Guidelines for 90 % Accuracy in Zone-Isolation Decisions
Authors: Fitzgerald D.D. 
Published: December 2007

Recent technological advances in the logging industry have caused marked improvements in cement-bond log (CBL) recordings. New techniques for measuring travel time, amplitude, and wave train have increased the reliability and effective use of CBL instrumentation. Examples show how these new techniques can be used more effectively, especially in hard formation applications. A new, fast, and simple quantitative interpretation method has been developed. It is a graphical solution that allows easy conversion of direct receiver output to percent bonded cement by the acknowledgment of percent bonded cement by the acknowledgment of logarithmic response of sound attenuation. Pipe size, weight, thickness, or cement compressive strength, as long as they are constant, do not affect the interpretation. Also, it is adaptable to practically all logs run by the various service companies if the recommended guidelines are followed closely. Furthermore, it makes little difference whether the amplitude curve is recorded in millivolts, percentages, chart divisions, etc. After several years of field tests, decisions based on the results of CBL's run within the guidelines have been more than 90% successful.

Introduction

For more than 20 years, CBL's have been used to evaluate cement jobs in oil and gas wells. Unfortunately. the lack of standardization, poor planning, poor quality control, and inaccurate measurements and displays have resulted in a lack of believability and confidence in CBL'S. We have determined that, when properly recorded and interpreted, this log can provide properly recorded and interpreted, this log can provide accurate and useful information for drilling and production personnel. Establishing the correct downhole logging personnel. Establishing the correct downhole logging conditions, choosing the correct equipment, using log formats that verify the accurate recording of the log data, and interpreting data with this new method prove that CBL's are capable of giving consistently accurate answers in making squeeze/no-squeeze decisions. Several commonly accepted oilfield practices can cause either expansion or contraction of casing after cement cure. These changes in casing diameter can break the bond between the outer casing surface and the annular cement, resulting in the creation of a microannulus at the outer casing surface. If a microannulus is present, the casing signal information recorded on the CBL is not representative of the annular cement fill. The use of correct logging techniques overcomes this adverse effect and allows meaningful cement fill data to be recorded. A new technique for interpreting casing signal amplitude data uses a semilog plot to define the annular cement fill quantitatively. Success with this technique exceeds 90% accuracy on squeeze/no-squeeze decisions.

Equipment Selection

The selection of logging equipment from the many available designs is critical in obtaining meaningful log data. Several key equipment design factors should be considered-such as transmitter/receiver spacing. accurate minimum amplitude measurement capability, and accurate travel-time measurement capability at low signal amplitudes. For example, equipment with a 3-ft [0.914-m] transmitter/receiver spacing can more easily provide the resolution necessary to define cement provide the resolution necessary to define cement channeling in all ranges of cement compressive strength. As the transmitter/receiver spacing becomes longer, the capability to detect cement channeling decreases. Moderate- to high-compressive-strength cements, well bonded to the casing, greatly reduce the amount of sound energy available at the receiver. Hence, equipment with minimum amplitude measurement capability down to 0.2 mV must be used to detect channeling in high-compressive-strength cements-i.e., neat Class H or G cement. Fig. 1 was recorded with equipment that registers zero amplitude for casing energy of less than 7.0 mV. This equipment will not detect channeling in Class H cement. Fig. 2 shows that as receiver amplitudes decrease below about 10 mV. accurate travel-time measurements, without cycle skipping, become increasingly difficult to make. When in a cycle-skip mode, the travel-time curve cannot be used as a centralization indicator. Good travel-time measurements from signal levels below 5.0 mV should be within the instrumentation capability. Better instrumentation will still make good travel-time measurements without cycle skipping at amplitudes less than 2.0 mV. Also in Fig. 2, travel time does not always cycle skip at casing energy levels less than 10 mV. This indicates an unstable trigger level in the travel-time circuit.

Well Preparation

Well preparation for cement logging is extremely important.

JPT

p. 2013