Integrated Forecasting and Optimisation of an LNG Project—The Case of the North West Shelf Project

Forecasting the production of large LNG projects is challenging and must consider:

• Optimising the by-products associated with the gas production, like condensate and LPG as these have significant value;

• Gas from a bundle of fields with often different gas compositions (impurities and LPG/condensate yield) is fed to the LNG plant;

• The complex operating envelope of the processing plant often with many constraints acting on either the full gas stream or on individual component and product streams;

• Stringent gas contracts stipulating strict gas delivery schedules. Predicting total system deliverability is important as Contracts are often long-term, prescribe gas quality constraints and set limits for the concentration of impurities;

• Complex ownership structures and operating arrangements, due to high capital intensity.

These challenges establish the need for forecasting tools that cover the full supply chain of a project, from the reservoir to the physical point of sale and the sales contracts. Tools need to be fully integrated taking into account business, strategic and commercial decision-making processes.

Woodside's bespoke Database Integrated Modelling Environment (DIME) has been built specifically for forecasting and optimising complex LNG projects producing from a diversity of fields with significantly different condensate and LPG yields. The tool provides fast, accurate forecasts for:

• sales products - LNG, domestic gas, chemical feedstock (e.g. ethane), LPG, and condensate;

• losses; and

• emissions - water, flue gas and CO₂ emissions.

Because DIME enables modelling from the reservoirs to the plants and sales delivery point, it forecasts sales products and therefore directly provides a forecast for revenue. The model is fully compositional and pressure-sensitive, and designed for system optimisation.

This paper describes the tool's functionality, focusing on the technical challenges of forecasting and optimising the complex production system of the North West Shelf project. Important functionality incorporated in this production forecasting tool includes:

• Achieving the highest condensate and LPG product yield – optimally blending production within the technical constraints and contractual product specifications.

• Imposing system constraints and contractual product specification, which can be dependent on the gas composition.

• Enabling control over distributing the hydrocarbon components over the products by modelling the LNG and domestic gas plant using a parametric description.

• Establishing the total system production capacity based on a complete pressure sensitive supply chain.

• Assessing the impact of Compression based on limiting power usage.

• Maximising valuable by-products while simultaneously honouring export pressures, flow rate constraints, plant and product constraints.

DIME integrates well with other tools and supports a range of technical, business and commercial processes. It has been used by Woodside over the last 10 years, and has established optimum project timings, sizing compression requirements and identified system constraints.