determine if a well can sustain optimal hydrostatic pressures and prevent fluid migration. This is a necessary but underappreciated aspect of assessing potential storage sites when optimal and sustainable pressure conditions are critical for success. To apply the systematic wellbore integrity evaluation to realistic CO2storage conditions, seven fields in the Michigan Basin were selected which were actively being used for CO2injection for enhanced oil recovery. Data on wellbore construction were collected for wells on and surrounding the reefs and were evaluated to determine the overall condition of boreholes in the study area using EPA guidelines and industry standards. Available cement bond logs were also reviewed to assess the quantity and quality of cement in the wells. The majority of the wells in the study area were shallow and did not penetrate the confining layers, thus, posing little to no risk. Most of the deep wells demonstrated sufficient plugs and cement quality that meet the desired standards. Some wells have missing or incomplete records and inadequate cement and/or plugs making them candidates for further evaluation, should large‐scale CO2storage be considered in the area. The results of the study show that readily available well records and cement bond logs can be used to efficiently evaluate and characterize CO2storage study sites for overall wellbore integrity. © 2016 Society of Chemical Industry and John Wiley & Sons, Ltd.

Decomposing full-waveform borehole acoustic data with application to data from a North Sea well

The objective of this study is to establish whether sonic data acquired in a cased hole can be used to estimate the material behind a second casing when the annulus between the two sets of casings is fluid filled. We have analyzed full-waveform data acquired using the Schlumberger tool Sonic Scanner for a double casing with a fluid between them, and where the outer annulus, outside the outer casing, might be cement filled or fluid filled. The sonic tool uses a cylindrical array of 104 omnidirectional receivers. The cylindrical array — approximately 4 in. in diameter and 1.8 m (6 ft) long — allows a formal decomposition of the acquired data into quasi-plane waves. Analyzing these plane waves, we have identified subtle but distinct changes in the waveforms. These changes appear to be dependent on the material filling the outer annulus, allowing for the determination of the fill material. The most significant changes relate to the propagating Stoneley waves. The identifications made are confirmed by cement bond log (CBL) analysis done on the exposed outer pipe after pulling the inner pipe. In one instance, for single-casing logging and a clean top-of-cement, like with conventional CBL analysis, we were able to confidently and accurately identify the cement/fluid boundary. In another instance, we were able to identify it as a nondistinct/smooth transition zone. For double-casing logging, we could confidently and accurately identify the cement/fluid boundary behind the second casing. For one case, we have substantiated that this zone has a longer interval of smooth transition from cement to consolidated/unconsolidated barite to a fluid-filled annulus.

Understanding acoustic methods for cement bond logging.

Well cementation is important for oil/gas production, underground gas storage, and CO2 storage, since it isolates the reservoir layers from aquifers to increase well integrity and reduce environmental footprint. This paper analyzes wave modes of different sonic/ultrasonic methods for cement bonding evaluation. A Two dimensional finite difference method is then used to simulate the wavefield for the ultrasonic methods in the cased-hole models. Waveforms of pulse-echo method from different interfaces in a good bonded well are analyzed. Wavefield of the pitch-catch method for free casing, partial or full bonded models with ultra-low density cement are studied. Based on the studies, the modes in different methods are considered as follows: the zero-order symmetric Leaky-Lamb mode (S0) for sonic method, the first-order symmetric Leaky-Lamb mode (S1) for the pulse-echo method, and the zero-order anti-symmetric Leaky-Lamb mode (A0) for the pitch-catch method. For the sonic method, a directional transmitter in both the azimuth and axial directions can generate energy with a large incidence angle and azimuth resolution, which can effectively generate S0 and break out the azimuth limitation of the conventional sonic method. Although combination of pulse-echo and pitch-catch methods can determine the bonding condition of the third interface for the ultra-low density cement case, the pitch-catch cannot tell the fluid annulus thickness behind casing for the partial bonded cased-hole.

