Gas Well Production Optimization Using Expert Systems Technology
Authors: McLean Dan , Goranson Heather 
Published: December 2007

Producing gas wells with high GLR (Gas Liquid Ratios) have been the source of many operational problems, especially during Canadian winters. Automation technology now allows us to relieve the wellbore of these liquids in a controlled manner, thus freeing production casing capacity where problem wells are produced by the tubing string. This paper deals with the implementation of a field-wide optimization program based on a SCADA installation of RTU's (Remote Terminal Units) at the well sites and an Expert System shell running in the SCADA host to optimize the production.

Introduction

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. In addition to the sensing and control equipment, EFM (electronic flow measurement) transmitters are installed on the orifice meter run after the separator, to accurately measure the flow rates (AGA3 pressure and temperature compensated). The G2 software program compares the pressure profiles of the tubing and casing and controls the flow from the well accordingly. Figure 2 illustrates this pressure profile.

SCADA System Infrastructure

All of the wells are part of a field wide SCADA system which enables production operators to view and control well production from a central location. Each well is monitored over a licensed radio communications system.

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