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Morris, Charles W. (Schlumberger) | Sabbagh, Lou (vMonitor, Inc.) | Wydrinski, Ray (BP plc) | Hupp, Jeffrey Lee (BP) | van Kuijk, Robert (Schlumberger Riboud Product Center) | Froelich, Benoit (Schlumberger Riboud Product Center)
Abstract The hydraulic isolation of the wellbore casing and cement is critical for the completion of production and injection wells. Zonal isolation prevents the production of fluids from non-completion intervals, contamination of ground water by fluids in the wellbore, and allows conformance control of injected fluids. Current acoustic evaluation techniques may be limited by the acoustic properties of the material behind casing and by the inability to see beyond the cemented region near the casing. A new ultrasonic imaging tool has been developed to address these limitations. The new imager tool combines the classical pulse-echo technique with a new ultrasonic technique that provides temporally compact echoes arising from propagation along the casing and also reflections at the cement-formation interface. Processing these signals yields unprecedented characterization of the cased-hole environment in terms of the nature and acoustic velocity of the material immediately behind casing, the position of the casing within the borehole, and the geometrical shape of the borehole.5 In order to provide answers to the casing/cement evaluation questions, a field study was performed to evaluate the results provided by both sonic and this new ultrasonic tool in the different cement materials, drilling fluids, and casing sizes. Field examples are presented to illustrate the actual response of the new ultrasonic tool to these various completion environments including wells cemented with conventional and lightweight cement. The results demonstrate enhanced cement evaluation for all cement types and a significant reduction in the uncertainty in making a squeeze or no-squeeze decision. The new cement evaluation tool implements both the traditional pulse-echo technique and the new flexural wave concept. The flexural mode enables deep imaging of the cement sheath up to the cement-formation interface. In addition, the measurement of the borehole geometrical shape makes it possible to evaluation double casing string conditions for potential damage. Introduction Sonic logging tools have been used since the 1960's to evaluate the placement of cement for hydraulic isolation of formations. There have been several advancements in the logging tools that improved the ability to evaluate the cement sheath since that time. During the same period of time there has been little change in the types of cement. In the past few years, however, there has been an emphasis on optimizing the cementing operation and reducing the overall cost of the completion. To the cementing operation, this meant developing lightweight and specialized cements that would allow setting casing strings deeper without worrying about lost returns. Other gains in efficiency were also achieved using lighter cements while drilling and completing weak formations. Changes in these cements, and their properties, have also brought about the need for re-evaluating the techniques and tools used for the evaluation of these cements with the sonic logging tools currently available. The Cement Bond Log (CBL) type tools, which include all tools that measure amplitude or attenuation, have a common theory of measurement, interpretation principles, strengths, and weaknesses. The principle of measurement of these tools is to measure the amplitude of a sonic signal, produced by a transmitter emitting a 20 kHz acoustic wave, after it has traveled through a section of the casing as an extensional mode. This amplitude is then converted into attenuation by either using a ratio of multiple transmitter and receiver amplitudes, or using chart book conversions. At this point the interpreter has to select a value for the attenuation of a 100% bonded interval. This can be done based on the CBL data collected in the well or it can come from the predicted cement properties. The value for the attenuation in a 100% bonded interval is the key to the interpretation of this type of log. Zonal isolation is estimated from an empirical data base. These tools also provide a qualitative indication of bond to the formation through the use of a Variable Density Log (VDL) waveform.