Behind-Pipe-Log-Evaluation Study: Deepwater Subsea Abandonments

This article, written by Special Publications Editor Adam Wilson, contains highlights of paper SPE 174777, “Behind-Pipe-Log-Evaluation Study: Deepwater Subsea Abandonments in the Gulf of Mexico,” by Glenn Donovan, SPE, Peter Fadesere, SPE, George Ware, and Lou Daigle, SPE, Shell Offshore, and Suparman, SPE, Gary Frisch, SPE, Phil Fox, Mike Englar, Gordon Moake, SPE, and Weijun Guo, SPE, Halliburton, prepared for the 2015 SPE Annual Technical Conference and Exhibition, Houston, 28-30 September. The paper has not been peer reviewed. Techniques for well-abandonment log evaluations have been studied in the Gulf of Mexico since 2012. Legacy practices typically used acoustic methods consisting of cement-bond-log (CBL) and ultrasonic-scanner devices. The new methods described in this paper consist of adding nuclear sensors to supplement the acoustic measurements and introduce novel processing methods. The overall solutions package consists of behind-pipe- evaluation techniques (BPETs). When properly modeled and analyzed, these data have the potential to reduce the cost of removing casing strings significantly. Introduction Much of the nonproductive time associated with well abandonments is attributed to difficulty circulating the stagnant material left behind in the casing-to-casing annuli during well construction. Many improvements have been made in the design of these fluids; however, drilling muds using barite as the weighting agent still suffer phase degradation over time, leaving a column of material with a density that increases with depth, except when solids bridging occurs. In extreme cases, the precipitated barite adheres to the outside of the casing wall, impeding the removal of the severed casing section. When traditional sonic and ultrasonic logs are run over concentric casing strings, the results can be misleading. Sonic logs such as the CBL do not yield quantitative results in concentric strings of casing because of the inability to predict the influence of the outer casing on the log response. Misleading data and experiences in well abandonments have left the industry actively seeking alternatives to the conventional method of identifying the material in the annulus, to increase the efficacy of the abandonment process. Abandonment Log-Evaluation Methodologies Historically, it has been common practice to use ultrasonic imaging technologies in combination with traditional sonic-waveform data to characterize the annular space of wells that are candidates for casing-retrieval operations. However, this approach is inadequate for evaluating a phenomenon that is often observed in older wells that contain concentric casing strings that were not cemented to the top of the string. In those cases, the original drilling fluid between the casing strings experiences degradation. As the drilling fluid degrades, solids in the fluid begin to settle out and migrate toward the bottom of the well or toward areas of high deviation or dogleg severity. The settling of these solids is commonly referred to as barite sag, mud sag, or barite settling.

Cement Evaluation—A Risky Business

Summary Cement evaluation is often limited to running a cement-bond log (CBL) and making some attempt to interpret the results and determine whether there is isolation in the wellbore. Often, that interpretation is made in isolation with little or no information on what occurred during the drilling and cementing of the well, or the cement systems used. Evaluating cement in older wells where the drilling report states “ran casing, cemented same” can be particularly challenging. Cement evaluation is much more than just a CBL. Understanding the objectives of the cement job, the design limitations imposed by those objectives, and the resulting slurry and job designs are integral parts of cement evaluation. Often, the selection of a specialty cement system to meet specific job objectives can dictate how one can evaluate the cement. To properly evaluate a cement sheath, knowledge of the cement job, slurry designs, and the limitations of the evaluation technique. To attempt to perform a cement evaluation that is based solely on the log output from a CBL, or any log, invites considerable error and bias into the resulting interpretation. This paper reviews various methods of cement evaluation—from job data, casing- and formation-pressure testing through sonic and ultrasonic logging. The assumptions associated with each technique are outlined, and the discussion includes some of the limitations of the various techniques, along with cautions on how misinterpretation of the results can lead to assumptions of cement integrity that may not be appropriate. The impact of new specialty cement designs, which incorporate high concentrations of inert materials to give the set cement unique properties, is discussed. The ability of specific logging methods to evaluate the presence of these slurries is presented. Data on selected specialty cement systems in which conventional ultrasonic cement-analyzer-strength data are not representative of the crush strength of the cement caused by the incorporation of specialty materials are included. Both overview of cement evaluation and a risk-based discussion of the technique that may be most appropriate on the basis of the cementing objectives are presented. Methods of reducing risk uncertainty in cement evaluation are discussed along with the “validity” of the various data sets available to the engineer to perform a proper cement evaluation on the well. Understanding the objectives of the cement job sets the boundary conditions for the designs, and from those designs, one can determine the ability to evaluate the resulting cement placement and well isolation. Setting the evaluation methodology and understanding the type of information required to apply that methodology can improve the quality of the evaluation.

