Horizontal well production logging (PL) measurements, especially center-sample devices, have been misleading in evaluation of the flowing stream. These inaccuracies are due to the natural segregation of the fluids owing to the difference in the phase densities. A new multiphase holdup tool and interpretation method has been developed that provides accurate determination of holdups and flowrates in horizontal wells. The multiphase holdup tool uses 12 capacitance-sensing circuits with multiple arm arrangements to provide an excellent image of the holdup pattern. Since these sensors are on the same cross-sectional plane, depth inconsistencies are not a factor in the interpretation. Each sensor responds to the permittivity (dielectric constant) of the surrounding fluids, and the response can be converted to a phase holdup by applying the known sensor calibration. Consideration of each sensor's position relative to the wellbore allows for prediction of the total phase holdup across the entire wellbore cross sectional plane. Advanced analysis packages allow the user to interact with the computed holdups, providing an easy method of interpretation. Different views of the wellbore, cross-sectional displays, and displays incorporating the wellbore trajectory allow accurate and detailed analysis that is critical for understanding flow in horizontal wells. Once the holdup of each phase (gas, oil, and water) is determined, an additional interpretation package allows a complete production log analysis including flowrates in a very difficult environment. The results of these analysis packages allow an operator to understand, modify, and improve the productivity of a horizontal well. Tool Design and Operations Phase separation makes it extremely difficult to get a good view of holdup in horizontal wells by using standard center-sampling tools. The Capacitance Array Tool (CAT™) solves this problem by using a circular array of 12 micro-capacitance sensors. They are radially distributed in the wellbore to measure accurately phase holdups. Fig. 1 is a picture of the tool showing the sensors with the arms in the out position. These cylindrical sensors have a diameter of 0.157 inches and are 0.055 inches long located 0.35 inches from the end of the motorized arm as shown in Fig. 2 and Fig. 3. Each sensor forms part of a circuit that resonates at different frequencies in gas, oil, and water. This variation in resonance allows the tool to determine what phase exists at a given region across the wellbore. Responding to the capacitance value around the probe, the oscillator circuits produce a low frequency in water, high frequency in oil, and a higher frequency in gas. Sensor frequencies are typically sampled 72 times per second (dependent upon the telemetry) and relayed to surface where they can be processed for presentation.1 The sensors have a radius of investigation of about 0.01 inches. Sensors are electrically isolated from other CAT components so that they only register capacitance values from their immediate environment. These micro-capacitance sensors are expected to measure the individual segregated phases (water, oil, or gas) that are assumed to surround them. However, due to the sensor and calibration response the holdup mixtures of any two phases can be accurately determined. Therefore, each of the 12 sensors will measure gas, oil, or water, or a mixture of two phases depending upon software parameter selection. The CAT's geometry makes it especially well suited to measuring holdup near the top and bottom of any cross section normal to the axis of the wellbore since all sensors are in this plane. The array provides full coverage across the wellbore, making it possible to accurately identify fluid in horizontal or highly deviated wells as shown in Fig. 2.
Methods to evaluate conventional cements with logging data may not yield valid results in foamed cements. Logging tools commonly used in cement evaluation are the traditional cement bond log (CBL) tools and the modern ultrasonic scanning tools. CBL tools provide waveform images and acoustic amplitudes that together help describe cement-to-pipe and cement-to-formation bonding. Ultrasonic scanning tools contribute circumferential images and detailed information regarding the cement-to-pipe bond. The major problem in evaluating foamed cements is that their impedance values can be below that of annular fluids, such as mud or water. Thus, a standard interpretation of ultrasonic images and data in foamed cements may provide an incorrect diagnosis of the cement bond and could lead to unnecessary remedial cementing activities. A new interpretation method expands and improves on previously published methods to effectively evaluate foamed cements with the common cement-evaluation tools. This method uses ultrasonic scanning data to provide detailed information regarding the cement-to-pipe bond and allows foamed cement to be distinguished from mud or spacer fluid behind casing. Cement-slurry weight and composition do not affect this technique. The technique does not require additional logging tools or passes and thus does not introduce additional logging or rig expenses. Examples are presented showing that the new method is valid and is effective in both time and cost. These examples will illustrate several new ultrasonic cement-bond curves that were developed as a result of the new method and that, when used in conjunction with CBL amplitude data, improve foamed-cement evaluation. In the examples, the interpretation is focused to answer the basic question, "Should remedial cementing be performed, or should the well be perforated for production?"
