1. Field of the Invention
This invention relates to the field of analyzing wellbore gases and more specifically to the field of analyzing annular gas to identify the annular gas zone of origin.
2. Background of the Invention
A natural resource such as oil or gas residing in a subterranean formation can be recovered by drilling a well into the formation. The subterranean formation is usually isolated from other formations using a technique known as well cementing. In particular, a wellbore is typically drilled down to the subterranean formation while circulating a drilling fluid through the wellbore. After the drilling is terminated, a string of pipe, e.g., casing, is run in the wellbore. Primary cementing is then usually performed whereby a cement slurry is pumped down through the string of pipe and into the annulus between the string of pipe and the walls of the wellbore to allow the cement slurry to set into an impermeable cement column and thereby seal the annulus. Secondary cementing operations may also be performed after the primary cementing operation. One example of a secondary cementing operation is squeeze cementing whereby a cement slurry is forced under pressure to areas of lost integrity in the annulus to seal off those areas.
After the well is completed, the pressure of gas in the annulus of the well (referred to as annular gas) is typically monitored. Annular gas pressure is a concern in wells. For instance, sustained annular gas pressure may increase to pressures that cause safety issues in active and abandoned wells. The gas that causes high annular gas pressures may originate and enter the annulus from any depth and zone in the well. As the gas enters the annulus, it may migrate up through the well to its surface and thereby increase the pressure in the annulus. As an example, the gas may migrate through old or damaged cement jobs to the surface of the well. Conventional methods for mitigating the annular gas pressure include testing gas at the surface of the well to determine its chemical composition. The chemical composition of the gas is then correlated to well logs to determine the origin of the gas. Drawbacks to the conventional methods include instances in which the chemical composition of the gas is insufficient to associate the annular gas with a specific zone in the well. For example, in some instances, the well logs containing chemical compositions of the mud gas found in wellbore zones do not have sufficient variances in compositions to identify the zone of origin. For instance, the chemical composition of hydrocarbon gases in a wellbore typically include carbon dioxide, nitrogen, methane, ethane, propane, iso- and normal butane, iso- and normal pentane, and sulfur gases, which are components analyzed in determining the origin of the gas. However, the content of heavier hydrocarbons typically varies slightly, which may reduce the number of available parameters in the annular gas suitable for correlating to the well logs. Further drawbacks include the time involved in determining the chemical composition and making a sufficient correlation to identify the zone of origin.
Consequently, there is a need for a more efficient method in identifying the zone of origin of annular gas. In addition, there is a need for monitoring annular gas pressure and identifying the zone of origin of annular gas in real-time. Further needs include improved methods for mitigating annular gas pressure.