A Field Study of Microbiological Growth and Reservoir Souring

The souring of normally sweet production systems is a significant problem which can have implications to continued oilfield operations. Such problems are commonly approached by gathering of field sample and laboratory analysis or by simple test kits. This series in Hot Topics describes an alternative approach which includes the use of specialized field sampling and analysis procedures and portable equipment that can be moved from site to site. The current article provides an overview and introduction to how MIC growth can be studied and evaluated.

The souring of normally sweet production systems is a significant problem. It can have implications in terms of

1. reduced quality of produced hydrocarbons relative,
2. the reduced productivity of wells,
3. increased corrosivity of produced fluids.

In cases where remedial action is not taken, it can also have implications relative to the potential for sulfide stress cracking and selection of materials for downhole, flowlines and surface facilities. Therefore, it is important to be able to properly characterize field situations and make accurate recommendations for remedial actions to minimize the impact of souring and to prevent the occurrence of similar occurrences in other related field operations.

In an assessment of reservoir souring, field analysis provides an important technical basis for engineering decision making through their supporting scientific evidence. Such tests can yield information useful in determining the sources and the potential severity of souring and the selection and confirmation of successful remedial actions. Some of the significant questions that can be addressed through field investigation are indicated below:

• Is the souring a direct consequence of bacterial action?
• Were the bacteria introduced by the water injection system?
• Are the bacteria naturally occurring in the reservoir?
• Can the bacteria survive and grow under reservoir conditions?
• How adaptable are they to varying field conditions?
• Are there sufficient nutrients to sustain bacterial growth in the system?
• What is the potential for controlling MIC and bacterial growth?

Field souring is often studied by either of two approaches:

1. field sampling followed by transportation of the samples to the laboratory for analysis, or
2. analysis directly in the field using simplified field test kits.

In many cases, the field sampling/laboratory approach is difficult due to the remote nature of many onshore and offshore production and injection facilities and the inability to transport, maintain and analyze cultures. Additionally, most simple field tests lack the sensitivity required to properly assess, characterize and differentiate marginal cases.

Issues discussed in this series include:

• Description of MIC systems and methods of field investigation
• Specialized procedures and equipment for field or in-plant investigation of souring, scaling, corrosion, fouling and bacterial action in water handling systems. This includes a survey of planktonic micro-organisms and bio-films as well as measurements of total dissolved solids.
• A case study to validate this field testing capability
• Analysis of data obtained for characterization of reservoir souring in petroleum production operations

Studies of the amount and nature of accumulated biofilms on metal surfaces were also performed using the portable apparatus shown in Figure 1. The use of this equipment is shown in greater detail in Figure 2. It consists of a tube that contains ten metal coupons, each with an exposed surface area of 2 cm2. The system was sampled at various locations and the water flowed through the annulus of the biofilm cell around the coupons. The flow velocities through the cell were controlled in the range of 0.5 to 1.5 m/s.

FIGURE 1 - Portable Microbiological Survey Instruments

FIGURE 2 - Method for Sessile Bacteria Studies