Analysis of Gas Migrationin Well Cementing and Cementing Technology for Shallow Gas Wells in ANACO Gas Field

The ANACO gas field is located in the northeast of Venezuela, and has huge natural gas reserves, almost 60% of the total natural gas reserves in Venezuela. The reservoir formations of the ANACO gas field are of great complexity, with gas zones and water zones extensively distributed from the shallow formations and to the deep formations. Several gas zones with different pressure systems coexist, and have low fracturing pressures, high pore pressures and narrow safety pressure windows. In some parts of the gas field, high pressure gas zones and water traps form high pressure gas pockets and water belts. Wells drilled in this area generally have five intervals to seal off zones of different pressure systems. From the first interval to the third interval, high pressure shallow gas zones exist, and the control of the shallow gas zones is limited by the pressure bearing capacity of the formations. Cement slurries with multiple thickening times cannot be used in cementing the top formations. Several factors, such as large hole sizes, low displacement efficiency, mixing and channeling of cement slurries in annular space, contamination to cement slurries, changes in anti-migration performance, as well as weight loss by gelling of cement slurry, all contribute to poor cementing job, negatively affecting the development of the gas field. The cementing service team hasstudied the stability of cement slurry, static gel strength and SPN coefficient to try to find a way to solve the problems encountered. In field operations, time interval for the development of the static gel strength of cement slurry was controlled within 25 min, and SPN controlled to less than 3, and in cementing casing strings with large sizes, a low flow rate of 0.55 m³/min was used and plug flow maintained during injection and displacement. High density anti-migration cement slurry was also injected into the annular space to displace the contaminated cement slurry, in an effort to thoroughly seal the formation with double thickening times. Using this technology, loss of cementing slurry has been avoided, formation pressure completely brought under control, and gas migration prevented. The quality of the cementing jobs in ANACO gas field has been improved using this technology.