Natural gas fields are not usually at high risk for bacterial-activity related problems like minimum inhibitory concentration (MIC) and reservoir souring. But the situation when developing unconventional gas reservoirs is significantly different.

Unconventional gas reservoirs are produced by designing fracturing programs that allow the gas contained within very low permeability shale and sands to be produced. This process of fracturing generally uses large volumes of water with typical volumes around 10,000 to 50,000 barrels per well. Additionally, unconventional gas fields require large numbers of wells, as many as 1,000 or more.

The water used for unconventional gas fields comes from freshwater supply wells, chlorinated city water, rainwater, pond water and lake water. Occasionally, operators re-use flow back water from a previous frac job. Each water source contains some bacterial contamination and bacterial growth can be initiated, thus introducing corrosion risks.

Guar gum, polyacrylamide or other organic components are present in frac fluids and can serve as food sources for bacteria. Bacterial control is required to avoid souring of the reservoir, as well as to prevent the problems associated with MIC.

Case study

In 2001, Shell Oil operated acreage in the Pinedale field in Wyoming. At the start of the field, concern existed that improper treatment of the fluids used for the fracs would result in bacterial contamination and eventually in MIC and reservoir souring.

Following the review by a chemical company, the treatment program was adjusted to biocide treatment of the frac fluids, identification of bacterial contamination of producing wells (and if contaminated, treatment with biocide) and treatment of a water disposal facility with biocide.

By 2009, total costs to control the bacterial contamination were above $2 million and the risk of MIC in the Pinedale field was re-evaluated and identified as medium or medium-high. So far there was no MIC problem experienced in the Pinedale wells and surface facilities confirmed a low risk for MIC.

Consequently, the bacteria treatment program was optimized. The biocide treatment program for frac water was stopped, and only a bleach treatment program for frac water was continued. Also the bleach treatment of the frac water at central process facility was continued.

To ensure the new treatment regime would not result in future problems, a study was performed to review the new treatment strategy. The project started in July 2009 and was closed in April 2010. The study was divided in two parts. First, understand current situation and perform risk assessment. Second, formulate possible solutions and implement in field. The study helped better understanding of the threat of bacteria and MIC, also improved the corrosion-leak-spill mitigation program for all key business processes, including drilling, completion, surface production, water disposal and water recycle. The direct business impact is to improve the effectiveness of bacteria-MIC control and optimize the use of the annual OPEX on bacteria control. The long-term business impact is to comply with governmental regulations related to health, safety and environmental.

Pinedale process

Pinedale facilities design began simple and, during further development, the same philosophy was followed. Initially each well had its own separators, condensate and water tanks, gas dehydration and gas sales meter. More recently, wells have been combined into bulk-test facilities to simplify the installations and reduce the capital and operating costs.

Condensate is sold by the stock tank and is trucked to the sales organization. The majority of the produced water is treated at a liquid gathering system (LGS) and central liquid processing facility (CLPF). The current capacity of the facility is 30,000 barrels of water per day through five disposal wells.

The flowback water is treated via a dedicated treatment unit also at the CLPF. The early well pads may have one well and corresponding production unit. The more recent well pads can have up to 20 wells and one to two bulk production units or bulk production units plus single units. The gas from this field has low CO2 and negligible H2S.

The Pinedale water processing systems includes well pad facilities, wells, manifold, well testing, three-phase separation, gas dehydration, selected produced water trucked for frac jobs, liquid gathering lines, condensate and water together and small diameter spoolable fiberglass reinforced pipe (FRP).

The feeds include produced water from well pad through LGS, produced water from well pad through truck and trucked-in flowback water to flowback water treatment unit at CLPF.

The processing includes inlet separation, metering, flash gas sales, stabilization, water treating, storage and frac flowback treating and disposal

The products include water export from CLPF to disposal wells and clean water redistributed for frac jobs.