Technology-Driven Approach To Develop Shale Gas in Saudi Arabia

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 172695, “Technology-Driven Approach To Develop Shale Gas in Saudi Arabia,” by Kirk M. Bartko and Brian Coffin, Saudi Aramco, and Roberto Tineo, Schlumberger, prepared for the 2014 SPE Middle East Oil and Gas Show and Conference, Manama, Bahrain, 8–11 March. The paper has not been peer reviewed. Shale-gas resources were explored in Saudi Arabia by adopting a “technology-driven” approach against the US-based approach that might be termed the “factory” or “statistical” approach. The technology-driven approach and the customized work flow applied in this project have minimized the time required to understand the reservoir and apply the appropriate technologies. To date, this work flow has been used and calibrated with single-well analysis. Introduction The technology-driven approach to acquiring data was accomplished by maximizing data collection and developing a customized work flow to rapidly incorporate lessons learned for the next wells drilled. The overall challenge was to perform the assessment quickly and execute operations efficiently because of the quick drilling program with multiple rigs working. Typically, a resource rock is evaluated, tested, and produced with individual tools and optimized by a statistical approach because of the number of wells drilled and completed in the area. In the study case, the well is an exploration well 1500 km away from the oilfield infrastructure with no offset-well information to support evaluation of the well. The approach was to build a single functioning model, combining a basin resource work flow with a hydraulic fracture- design work flow. The foundation pieces for this model must contain, at minimum, the following: Core: Acquiring a core is essential because rock properties are required in geology, petrophysics, geomechanics, drilling, and fracturing. Both resource and bounding layers need to be included in this work. Petrophysics: Logging programs need to deliver all data for the models and to be recalibrated on the basis of core and performance of the well. Geology: A depositional model is critical because tectonism influences the trapping potential of the source rock. Recalibration is performed on the basis of the core and petrophysics. Geomechanics: Both isotropic- and anisotropic-based models need to be constructed for drilling and stimulation requirements. The models need to integrate static core data and dynamic logging data, recalibrated with vertical well stress testing through microfracturing, if possible. Drilling: Geomechanics is necessary for proper landing and drilling of the horizontal well along with the proper construction of the completion to handle pump rates and pressure. Stimulation modeling: The model needs to be built on geological information of the area, and petrophysical and geomechanical data. The model should also incorporate mechanical limitations on the basis of completion capabilities such as maximum treating pressures or poor cementbond logs. Microseismic: This provides an effective stimulation volume. The process needs to be integrated into the stimulation execution in realtime mode. Production Data: A simplistic production model should be developed initially to understand production declines associated with stimulated volumes and fracture geometries. Model complexity increases with additional data.

Reaming With Casing and Rotating While Cementing

During Cementing operations, rotating and reciprocating methods plays major role in Casing system. Casing, once landed which cannot be moved prior to cementing, is a positive indication that something is wrong. The use of a single ball launcher and cement plug coupled with a modern casing-running tool allows the casing string to be run, washed, reamed, and cemented as part of a seamless operation. This adds the benefit of getting casing to bottom despite difficult formations, and the ability to transition immediately to cementing operations while retaining the ability to rotate and reciprocate. During Cementation, Pipe movement has recently become a simple operation for any operator to include in a drilling program: there is minimal cost and planning required. A cement plug is attached to the bottom of the proprietary Casing Drive system(CDS) and it is a trademark of TESCO Corporation. A side entry swivel sub is included between the CDS and the top drive. A ball launcher, also provided, is rigged up in line with the chicks an lines running from the cementing pump truck to the side entry swivel sub. The solution allows the operator to go directly to cementing after the casing has been run. It also enables the rotation and reciprocation of the pipe during cementation, which prevents channeling of cement and leads to a notably superior cement job as proven by cement bond logs. Using a hybrid cementing tool with a modern casing running system allows wells to be engineered more aggressively, thus the ensure casing can be run to bottom in difficult formations and guarantee an improved cement job. In turn, this provides greater production and returns for the operator.