Evaluation of cement bonding and zonal isolation is a challenge that the oil and gas industry continues to face as wells are drilled deeper within more hostile environments. The complexity of this task has increased as these wellbores have more challenging trajectories and being drilled in formations for which there is little drilling and completions experience. In addition, cement slurries have become more complex with the addition of inert gases, micro-spheres, non-traditional liquids, and many other additives designed to improve the cement sheath quality. These slurries require non-traditional interpretation methods to effectively evaluate the cement sheath because older methods do not yield accurate results in these situations. This paper will present information concerning the existing cement evaluation logging tools, basic interpretation techniques, and an overview of the new, advanced methods for existing tools available from a variety of vendors in the industry. Progress is continuously being made in the development of more effective cement evaluation tools and evaluation techniques. Standard cement evaluation logging tools consists of two major classes, sonic and ultrasonic. The standard cement bond log, segmented bond log, and radial bond log are all part of the sonic logging family. The ultrasonic family consists of tools with either a rotating transducer or a stationary array of transducers. This paper, however, will not focus on the hardware but will focus on the interpretation of available measurements and on facilitating optimized decisions using measurements from both families. Advanced interpretation methods discussed in this paper broaden and refine previously published methods in order to effectively evaluate wellbore conditions with the commonly available cement evaluation tools. The original processes developed in the early 1990s now incorporate a statistical variance mapping display for both the sonic and ultrasonic tools. The resulting variance image from the ultrasonic tools allows detection of minor changes in cement or fluid composition and aids in the interpretation of the pipe-to-cement bond. This technique provides a robust answer product helpful in diagnosing zonal isolation and highlighting channeling and quality of materials behind pipe for all cement compositions. It is also possible today to process and interpret non-standard sonic data, such as refracted monopole and flexural dipole from logging tools not specifically designed for cement evaluation. Correct application of the newer interpretation techniques described in this paper can lead to fully evaluated cement sheath quality and distribution behind pipe. Several examples using the new advanced interpretation methods will be presented including comparisons (1) between a scanning ultrasonic tool and a radial bond tool (2) a sequence of evaluations using a cement bond log combined with an ultrasonic tool before and after several multiple remedial squeeze operations, and (3) also a comparison between two different scanning ultrasonic tools and a segmented bond tool. The final example shows a successful use of the new technique in a completion using titanium casing. Basic Cement Evaluation Tools There are two major classes of standard cement evaluation logging tools (or logs), sonic and ultrasonic. The sonic logs include cement bond (CBL), segmented bond (SB), and the radial bond (RB). The ultrasonic logs consist of two types of tools, one having a rotating transducer and the other has eight stationary transducers. Several previously published papers in the bibliography Frisch et al.[1,3,4] provide more detail about tool theory and applications.
TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractUnwanted water production may adversely affect well economics because of water-disposal costs, environmental issues, and reduced hydrocarbon production. This paper presents examples of how openhole and cased-hole logs can help determine the type and location of water entry, and indicate appropriate conformance treatments. Selected logs run following the treatments are used to evaluate the treatments. The cost of running the logs and applying the indicated conformance treatments is compared with the cost of water disposal and declining petroleum production.Specific examples show that casing leaks, cement channels, coning, and watered-out reservoirs may all be easily identified and located through the use of openhole, production, pulsed neutron, casing inspection, and cement evaluation logs. Not all of these logs need be run in every well; thus, in each well, existing information was thoroughly studied to determine the most cost-effective diagnostic logging program.The diagnostic information provided by the logs was useful in selecting appropriate conformance treatments. Many of the same logs used in diagnosing the water-production problems were found to be valuable in evaluating the effectiveness of the performed treatment. In some cases, follow-up treatments were suggested by the evaluation of the initial treatment and were the difference between successful rejuvenation of hydrocarbon production and continued declining well performance.The costs of the logs used for conformance diagnosis and treatment evaluation were compared to the possible long-term costs of water disposal, environmental considerations, and reduced production from the well and were found to be relatively inexpensive. Using Well Logs for Conformance DesignOpenhole logs offer a wealth of information for reservoir characterization, well planning, and stimulation design, each of which is an integral component of conformance design.
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