Centeno, Manuel (Schlumberger) | Krikor, Ara (Schlumberger) | Herrera, Delimar Cristobal (Schlumberger) | Sanderson, Martin (Schlumberger) | Carasco, Anant (Schlumberger) | Dundin, Alexander (Schlumberger) | Salaheldin, Ahmed (Schlumberger) | Jokhi, Ayomarz (Schlumberger) | Ibrahim, Sameh (Schlumberger) | Wehaidah, Talal (Kuwait Oil Company)
Abstract The complexity of drilling highly deviated wells in Kuwait drives the need for step changing in the well construction mindset, where severe to complete loss of circulation in Shuaiba formation significantly deteriorate the shale layers in Wara and Burgan formations leading to uncontrolled wellbore stability events. Casing while drilling (CWD) and two-stage cementing with a light density cement slurry were introduced as a technology system to drill the highly deviated complex wells through unstable and highly fractured formations. Fit for purpose engineering processes, advanced software solutions, a tailored bit and a bottom hole assembly dynamically simulated for drilling stability and directional tendency behavior were designed. A special light density cement slurry with high compressive strength was also designed to tackle the lost circulation issues when cementing the casing string. The paper will describe how the technologies can work as one system to solve complicated wellbore problems and address the problematic challenges of drilling unstable shales and fractured formations in the same section of the wellbore. This strategy enabled a significant time saving compared to drilling the section conventionally, removing Non-Productive Time (NPT) resulting from additional trips, cement plugs, stuck pipe, and subsequent sidetracks.
Abstract This paper describes the deployment of a new circumferential ultrasonic tool for cement evaluation used in a thick-casing environment. The operation was performed in a deepwater well, where massive loads often require heavier linear-weight casings with thicknesses greater than 1.0 in. A new-generation, circumferential, ultrasonic cement-evaluation tool was run in combination with a cement bond-log (CBL) tool to evaluate a primary cementing operation and assure zonal isolation in an ultradeepwater well with 10 3/4-in. casing set in the 14 3/4-in. hole section. Casings with thicknesses greater than 1.0 in. are outside the operating range of current circumferential ultrasonic tools. The improvement features, main specifications, and measurement physics comparing the new tool with previous-generation technology are presented. The operation was performed in a well containing two sizes of 10 3/4-in. casing in the same casing string. Both 85.3 lbm/ft (0.8-in. thick) and 109.0 lbm/ft (1.0-in. thick) casing sections were present and evaluated in the same pass. The sonic/circumferential ultrasonic combination was able to effectively evaluate the quality of the primary cementing operation behind both casing weights, as well as positively detect the top of cement (TOC). The combination of the ultrasonic tool with traditional bond-log technology provides independent and complementary measurements of cement bond quality. This presented the operator with the ability to radially analyze the zonal isolation using an image map and to identify cement quality issues, such as channeling and the presence of microannuli. In addition, the casing integrity was evaluated in the same pass using the ultrasonic tool. The new ultrasonic tool makes it possible to achieve confirmation of well integrity in complex, deepwater environments, clearly identifying zones ranging from free pipe to fully cemented conditions, including radial mapping. Improvements in the measurement physics enables the analysis of the annulus in cases of heavy, thick-walled tubulars, as well as in the presence of heavier drilling fluids.
Boyd, Douglas A. (Zakum Development Co.) | Al-Kubti, Salah A-Rahman (Zakum Development Co.) | Khedr, Osama Hamdy (Zakum Development Co.) | Khan, Naeem (Zakum Development Co.) | Al-Nayadi, Kholoud Ghareeb (Zakum Development Co.) | Degouy, Didier (Total S.A.) | Elkadi, Antoine (Schlumberger) | Al-Kindi, Zaid Laith (Schlumberger)
Abstract Two classes (sonic and ultrasonic) of cement bond log tools are run in tandem as part of ZADCO's standard cement evaluation program. The effectiveness of these tools and their evaluations are often challenged and are not regarded as a replacement for reservoir inter-zonal communication tests performed between producing reservoirs on every well. Consequently the value of continuing to run these tools was raised by management. In response, the reliability of these tools and their interpretations for determining the existence of poor behind casing cement quality and possibly hydraulically communicating layers was critically and systematically examined by a dedicated team of ZADCO and Schlumberger technical professionals. The criteria used to judge the usefulness of these logs was the present or not of communication behind casing as determined by the physical test. For the twenty-eight wells examined, twenty-five of the cement log interpretations matched the communication test results. One well which communicated had 10 feet of apparent good cement with the remainder poor. Its cement log suffered from eccentralization which negatively affected the cement