New Cement Spacer Chemistry Enhances Removal of Nonaqueous Drilling Fluid

Technology Update An efficient removal of drilling fluid is essential to successful cementing operations. When a cement slurry comes into contact with mud residue, the cement may not set properly or adhere to the casing and formation, thereby preventing the isolation of permeable zones under different pressure regimes. This can cause stimulation out of zone, production of unwanted fluids because of communication between zones, loss of hydrocarbons into lower-pressure formations, corrosion of casing, and blowouts. In recent years, the industry has focused considerable attention on enhancing cementing practices to ensure well integrity and zonal isolation. To clean all well surfaces after the placement of casing, an intermediate water-based fluid, or spacer, is pumped between the drilling fluid and cement slurry. Typically, a package of surfactants or solvents is added to the spacer to improve cleaning and displacement efficiency. However, over the past decade, spacer formulations have not evolved as rapidly as the chemistry of drilling fluids. Every year, operators drill deeper, more complex, and higher-temperature wells under a growing range of downhole conditions. To boost performance, drilling fluids incorporate a greater variety of nonaqueous fluids (NAFs) based on synthetic and natural oils—mineral, paraffinic, and olefinic oils—with much longer carbon chains than traditional diesel-based muds. As such, NAFs are much more difficult to clean. Although spacers are intended to separate drilling fluid from cement, a small amount of spacer fluid—from 5% to 10%—typically contaminates a portion of the cement slurry. Many spacer chemistries affect cement properties, such as thickening time, rheology, and compressive strength, often precisely where zonal isolation is most essential. If the cement has not fully hardened by the time a cement bond log is run, it may not show up clearly and the top of cement (TOC) may be uncertain. Therefore, it is vital to ensure that spacers remove the maximum amount of mud while having a minimal effect on cement properties. Since NAF compositions vary widely, there are no universal spacer formulations. Most are designed and tested on a case-by-case basis using procedures that differ from one operator or location to another. Existing American Petroleum Institute (API) recommended tests for evaluating the suitability of a particular spacer have been found nonrepeatable. In addition, no test protocols exist for well conditions above 85°C, despite the large number of cement jobs in wells with bottomhole temperatures of up to 150°C. Along with salinity and the type of drilling fluid, temperature is one of the most critical factors influencing cement integrity. Thus, enhanced test procedures and spacer formulations are needed.

Acoustic evaluation of cementing quality using obliquely incident ultrasonic signals

Ultrasonic cement bond logging is a widely used method for evaluating cementing quality. Conventional ultrasonic cement bond logging uses vertical incidence and cannot accurately evaluate lightweight cement bonding. Oblique incidence is a new technology for evaluating cement quality with improved accuracy for lightweight cements. In this study, we simulated models of acoustic impedance of cement and cementing quality using ultrasonic oblique incidence, and we obtained the relation between cementing quality, acoustic impedance of cement, and the acoustic attenuation coefficient of the A0-mode and S0-mode Lamb waves. Then, we simulated models of different cement thickness and we obtained the relation between cement thickness and the time difference of the arrival between the A0 and A0′ modes.

