RAWWATER: Sour pointers: Understanding the impact of microbiological oilfield reservoir souring

By Matt Streets, Head of Division and Senior Research Scientist at Rawwater – the UK-based specialists in problematic microbiology.

Compared to sweet crude, sour crude oil is more expensive to refine, presents a significant corrosion risk to topsides and refinery equipment, incurs higher chemical treatment costs – and is often associated with foul-smelling hydrogen sulfide gas (H₂S).

Whereas crude oil from some producing regions of the world is naturally sour, the process of secondary recovery – introducing water, under pressure, into oil reservoirs through injection wells, to ensure a steady flow of crude oil at the producer – can frequently result in previously sweet oilfield reservoirs turning sour.

A MULTI-MILLION-DOLLAR PROBLEM

The phenomenon is known as microbiological oilfield reservoir souring. Unchecked it can lead to many millions of dollars being spent annually on souring control, especially where topsides facilities were originally set up for sweet – non-corrosive – oil service. That said, when forecasted early, microbially induced souring can be dealt with far more cost effectively and efficiently. In some circumstances, forecasting can even indicate that downhole conditions will not enable souring to reach problematic levels.

As the global leader in microbiological oilfield reservoir souring forecasting and simulation services, Rawwater helps oil majors and service companies globally to plan ahead and, as a result, save vast sums on the cost of souring control. However, before we look into the technologies used by Rawwater to forecast and simulate microbiological reservoir souring, let’s learn a little more about its causes.

As already mentioned, additional downhole pressure is needed to extract oil from a reservoir during secondary recovery. This is normally achieved by ‘water-flooding’ – introducing water (typically sulfate-rich seawater, or sulfate-containing produced water) into the reservoir under pressure. The injected water will, however, introduce sulfate-reducing microorganisms (SRM) – also called sulfate-reducing bacteria (SRB) ­– into the reservoir environment. It can also encourage the growth of any microbial lifeforms that are already present in the subsurface. Put simply, the naturally occurring microbial life that causes crude oil to sour, consumes the sulfate that is present in both the injection water and the oilfield reservoir.

THE CONDITIONS NECESSARY FOR SOURING TO OCCUR

To flourish, sulfate-reducing microorganisms require water, sulfate (that can be chemically reduced by them and converted to sulfide), an oxygen-free environment, and a carbon source for energy. This is normally the volatile fatty acids and dissolved components of the native crude oil. If the reservoir environment has a pH of between 4 to 9, a temperature range of 10°C to 80°C and a pressure range from 1 psig to 8,000 psig, sulfate-reducing microorganisms will invariably thrive.

Left unchecked, microbiological oilfield reservoir souring can remain unnoticed until higher levels of H₂S are detected in crude oil production. This is because it may take several years of secondary recovery operations before H₂S levels are significant enough to be observed. Inevitably, treatment at this stage is costly and involves substantial chemical dosing and even vastly expensive sulfate removal facilities.

WORLD-CLASS TECHNOLOGIES

To assist operators in understanding whether or not their oilfields will sour – and to ensure any risk of souring is identified early enough to allow for economical treatment – Rawwater uses a range of industry leading technologies. These include DynamicTVS^© (TVS = Thermal Viability Shell) – its advanced predictive desktop modelling tool, and its purpose-built pressurised bioreactor facilities.

FORECASTING FUTURE LEVELS OF H₂S

Using operational, planning and survey data from all stages of oil production, DynamicTVS^© offers a highly cost-effective means of forecasting future levels of H₂S within an oilfield reservoir. The software can indicate if reservoir conditions will support microbiological souring and resultant H₂S production – and to what extent. A recent upgrade to the modelling tool now means that Rawwater is also able to provide a good indication of future souring in new discoveries, at the earliest stages of field planning and development, and long before production commences.

ADVANCED PRESSURISED BIOREACTOR FACILITY

To confirm any risk of souring sufficiently early for cost-effective treatment, Rawwater carries out extreme environment simulation testing in high-pressure bioreactor studies. The company has built and operates what is widely considered to be the world’s largest, most advanced laboratory facility to study and evaluate microbiological souring in simulated reservoir conditions. As many as 85 pressurised bioreactor columns, ranging from 25 cm to 4 metres in length are in operation at any one time at Rawwater’s UK-based laboratories. Specially designed corrosion-resistant columns that are filled with sand, seawater and native crude oil, and inoculated with oilfield bacteria, Rawwater’s pressurised bioreactors are used to simulate the pressure and temperature (P/T) conditions that are found in water-flooded oil reservoirs, in order to establish whether or not the microbial life that causes souring can thrive or even survive in such conditions.

PRICELESS OILFIELD SOURING DATASET

Testing pressures range from atmospheric pressure to 12,000 psig, with testing temperatures ranging from 5°C to just below the boiling point of water. To date, Rawwater has accumulated the equivalent of almost 600 bioreactor years’ worth of oilfield souring data. A priceless dataset that is updated on an almost daily basis, it provides oil majors and service companies globally with an unrivalled insight into the microbiology of their onshore, offshore and subsea assets – and the opportunity to act in a timely manner should a risk of future, problematic souring be highlighted.

INVESTIGATING THE MICROBIOLOGY OF CARBON CAPTURE ENVIRONMENTS FOR NET ZERO 2050

Of course problematic microbiology isn’t exclusive to oil production in the energy sector. The scientists at Rawwater are acutely aware that the same microbiological lifeforms that can cause crude oil to sour could also impact the successful long-term underground storage of powerful greenhouse gas, carbon dioxide (CO₂). With that in mind, and in collaboration with the Manchester Institute of Biotechnology at The University of Manchester, Rawwater is establishing a pioneering Carbon Capture Storage R&D Hub at its UK HQ.  The project will benefit from Rawwater’s extensive understanding of oilfield reservoir souring and use a specially developed suite of pressurised bioreactors to model CO₂ injection and storage into depleted oil & gas reservoirs and saline aquifers. Knowledge gained from the Hub’s activities will assist the UK in becoming a global leader in understanding the microbiology of carbon capture storage environments and help expedite the UK government’s Net Zero 2050 strategy.

Rawwater offers cutting-edge expertise in the study of problematic microbiology across a range of industries, in addition to the forecasting and simulation of microbiological oilfield reservoir souring. To find out more about Rawwater’s services and capabilities, email info@rawwater.com or call + 44 (0) 1925 768 910.