bond log interpretation. Two wells with long intervals of excellent cement on logs suffered behind casing communication. The reasons for this remain unresolved but are a source of constant discussion. Possible explanations for the mismatch between cement evaluation log interpretation and the physical test results are explored. Introduction Evaluation of cement integrity and its ability to prevent fluid and gas migration in the annular space behind casing is a challenge. Historically, three measures are evaluated to check for isolation between reservoir units:pressure testing, physical communication tests between formations, and cement bond log interpretation. Pressure tests are restricted to localized areas of the casing such as the casing / liner shoe and squeeze perforations. Communication tests, regarded as the most definitive method of testing behind casing isolation jeopardize casing integrity, are costly, and directed at preventing reservoir to reservoir communication. Cement evaluation logs are time efficient, cover the majority of the casing and inexpensive compared to communication tests. Their interpretations however occasionally do not predict behind casing communication, creating a perception of unreliability. Determining the confidence which can be placed upon cement log interpretations is an objective of this task force. Behind casing communication of hydrocarbons via the cemented annular space means expensive remedial cement squeezes to cure the problem. The expense is considerably higher if the communication problem is discovered after the well is drilled and the rig has left location. Because of the importance and criticality of the zonal isolation between oil bearing formations for reservoir management, ZADCO's policy is to confirm isolation by physical communication tests regardless of the quality of cement interpreted from cement bond logs. The policy is applied selectively however. Only cement barriers between formations where production is planned are tested (Figure 1). Intervals between reservoirs not planned for production are not tested. Since the majority of cement squeeze decisions are based on the results of physical communication tests, the value of continuing to run cement bond logs was questioned. Conversely, applying the policy of communication tests was costly and time consuming and could not be practically applied to wells covering multiple reservoirs not intended for production. Consequently a task force comprised of representatives from drilling, well integrity, petroleum engineering, petrophysics and the primary service company was formed to review ZADCO's communication test policy and access the reliability of cement bond logs for evaluating behind casing cement quality and from this inferring the presence of or lack of isolation between formations.
van Kuijk, Robert (Schlumberger) | Zeroug, Smaine (Schlumberger) | Froelich, Benoit (Schlumberger Riboud Product Center) | Allouche, Michael (Schlumberger) | Bose, Sandip (Schlumberger) | Miller, Douglas (Schlumberger-Doll Research) | Le Calvez, Jean-Luc (Schlumberger Riboud Product Center) | Schoepf, Virginie (Schlumberger) | Pagnin, Andrea (Schlumberger Riboud Product Center)
Abstract Current acoustic (sonic and ultrasonic) techniques for cement evaluation have proved to be limited in providing unambiguous answers to the zonal isolation issue. This is especially true in lightweight cements where they often fail to differentiate cement from mud. Also, as far as imaging of the cement sheath is concerned, ultrasonic pulse-echo tools fail to image beyond the cemented region adjacent to the casing, thus providing limited diagnosis of the annulus. A new ultrasonic imaging tool has been developed to address these limitations. The new imager combines the classical pulse-echo technique with a new ultrasonic technique that provides temporally compact echoes arising from propagation along the casing and also reflections at the cement-formation interface. Processing these signals yields unprecedented characterization of the cased hole environment in terms of the nature and acoustic velocity of the material filling the annulus between casing and formation, the material immediately behind casing, the position of the casing within the hole, and the geometrical shape of the hole. Different wells cemented with conventional and light cements were logged with the new experimental tool. The results demonstrate enhanced cement evaluation for both cement types and significant reduction in the uncertainty in making a squeeze or no-squeeze decision. Introduction Cement evaluation logging tools have been used successfully for many years to evaluate casing and cement conditions. These tools, which use sonic or ultrasonic1 techniques, are designed for conventional steel casing and cements. The sonic tools, commonly known as Cement Bond Log or CBL, operate at frequencies of about 20 kHz and measure the amplitude or the attenuation of a wave traveling along the casing. The wave loses energy mainly though shear coupling to the surrounding cement, so that well-bonded solid cement attenuates more quickly than a fluid. Due to the low frequency, the CBL logs made with these tools lack azimuthal resolution, which makes it difficult to distinguish channeling from poor cement properties.