Numerical simulation of an acoustic field generated by a phased arc array in a fluid-filled cased borehole

Transducers that are widely applied in cement bond evaluation tools, such as cement bond logs and variable density logs, cannot radiate acoustic energy directionally because of the characteristics of monopole sources. A phased arc array transmitter, which is a novel transducer that differs from monopole and dipole transducers, is presented in this study. To simulate the acoustic field generated by a phased arc array in a fluid-filled cased borehole with different channelings, a 3D finite-difference time-domain method is adopted. The acoustic field generated by a traditional monopole source is also simulated and compared with the field generated by the phased arc array transmitter. Numerical simulation results show that the phased arc array radiates energy directionally in a narrow angular range in the borehole, thereby compressing the acoustic energy into a narrow range in the casing pipe, the cement, and the formation. We present the analyses of first-arrival waveforms and the amplitudes of casing waves at different azimuthal angles for the two different sources. The results indicate that employing a directional source facilitates azimuthal identification and analysis of possible channeling behind the casing pipe.

Well Integrity Assessment of a 68 year old Well at a CO2 Injection Project

Abstract As part of the United States Department of Energy (DOE) National Energy Technology Laboratory (NETL) sponsored project, The Quantification of Wellbore Leakage Risk Using Non-destructive Borehole Logging Techniques, the construction and integrity of a 68 year old well was studied. This study builds upon previous work examining the integrity of existing wells through shale formations. The objective of this study was to measure well integrity through potential caprocks and aid in understanding the potential leakage risk posed by old wells that intersect CO2 injection projects. The well was originally completed as a production well in the Gulf Coast region in 1945 and then plugged and abandoned in 1969. The well was re-entered and re-completed as an observation well for a CO2 injection project in 2008. It was replugged and abandoned after completing its observation role in 2013. For this study the well was logged using cement bond log and ultrasonic mapping tools, tested and sampled using a dynamic tester, and cored using a sidewall coring tool. The age of this well makes it the oldest well to be studied in this manner and this is the first well to be studied this way in the region. The results of the study indicate that much of the material behind the casing is unconsolidated cement. Logging results in many places in the well show poor isolation potential and indicate a microannulus. The logs also indicate removal of material during hydraulic testing which was confirmed by laboratory analysis. Of the six cores collected, four consisted of unconsolidated, soft, cement or rock, one consisted of heavily altered cement, and one consisted of slightly altered cement. The results of study are an interesting contrast to earlier field studies because they show an old well that lacked integrity at most of the test and sample points. However, the logging and a core sample in the squeezed zone indicate that there was zonal isolation between the injection and monitoring zones. The logging and mapping data along with the analysis of the “core” material collected provide insight into the potential leakage pathways within the well.

Investigation of Wellbore Integrity Factors in Historical Oil and gas Wells for CO2 Geosequestration in the Midwestern U.S.

Abstract A systematic investigation was completed to evaluate wellbore integrity factors for oil and gas wells in the Midwestern U.S. in relation to CO 2 geosequestration. In this region, over one million oil and gas wells have been drilled, and there is a perception that many areas are not suitable for geosequestration. The project focused on Michigan and Ohio as study areas. Records were obtained for over 280,000 wells. Plugging and abandonment reports were tabulated for 1,730 wells, and 278 cement bond logs were analyzed with a systematic method to grade the cement present in the well. Sustained casing pressure was monitored and analyzed for cement defect factor on thirteen wells in various configurations. The data was summarized with maps and graphs. The test site assessments provide examples of the steps and costs necessary to address wells in the region. Results indicate that a variety of well construction and plugging approaches were used over time, but many of the geologic formations being considered for CO 2 storage are not penetrated by many wells. The project provided practical tools for CO 2 storage applications including: database of well parameters, systematic cement bond log evaluation tool, and sustained casing pressure analysis method for cement defect factor.

Effects of tool eccentralization on cement-bond-log measurements: Numerical and experimental results

