Summary
Studies were conducted with a large-scale displacement model containing manmade tar and competent (hard) manmade sandstone formations. This paper investigates the cement placement process and the acoustic response with standard logging tools. This investigation revealed that the formations behind the cement make a significant contribution to the pipe-attenuation rate. The large-scale acoustic experiments indicate that lower pipe-attenuation-rate levels are measured when tar formations are pipe-attenuation-rate levels are measured when tar formations are cemented. This occurs even with the same type of cement and the same annular clearances. The tests suggest that low-attenuation-rate levels also may be measured across other low-restraint formations.
Introduction
On the basis of previous studies, it is clear that even with good quality control the cement bond log (CBL) is strictly a measurement of the degree of acoustic coupling between the cement and the pipe. Cement presence and quality are inferred from this measurement. Because true acoustic coupling is rare, and, when obtained, is very fragile, the presence of microannuli between the pipe and the cement is a real problem. The industry generally has pipe and the cement is a real problem. The industry generally has assumed that microannuli are caused mainly by the condition of the pipe surface and pressure and temperature changes in the wellbore pipe surface and pressure and temperature changes in the wellbore after the cement is set. It has been assumed that application of pressure to a casing in the presence of a microannulus eventually pressure to a casing in the presence of a microannulus eventually would close the microannulus enough to achieve a reasonable level of acoustic coupling. Although the significance of a microannulus to good zone isolation is still unclear, the microannuli created by normal temperature and pressure changes generally are considered too small to compromise cement sealing efficiency. Once adequate pressure has been applied, persistent low responses from the log pressure has been applied, persistent low responses from the log along certain critical zone lengths are attributed to channels and recommendations to cement squeeze frequently are considered. We began this research project to investigate the cement placement process and the acoustic response across heavy-oil sands placement process and the acoustic response across heavy-oil sands to explore some of the problems experienced by a major oil company when cementing casing across heavy-oil sands in a field in Canada. These problems were related to poor quality of some primary cementing jobs across the heavy-oil sands, as suggested by CBL'S. Selective production tests in some wells, which resulted in unexpected high production of water and gas, tended to confirm some of the CBL interpretations. On the other hand, serious questions also were raised regarding the quality and the interpretation of a number of the logs; these issues related to calibration procedures, tool centralization, fast formations, and overall logging techniques. Studies investigated the cement placement process and the acoustic response across both manmade tar and competent manmade sandstone formations with a large-scale displacement model. Constructed from Athabasca tar sand, the formations were packed by hand around the inside of a 10 3/4-in. outer casing (Fig. 1). The formation, which comprises sand of several sizes and a catalyzed epoxy resin, has a cured compressive strength of 2,000 to 3,000 psi and a permeability of 100 to 700 md. Conventional mixing and pumping equipment, including spacers and cement slurries, was used pumping equipment, including spacers and cement slurries, was used to displace mud from the simulated annulus. Fig. 1 shows a typical configuration of the test model. After the cement was allowed to set (for most tests, from 2 1/2 to 4 days' curing, with cement compressive-strength development of more than 3,000 psi), the test sections were logged with acoustic-type tools to compare the attenuation across the tars with the response across the competent sandstone formations. The sections then were cut perpendicular to the main axis of the test model to allow visual observation and to measure the test's displacement efficiency. This permitted examination of the predicted condition of the cemented interval through comparison of observed pipe-attenuation rates with the actual situation. After cutting the sections, photos were taken to document the findings. With one exception, all the tests were run with Class H cement containing up to 0.3% bentonite, mixed at 15.7 to 15.8 lbm/gal. One test was performed with a commercially available expanding cement. The test facility did not allow a continuous log to be run along the length of the test model. Measurements could be made only at fixed locations in the model. Most of the acoustical observations were made under pressure at 100-psig increments, from 0 to 500 psig. Two different acoustic tools were used for the investigation. Both tools measured the amplitude of the E1 peak detected at the receivers. The first tool (30,000 cycles/sec and 1.7 in. in diameter), consisted of two receivers spaced 1 ft apart and two transmitters spaced 3 ft from the nearest receiver. The second tool (16,000 cycles/sec and 3.4 in. in diameter) consisted of two receivers spaced 2 ft apart and one transmitter spaced 3 ft from the top receiver. The signal's attenuation was calculated normally with the logarithm of the ratio of the measured amplitudes.
