Economical decisions acceptable if constraints met

Paul Boughton

Doug Kolak explains how crucial the role of water injection systems are to effective flow supply effective extraction.
World demand for oil is set to increase 37 per cent by 2030, according to Energy Information Administration's (EIA) annual report. Taking this into account and considering that the number of known wells is shrinking it comes as no surprise that it is becoming increasingly expensive to locate and extract supply. This highlights the critical roles efficiency and complex extractions have to play in each operation, especially when dealing with subsea wells which can cost upwards of $100 million to initiate production.
Increasing supply from an existing well or drilling into a new pocket can be an option for many, but without taking the proper considerations into systems and production efficiency it may not profitable.
The need for a constant and longer flowing supply is crucial to effective extraction. Achieving optimum extraction relies on a number of systems working together effectively and none more so important than an efficient water injection system. Correctly designed, it can prolong the oil extraction and provide a constant flow of supply.
Any water injection system should be optimised to deliver the correct amount of water to the current set up and take into consideration any unforeseen eventualities that may occur due to the increase in production. A major factor to consider in the design of water injection systems is surge pressure, also known as 'Water Hammer'. The consequences of which can be as catastrophic and irreparable. A pressure surge can happen, but engineering your system to minimise the instances and impact should be a must.
Identifying surge cases
Surge analysis is not only pertinent to conceptual designs but also to expansion of existing systems. Surge analysis on water injection systems is performed to determine the maximum pressure surges that can occur as a result of transient events such as rapid valve closure during pipeline operation, or pump trip and restart operations.
The first step in any surge analysis is to identify credible cases where surge can arise. For example, closure of a single valve in a complex water injection network is unlikely to be an issue; however, closure of all wells while water injection pumps continue to operate (eg, due to malfunction of a facility’s shutdown system) is potentially a serious concern. As is the loss of LP (Low-Pressure) hydraulic fluid as a result of umbilical failure which occurs if all wells are closed at the drill centres. Of course there is also operator error to factor in that can result in mistaken valve closures.
Build a model
Only after the surge cases have been identified, can an accurate simulation model be built. Using a simulation tool like Flowmaster ensures all eventualities are considered. For instance both the Topsides and Subsea systems need to be incorporated to ensure confidence in any results obtained. It is critical that control systems (eg, minimum flow controllers), safety devices (eg, Topsides bursting discs), shutdown systems and elevation changes are included as they can have a significant impact on the results. For Subsea systems all the pipelines and wells should be included. Choke valves can be modelled to control the flow rate to each well and the injection wing or master valves slammed shut to create a surge event.

Simulating each potential surge case in detail is vital to reliable data for evaluation. It is especially important to determine the maximum allowable surge pressure that can be reached in each case.
Some important considerations for running a surge study:
* Start with a well tuned steady state model that accurately reflects the normal operation
* Use a pipe model that accurately models pressure wave propagation
* Choose an analysis time step to effectively capture the effect of the transient event
* Run the simulation long enough to investigate the initial event as well as any secondary events that could be generated by system interactions
When surge analysis is carried out at an early stage in the design of a water injection facility, simulation software can be used to predict the maximum surge pressure and this can then be used to select system design pressure. When surge analysis is carried out on an existing system simulation, it can be used to investigate measures to mitigate surge (e.g. extending valve closure time) to ensure that the system’s maximum allowable surge pressure is not breached. It is important that surge cases considered are realistic otherwise considerable expense may arise due to an overly-conservative design pressure on a new facility, or due to unnecessary modifications to an existing facility.
Mitigating Measures

Based on the evaluation it is essential to identify mitigating measures that can counteract the effects of any surge problems. These measures should in all cases either eliminate surge pressure entirely or reduce it to within acceptable limits. Flowmaster allows users to simulate possible mitigating solutions such as installing additional pressure transmitters, programming in trips if certain pressure points are reached or exceeded, or staggering the closure of wells in the event of a shutdown.
Thermo-fluid system simulation tools allow engineers to design, optimise, validate and troubleshoot Oil and Gas system designs. The wide range of solver capabilities and supplied loss and performance data enables users to build models and run simulations quicker and easier than ever before.
Within the Oil and Gas industry, the need to design safe, reliable and efficient systems across a range of applications is of paramount importance. Flowmaster provides system engineers with a powerful tool to investigate pressure surge, pressure drop, flow rate, temperature and system response times – removing the uncertainty from fluid flow systems.
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Doug Kolak is Process & Aerospace Product Engineer is Flowmaster Group, Towcester, Northants, UK.

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