The cement-bond log (CBL) is a conventional and widely used cement quality evaluation technology for vertical wells. With the increase in horizontal wells around the world, the existing cement evaluation technologies are not appropriate. We have explored the possibilities of utilizing CBL in horizontal wells through investigating the effects of a noncentralized tool on CBL measurements. The parallel finite-difference numerical simulation method and experiments in calibration wells were adopted in the study. The numerical and experimental results matched very well, and indicated that the CBL amplitude decreases linearly with increasing tool eccentricity in a well with free pipe (i.e., a cased but uncemented well). For a standard pipe with a diameter of 5.5 in (139.7 mm) and a thickness of 7.72 mm, an eccentricity of [Formula: see text] (17% of the maximum eccentricity) could cause the CBL amplitude to be reduced by about 20%. The numerical simulations of CBL in wells with fluid channels in the cement showed that tool eccentralization could either increase or reduce the CBL amplitude relative to a centered tool, depending on the channel azimuth relative to eccentered direction. To explain this phenomenon, we investigated numerically the polarizations of casing waves in a well with free pipe and in a well with a fluid channel, and casing waves at higher frequencies in a well with free pipe. The relationship between the CBL amplitude and the percentage of cemented area for a borehole-centered tool was also studied. Our results provided some insights into understanding CBL measurements in horizontal wells.

Assessment of Well Integrity at Nagaoka CO2 Injection Site Using Ultrasonic Logging and Cement Bond Log Data

This paper reports logging results for well integrity conducted at the CO2 injection site in Japan. Well integrity is one of the essential issues for safety in geological CO2 storage. Wells in the Nagaoka site have been used for monitoring logging of CO2 behavior for more than ten years, and have experience of large earthquakes near the site. Time-lapse ultrasonic and cement bond logging have been employed to investigate the well integrity of the observation wells. Results of the ultrasonic logging showed that there was no sever damage or deformation in the casing at the reservoir depth. No clear change in cement can be seen in either logging results, even at the depth where the cement was exposed to the CO2. These results showed that there was no clear evidence of CO2 leakage at Nagaoka.

Analysis of potential leakage pathways at the Cranfield, MS, U.S.A., CO2 sequestration site

A 1.5-million-ton CO2 sequestration project took place in a 3000-m-deep historical oilfield, combined with a CO2-EOR flood. The Cranfield reservoir is found within a multikilometer domal structure related to a deep-salt diapir and consists of fluvial sediments of the Tuscaloosa Formation. An earlier analysis determined that plugged and abandoned wells provide the most likely leakage pathways to aquifers and potentially to the ground surface. Fourteen Cement Bond Logs (CBL's) were used to assess the risk. The present quality of the cement bond ranges from excellent to poor.Geological insights, stochastic numerical modeling of the pressure field, analysis of the CBL's, and application of a wellbore flow model were used to conclude that the limited pressure increase and mostly intact wellbores result in a low CO2- and brine-leakage risk. Statistical estimates of well properties suggest that at most two (and possibly none) could be capable of conveying a total of 1800kg/yr CO2 to the surface (0.0002% of annual injection rate). Given that the oilfield is an active operation, it is improbable that well leakage to the surface will go unnoticed and certain that risks will be managed through active risk mitigation and remediation if necessary.

Well integrity and the correlation between borehole breakouts, drilling-induced tensile fractures and cement-bond log response

Traditionally, image logs are acquired for sedimentary and structural analysis and more recently geo-mechanical analysis. Caliper logs are usually acquired for cement volume information and borehole condition information with pad tools. In general, cement bond logs have been the only logs used for the assessment of cement bond quality and thereby well integrity at the end of the well construction phase. Cement bond logs occasionally produce puzzling results—the cement job parameters indicate a successful cement job, yet the cement bond log indicates otherwise. In some circumstances, the presence of a micro-annulus can cause the appearance of a bad bond. In others, the causes of such cement bond log responses are less obvious. Wellbore breakouts and drilling induced tensile fractures can cause drilling problems such as increased losses, washouts and cave-ins; however, their association with cement bond quality is not always appreciated. This dataset consists of LWD StarTrak and Lithotrak caliper data and EWL Wellbore Geometry Instrument (WGI) data and Segmented Bond Tool (SBT) data The data shows extensive zones of borehole breakouts as well as tensile fractures, some of which exist in areas without breakouts. The SBT pad data shows large areas of poor bond between relatively thin-cemented sections. The VDL data shows formation arrivals that became more pronounced with depth. When the data are compared, the zones that showed tensile fractures coincided with zones showing poor bond. The zones showing no tensile fractures or breakouts were also the zones showing good cement bond.