Optimizing production from gas wells that are prone to liquids (water) loading in the wellbore is a common operational problem in Western Canada gas fields.
Wascana Energy has implemented an innovative method to automatically unload the wellbore by alternating production between the casing and tubing strings. Wells with sufficient reservoir pressure are normally produced up the casing. When liquids accumulate in the wellbore, thereby restricting the gas flow rate, production is automatically switched from the casing to the tubing string, to lift the water out ojthe wellbore.
The Gensym G2 expert system is used to optimize the point at which this production switching occurs to maximize daily gas production. Well performance is monitored remotely over a field wide, radio based. SCADA system.
Wascana implemented a field wide SCADA (Supervisory Control and Data Acquisition) system in the Pierceland area of northwest Saskatchewan during 1996. The SCADA system remotely monitors the performance of some 75 natural gas wells scattered over an area of approximately 70 miles by 30 miles. A Gensym, G2 application has been developed to optimize the gas production from wells subject to wellbore loading. The expert system will manage production from 18 of the 75 gas wells that have local control systems for the automatic recovery of liquids from the well-bore. These wells are relatively low producers with low suction pressures, hence restrictions in the wellbore due to liquids accumulation significantly affects production rates. Typically manual operator intervention is required to unload these wells, which is complicated by the remote well locations and difficult accessibility. This practice is typically potentially damaging to the formation as wells are blow down to atmosphere to unload, and the severe drawdown can promote sanding problems. In addition, these wells must be produced through the smaller diameter (typically 2-5/8 inch) tubing string in order to create enough velocity to carry the liquids to the surface. Significant production can be gained by producing up the larger casing annulus (typically 4 1/2 inch). The difference in the cross sectional area alone allows a significant gain in well productivity, given that reservoir pressures are high enough and there are no other physical constraints in the gathering system and inlet separation at the compressor facilities to handle the increased water production. Figure 1 shows a typical well site configuration with the water loading symptoms illustrated.
To automatically remove the water from these wells, a technology was developed by Kenonic (patent pending) to monitor and automatically unload the wellbore based on flowing pressure differential between the tubing and casing. The local well site system is a combination of hardware and software elements including RTU's (Remote Terminal Unit), control (switching) valves and pressure transmitters located at the wellhead. The switching valves control the flow from either the tubing string or the casing and are pneumatically or electrically actuated. The pressure transmitters sense the flowing tubing and casing pressures and are connected to the local well site RTU.