This paper will give an overview of a laboratory gas flow model and testing procedure that allow real time measurement of gas migration during the transitional period of setting cement, Key parameters are continuously monitored and plotted on a Personal Computer such as: fluid loss, column hydraulics, matrix stability and set time. Repeatable results indicate potential gas migration even when proper slurry parameters are met which include low gelation, low fluid loss, minimal free water and short transition period. Three case studies from different West Coast fields, each of which showed improved bond logs, are reviewed to support the new testing procedure.
The problems of gas migration after cementing have been well documented. The costs associated with these problems, as well as stricter environmental regulations, have forced gas well operators to look for methods to minimize gas migration.
The most commonly accepted evaluation of success for cement job quality, for the following cases, has been the cement bond log. Varying degrees of success have been noticed, and mediocre bond logs have often been accepted. This may result in costly remedial cementing.
To help predict and overcome the potential for gas migration after cementing, a unique gas flow model was developed. The model evaluates both the potential and severity of gas migration at downhole conditions with the recommended slurry. This enables the cementing company to design the most economical and reliable cement slurry for the particular well.
Results from this model correlate closely with field results from two separate northern California fields and one southern California offshore field. Previously accepted slurries from each of these fields have been evaluated with the new model, and improvements in slurry design implemented in offset wells. Improvements in cement bond logs were seen in all three cases.
The mechanisms for gas flow after cementing vary from well to well. While a number of theories exist, the three most widely accepted are concerned with free water separation, filtrate loss to the formation, and reduction in hydrostatic pressure during initial hydration.
Based on the above mechanisms, the optimum design of a slurry for primary cementing in gas wells would include the following properties:
zero free water and settling (especially in deviated wellbores),
low fluid loss value,
adequate placement time and
early compressive strength development with the ability to prevent gas intrusion during the cement's transition stage.