GPA ’14: Noble revamps Keota gas design for LNG

By ADRIENNE BLUME 

DALLAS -- At the Gas Processors Association's Tuesday afternoon LNG forum, April Schroer from Noble Energy and Timothy Miller from Audubon Engineering discussed the conceptualization and design of Noble's Keota gas plant, which will be Colorado's first liquefied natural gas (LNG) plant.

The Keota facility, which was designed to process gas from Noble's Denver-Julesburg (DJ) basin operations, was originally conceived as a traditional gas processing plant. However, Noble began examining other options for Keota as it sought to bring in a clean energy alternative for fueling its DJ rig engines. 

Revamping the engineering plan. At the FEED stage, Noble studied the possible use of LNG as a means of displacing diesel usage, reducing emissions, assisting with obtaining the company's operating license and saving fuel costs. The abundant amount of shale gas available provided a low-emissions, secure source of energy to fuel Noble's drilling rigs.

Ms. Schroer recounted how Noble initially examined three options for natural gas fuel—fuel gas, compressed natural gas (CNG) and LNG. Fuel gas was not determined to be an optimal choice for the DJ basin operations due to its inconsistent product quality. CNG was also rejected due to a lack of CNG providers and because of the fuel's low energy density relative to diesel. LNG, however, has a higher energy density than diesel and consistent product quality, and a number of turnkey options are available. 

After choosing LNG as its main fuel source, Noble brought in two dedicated rigs from another field for the LNG pilot test, and it entered into a turnkey contract to provide a dedicated LNG fuel supply. The demonstration was successful overall; however, the company did experience a learning curve while powering the rigs with 100% LNG. 

Where Noble operates in the DJ basin, the nearest LNG source is 400 miles away, which poses a problem if weather-related LNG shipment delays cause rig downtime. Also, there is a limited number of LNG providers in the area.

Noble picks dual-fuel power solution. Noble then examined ways to continue using LNG while mitigating the risk of downtime, Ms. Schroer explained. The company decided to convert to a dual-fuel option, using both LNG and diesel to mitigate the risk of a possible supply interruption. LNG storage and vaporization units are used to transfer LNG to the engines at Noble's drilling rigs. 

The company purchased kits to run its generation sets on a mix of fuels, providing flexibility to its drilling operations. Now, instead of consuming 3,080 gallons per day (gpd) of LNG, the rigs can run on half of that amount with the dual-fuel generator sets, Ms. Schroer said. This required significant monitoring and adjusting, as the compressors and engines had to be loaded up and altered to accommodate both LNG and diesel. 

The introduction of LNG into the Keota plant, which took place during the FEED stage, will secure a cost-effective supply of LNG for Noble Energy and other operators in the DJ basin. It will also enable greater natural gas use throughout the supply chain. Chart Industries was selected to design and manufacture the LNG facility. Design changes made to integrate LNG include the addition of a gas pretreating section and an adjustment to the original site plan. 

The LNG facility will receive 8 million standard cubic feet per day (MMscfd) of gas to make 100 thousand gpd (Mgpd) of LNG, and the balance of the gas will be delivered to a gas pipeline. Approximately 360 Mgal of LNG tank capacity is required to support continued operations; six days of LNG fuel are needed at present usage rates. The balance of the LNG is available for bulk sales.

Facility overview. Mr. Miller explained how Noble Energy chose a nitrogen refrigeration cycle for its LNG process, which was determined to be less complex than the more complex mixed refrigerant (MR) process—albeit less efficient, requiring more horsepower. Mixed refrigerant was also rejected based on the requirements of increased monitoring, onsite storage of components, transport difficulty, and the associated fugitive emissions. After choosing the nitrogen refrigeration technology, Noble began working with Chart to develop the process design. 

Product specifications required CO2 to be less than 15 ppm in the LNG sales gas, due to freezing concerns associated with CO2 content above 50 ppm. The LNG also needed to contain 96% methane, and the LNG process was required to have a provision to remove ethane to meet LNG specifications. Depending on market conditions, Noble may need to operate the Keota plant in ethane-rejection mode.

Lessons learned and present progress. Mr. Miller stressed that code determination must be started early in the process for this type of project. Also, project partners must ensure that the air permit, the emissions permit and the amine unit can handle a stricter CO2 specification. Also, as the increased power requirements needed to power the LNG component came to light, Noble decided to source grid power from the local utility. 

Finally, since the implementation of the dual-fuel LNG/diesel concept at Keota was new for Noble, the company held many face-to-face meetings with all parties involved in the project, as a means of ensuring transparency and eliminating risk for mistakes in project design and execution. The company also participated in facility tours of existing LNG plants and gathered knowledge from other operators at trade shows, like the GPA Annual Meeting.

In closing, Mr. Miller noted that Noble is presently wrapping up the engineering phase of the Keota gas plant and is nearly ready to move into the full construction phase.