# SOE - Storage Operation Expert

## Product description

Storage Operation Expert (SOE) – is a software solution for optimizing the operation of underground gas storage plants. The main goal of the software is to maximize plant capabilities and to minimize energy usage, while ensuring compliance with operational safety barriers. Obtaining such a compromise requires taking into account several complex factors, e.g., without an accurate model of the withdrawal and injection flow potential, the overall plant capabilities might be artificially underutilized.

## Target Group

SOE is dedicated for Storage System Operators. It could be used by sales and engineers for finding the optimal way of operating gas caverns, porous storages and dedicated above ground facility.

SOE enables the operator not only to choose the optimal configuration of the plant components and caverns, but also to forecast the plant’s flow potential for a longer time-horizon (e.g. 7 days). The plant model implemented here is based on real operational data and fulfills geological boundary conditions defined by expert rock mechanical reports.

The software includes:

- a thermodynamic model of the gas caverns,
- rock-mechanic constraints for caverns operations,
- model of the above-ground facilities,
- several predefined strategies for caverns utilization.

SOE solution is intended to be used for a variety of gas demand scenarios by utilizing different configurations of gas caverns and choosing compressors with the optimal performance parameters accordingly. The simulations draw a detailed picture of the plant state, for example providing trajectories of cavern wellhead pressures, temperatures and the fulfillment of rock-mechanical restrictions. The trajectory of the plant flow potential can be easily converted into the expected available nomination limits, allowing the storage users a better matching between gas demand and supply.

Moreover, SOE solution is an appropriate application for calculating an optimal technical profile for a balanced and economic utilization of caverns for given gas nominations. It allows also to perform WHAT-IF simulations which facilitate the development of service plans for underground storage facilities for optimal plant maintenance.

For example, nonlinear characteristics of above ground devices such as compressors and preheaters, the complex characteristics of caverns and the necessity of balancing the rock-mechanics and safety constraints with market requirements. The plant’s operator has to find a compromise between using the plant’s maximal capabilities (e.g. the maximal amount of withdrawn gas) and the necessity to adhere to the rock-mechanic limitations (in order to avoid unwanted effect such as cavern convergence).

## The main practical functions of SOE:

- finding the time trajectory of optimal configuration of caverns and devices for an assumed nomination profile,
- calculating the current and the future maximal possible plant’s flow,
- providing a tool for WHAT-IF simulations (which could be useful for defining a -development plan), following the fulfillment of rock mechanics limitations

## Optimization process

The purpose of the optimization procedure is to find the maximal plant’s withdrawal/ injection flow for a given plant’s state and the external parameters (e.g. gas pressure in the system pipe). The optimization task is solved by finding:

The main blocks constituting the algorithm are:

- a thermodynamic model of the gas caverns,
- a model of the above-ground facilities,
- a set of predefined rules of plant’s operation which define a strategy of caverns utilization,
- an optimization procedure which selects the plant’s configuration providing the maximal flow,
- a set of timers dedicated to follow the limitations resulting from rock mechanic constraints of cavern operation.

## The strategy

In most cases there are several different plant configurations that could be used to provide the demanded flow. The purpose of the strategy is to find a configuration of caverns (or a set of configurations) that will be the most effective from the economical and operational standpoint. For example, it is preferable to withdraw gas from the caverns with the compressors turned off (if possible) and use the caverns for which the gas pressure is high enough so that the rock-mechanical constraints need not be considered. Obviously a preferable configuration will not always provide the desired flow, so the strategy generates a set of configurations, each with an assigned priority. The SOE application selects the action with the highest priority for which the plant’s flow is as close as possible to the desired flow. In the case the maximal flow should be provided, there is an additional procedure performed on the set of active caverns in order to select an optimal configuration. Hence, in each step, SOE chooses the action which satisfies the gas flow demand and minimizes the plant operation cost.

SOE application takes into account geological recommendations related with gas caverns operation. These anti-collapsing rules vary for different types of caverns (different cavern types may be used even in a single plant). SOE monitors the state of each cavern according to these recommendations and automatically detects the necessity to initiate an injection action that will protect a given cavern from violation of geological and operational constraints.

The most complex rock-mechanical restrictions are those that limit the time of a safe operation for a cavern below a certain pressure level (denoted by LLOUR-lowest limit of unrestricted operation). Following these rules, SOE application chooses a configuration that, when possible, avoids restrict pressure ranges. When it is not possible, and caverns must operate with pressures in the restricted ranges, a set of dedicated timers is used in order to determine when it is necessary to leave the restricted pressure range by injecting gas into the caverns (to start the get out action).

The diagram below presents a specific example with five caverns in three stages. It illustrates the strategy definition which is used in the gas withdrawal mode. The first stage decision was to withdraw the gas from:

- the class A caverns, when the pressure inside them was above the level of unrestricted operation (this level sets the target pressure for the gas withdrawal action),
- the class B caverns, which can operate in the restricted pressure range for unlimited time, unless the pressure reaches the minimal value. The target pressure was set to the compressor activation level.

When the amount of the gas in caverns has decreased, the class A caverns were switched off when the pressure inside them decreased to the minimal level of unrestricted operation. When all of the gas from the class B caverns was pumped out, further utilization of the class A caverns was forced. The class A caverns started operating below the level of unrestricted operation, and the rock-mechanics timers were initiated.

The SOE input parameters may be divided into four categories:

- parameters describing the plant characteristics, such as physical limitations (i.e. pressure and volume limitations, flow limits), details of the thermodynamic cavern model and nonlinear rock mechanics rules, technical data of compressors capabilities
- parameters defining initial state of the plant, such as values of pressure and temperature in caverns, state of the timers dedicated to monitor the fulfillment of the rock-mechanics limitations and current configuration of compressors. These parameters may be set manually or defined by the plant’s current state
- trajectories describing the future values of various quantities, such as prognosis of the physical gas parameters, gas demand scenarios, availability of caverns and compressors dictated by the service plan,
- parameters controlling the action of the algorithm and tuning the optimization procedure such as different pressure margins or the penalty factor which prevents frequent switching of the plant’s configuration.

The simulated plant model consists of four major parts:

- the system pipe connecting the plant with the external gas grid
- the set of above ground devices
- the field pipe connecting the caverns with the set of above ground devices
- caverns with pressure control valves

SOE simulates the storage plant state for each hour or day of a simulation. Output data are presented in the form of easy to interpret time plots. The plots present trajectories of:

- pressure and gas volume in each cavern
- caverns statuses (active / inactive)
- hourly or daily increment of the gas volume in each cavern but also for all caverns
- decisions on the gas caverns’ operations, related with geological constraints and the applied strategy of gas distribution between caverns
- state of special timers related with geological constraints that are activated when cavern gas pressure is below LLUOR or RCP level.