A Well Cementation Evaluation Method by the Azimuthal Gamma Combination With the Acoustic Logging in Horizontal Well

Horizontal well technology has been widely used in the exploration and development of unconventional oil and gas reservoirs to increase the exposed area of oil and gas reservoirs, and the well cementation is of great significance to the isolation of oil, gas and water layers. Using gamma-ray attenuation to determine cement density, combined with acoustic logging technology to evaluate the first and second interfaces, is an effective method for well cementation evaluation. Compared with well cementation in vertical wells, due to the gravity of the casing and the logging tool, there are problems in horizontal wells such as logging tool and casing eccentricity, cement slurry settlement, et al. These problems together affect the detection of gamma-ray, lead to the ambiguity of the azimuthal cement density evaluation. There is a logging tool that can be used to measure the azimuthal density of the cement, which is mainly composed of a 137Cs gamma source, a near detector and six far detectors arranged in a circumferential direction. In addition, the logging tool contains an acoustic section. Since the gamma-ray measured by the detectors is affected by the eccentricity and the circumferential cement density distribution, we use the difference in multi-detector gamma count of measurement and the reference condition to calculate the circumferential cement density. The reference condition here is the measured value of gamma count under the calibration pit with well-filled cement and no eccentric casing. Firstly, the degree of the logging tool eccentricity is determined by using the time arrival information of acoustic logging. Then, under different logging tool eccentric conditions, using the GEANT4 numerical simulation software to establish the spatial dynamic response functions with different casing eccentric conditions. The cement is divided into 6 sectors, and the spatial dynamic response functions are integrated to obtain the contribution of the cement density in the 6 sectors to the gamma-ray detected by different detectors. Based on the contribution of the circumferential cement density and casing eccentricity to the multi-detector gamma count, a multi-parameter calculation equation set is established, the Newton iteration method is used to invert the casing eccentricity degree and the azimuthal cement density in six sectors, and the regularization method is used to improve the accuracy of the solution, thus, the ambiguity of the azimuthal cement density evaluation in horizontal well cementation evaluation is eliminated. The effectiveness of the method is verified by a simulation example with logging tool eccentricity, casing eccentricity and uneven distribution of azimuthal cement density. The results show that the method can control the circumferential cement density accuracy within 0.05g/cm3. Compared with the calculation error 0.3g/cm3 of the cement density without considering the eccentricity, this method improves density accuracy while eliminating the misjudgment of well cementation quality caused by eccentricity. A field logging example also verifies the effectiveness of the method, this method provides technical support for the monitoring of the cementing quality of horizontal wells.