According to market studies, all indications are that the importance of the shale gas market is not going to diminish in the foreseeable future. On the contrary, significant growth is projected in many quarters. For example, a 2011 report issued by ICF International on behalf of the INGAA Foundation predicted that total United States and Canada shale gas production will jump from 2010 levels of about 13 Bcf/d to 52 bcf/d by the year 2035.
The report anticipates that more than 400,000 miles of gathering pipe will be constructed in North America by 2035. Nor is this growth limited to North America; shale gas plays are also among the fastest growing production areas worldwide.
As liquids-rich unconventional resource plays are developed, there are multiple challenges and implications for midstream system infrastructure, particularly around pigging and integrity. This is especially true for shale plays producing rich gas (also known as wet gas), which contains significant levels of liquefiable hydrocarbons (like ethane or propane) along with methane gas.
Liquids can accumulate at low elevation points along gathering systems where the high liquid concentrations in the gas streams cause significant issues with slugging, high differential pressures (liquids loading) and corrosion. In addition, crude oil containing high levels of paraffin and other flow-reducing contaminants (frac sand, chlorides and spent chemicals) presents flow restriction issues in these midstream pipeline systems.
Because many factors contribute to the overall performance and flow efficiency of pipeline systems, including the elevation profile, flow volumes, product quality, and temperature, all pipelines must be evaluated on an individual basis.
In order to prevent liquids from accumulating and to maintain production levels, routine pigging is required. Routine pigging removes liquids from the line, offers control on the volume of liquids removed at any one time, and sustains well production at consistent capacity. Routine pigging also removes contaminants associated with wet gas, including paraffin, asphaltenes, iron oxides, water, hydrogen sulfide and carbon dioxide.
Pigging frequency should be determined based on the specific characteristics of a pipeline system. Depending on the circumstances, pigging may be required as often as several times a week, or in some cases, three times a day. In most cases, pigging frequency should be based on flow efficiency over time as opposed to a pigging program based strictly on time.
So, the rules of the game are still undefined; producers and gatherers are still seeking scalable solutions to these issues. The high number of lines to be pigged and the need to pig these lines frequently may require the installation of multiple automated pig and/or sphere launchers. Automation of pigging systems offers compelling economic benefits when compared to traditional manual pigging systems.
Easing The Burden
Traditional manual pigging systems are time- and labor-intensive. A typical pigging system requires the opening and/or closing or three major valves, the draining and venting of a barrel, and the opening and closing of a closure door. In some cases, it can take up to four hours for a single crew to load and launch a single pig, which does not include the time to receive and remove the pig. Beyond the time and labor constraints, there are also wear and safety considerations.
Opening and closing valves several times a week can increase the risk of valve seat failure, as well as increase maintenance and replacement costs. Frequent cycling of launcher/receiver barrels can also create unnecessary safety risks that may include cyclic fatigue of the systems and additional exposure to hazardous gases produced in many of the shale plays.
Shale play operators have confirmed the need for pigging systems that make it possible to use automated sphere launchers to routinely pig their pipelines. In addition, operators need the ability to clean their lines with cleaning pigs and inspect their lines using inline inspection tools. Spherical pigs are quite effective at removing liquids from a line, but they do not clean the pipe, and typical sphere launchers are not compatible with either cleaning pigs or inline inspection tools.
By drawing upon experience gained designing automated pig and sphere launchers since the late 1970s, T.D. Williamson (TDW) developed an automated pigging system that combines all three pigging functions: pigging, cleaning and inspection.
The SmartTrap® automated combo pigging system releases spherical pigs individually at pre-set intervals through use of a dual launch pin system. Multiple spherical pigs can be loaded at one time. The system also enables operators to launch a single cleaning pig, batching pig or inline inspection tool.
The shale play pipelines are not typically regulated like transmission pipelines; however, the ability to continuously remove valuable liquids and maintain their integrity is equal to that of transmission pipelines.
The flow-through barrel design of the automated system removes the need to open/close valves for each pig launch. This design increases valve life and reduces maintenance costs. Hydraulically controlled launch pins are used to launch spheres without opening or closing any valves. All elements of the automation system (including its control panel, hydraulic power unit, cylinder limit switches and all electrical conduit/connections) are Class 1, Division 1 compliant.
The system can be used with cleaning pigs and inline inspection tools, but its primary design is based around launching spheres at regular intervals for the removal of liquids. One of the main components of the system is its automation, which relies on a logic-driven “smart system” with a programmable controller designed to be programmed to meet an operator’s specific needs. The programmable controller allows operators to launch spherical pigs on a time-based frequency to remove and control the liquids that are condensed and accumulated in the pipeline system.
For security reasons, the system cannot be activated without first entering a four-digit code. Once the system is activated, the operator works from four screen modes: local control, timed control, maintenance mode and configuration mode. Any or all of these modes can be used to launch spheres during the life of a production field. Local control mode allows the operator to load, launch and run pigs onsite.
Timed control mode makes it possible to set the time interval for automatic launches of spherical pigs. Maintenance mode enables manual cycling of launch pins. Configuration mode allows the operator to switch to remote controlled operation, as well as adjust the time, date and other system settings.
When a pigging system is placed in service, it is critical to monitor and document when (and how many) pigs have been launched to evaluate the performance of the pipeline flow efficiencies. The automated pigging system monitors and records the most recent hundred events, including user log-ins, launches (both time and date), and errors should a component not report an activity. This history log can be accessed by an operator either onsite or from a remote location to provide discrete monitoring of the pigging system.
The main advantage of the “smart system” is that it helps prevent operator errors. For example, the system will not allow a pig to be loaded if an unlaunched spherical pig remains in the barrel. The system displays the number of spherical pigs remaining in the barrel and transmits an alert as the barrel nears empty. Also, the system will not allow the raising of both launch pins when the barrel contains spherical pigs, and it reports system errors when they occur. Other inputs and monitoring can be configured as needed to meet specific operator requirements.
Looking to the future, there is every reason to expect that the scope and needs of the shale gas market will continue to grow. As the existing shale plays mature and new plays come on line, increasing production flow and manpower efficiency while decreasing operating and maintenance costs will be critical keys to success. Pigging solutions such as the automated system described here can help operators meet the daily challenges encountered in the field.
The automation of pigging systems has proved successful in the shale plays, where automated pigging systems can be monitored through operator SCADA and remote monitoring systems in addition to operational data that may include line pressures and flow rates. The integration of multiple monitored data sets can help an operator maintain optimum flow efficiencies by comparing the theoretical differential pressures compared to the actual differential pressures to establish the appropriate pigging frequencies from the performance-based conditions of the pipeline system.
Larry Payne is the manager of market development for the Pipeline Services & Pigging Solutions division of T.D. Williamson.
David Wint is the manager of midstream technical services for the Pipeline Services & Pigging Solutions division of T.D. Williamson.