Abstract
Junggar basin in west China is rich in oil and gas reserves in different geological structures that range in depth from 800 m to 7000 m. A variety of cement slurries is used to ensure the cement integrity in the wells. For instance, low-density cement as light as 1.1 g/cm3 with hollow glass microspheres has been used for low-pressure-gradient formations in long openhole wellbores, whereas high-density cement as heavy as 2.5 g/cm3 with iron mineral powder and other additives has been applied in high-pressure deep wells. This variety poses challenges to performing cement evaluation with traditional acoustic techniques that have limitations in providing unambiguous answers, and in some instances it is not possible to assess implications to zonal isolation behind casing.
A new-generation ultrasonic imaging tool was deployed to increase the confidence in cement evaluation in Xinjiang oil field over the past 2 years. This technique combines the traditional pulse-echo ultrasonic measurement with a new flexural wave measurement. The analysis allows a better discrimination of solid, liquid and gas for density-independent cement evaluation behind casing, additional information on casing centering, annulus geometry, and accurate casing inspection at high resolution in one logging trip. Based on this improved zonal isolation information, well testing design improved, and the testing success rate was significantly increased over previous operations. Moreover, the cementing technique improved from proper post evaluation. The examples in this paper demonstrate the value of the high-quality logging data that were acquired in different downhole conditions. This paper presents a new way to look at cement quality and placement.