Bacteria activity

Considerable bacteria monitoring has been done by the chemical vendor on a variety of water samples throughout the production and water disposal system, including water considered for re-use in fracturing and water disposal.

Major observations from these studies illustrated that bacteria was growing through the treating facilities, a small amount of H2S was detected confirming the SRB activity and FeS was detected throughout the surface production facilities,

Although bacteria counts using serial dilution techniques in the Pinedale facilities have been high with values for SRB as high as 1010, there are no reported MIC cases in the tubing. In 2009, several Pinedale wells had tubing integrity problems, but the corrosion mechanism was found to be most likely CO2 corrosion. Solids removed from the tubing indicated no bacteria activity and the inorganic composition were found to be most probably iron carbonate, iron oxide-hydroxide, manganese oxide and small to trace amounts of silica and aluminum.

The experience with MIC in the surface facilities is different as there are numerous examples where MIC is

experienced, but it is always observed in combination with other mechanisms like CO2 or under deposit corrosion.

Although severe MIC problems do not exist, it is obvious that problems do exists with biofilm development and, especially in areas with stagnant flow, the risk of MIC attack is high. An example of the risk of MIC corrosion in stagnant water flows is the water tanks on the individual well pads. Water is collected in these water tanks and when full a truck is emptying the tank via the water off-take line. Once empty, a small amount of water remains in the load off line and this water is highly active in bacteria and susceptible for MIC attack. At the moment this type of MIC is fully under control.

Production chemicals

Several different chemicals are used in frac fluids. The Pinedale wells are fractured in stages and in the upper part different fluids are used than deeper in the well. The upper stages are treated with slick water (SW) containing a linear gel to assist laminar flow while keeping the proppant in solution. In the lower parts of the well, friction reducers are used, which are similar gels cross-linked using borate. The gel used in both stages is a guar gum, which is known to be an excellent food source for APB5.

Another parameter related to the bacteria activity in fluids is pH. For the linear gel used in SW, the pH is 6 to 8, but for cross-linked gels (deeper zone), the initial fluid pH when pumped is 9 to 11.5. After the fluid is flown back after it broke to some degree, the pH is more likely 8 to 10. The flown back fluid pH also depends on how long the fluid has stayed at bottom hole static temperature (BHST). The longer it was at BHST, the more broken the fluid and the lower the pH.

The intermixing of frac fluid with produced water will likely lower the pH of the flow back fluid as well. Ammonium persulphate is a strong oxidant and when broken down sulfate is formed, which may serve as an indirect source of sulfate. There are no known cases of any SRB activity with persulphate.

The other chemical additives are not directly affecting the bacteria developments, although some of them (methanol especially) are known to be a carbon food source for bacteria. VFA are not expected to be present in the frac fluids. In addition to biocides, several other process chemicals are being applied in the surface facilities: demulsifier, foamer, corrosion inhibitor, oxygen scavenger. Only the oxygen scavenger may result in additional SRB activity as once reacted with oxygen, sulphate is being formed. It was confirmed that over injection of this chemical was minimal.

Study conclusion

The performed study identified several areas of concern regarding the bacteria control in the Pinedale gas production facilities. The control of bacteria is not only required to limit MIC and reservoir souring, but also to limit bio-fouling directly impacting the separated water quality either re-used for new frac job or disposal.

Several locations in both subsurface and surface have been identified, each with a unique treatment and monitoring strategy. SRB and APB have been identified as bad bacteria. Unfortunately, the current available data does not allow making any further description about the species of SRB or APB present in Pinedale resulting in (potential) problems.

It is also possible that there are other bacteria families present which are also bad bacteria. More detailed analysis is currently ongoing and, once the detailed analyses are available, this information may result in a change in the routine monitoring program for bacteria.

Based on the Shell risk assessment results, the bacteria treatment and monitoring strategy and actions should be imbedded into the business plan of different business units supporting the Pinedale assets, including the operation team, project team and well team.

Cor Kuijvenhoven is with Shell International Department and Hongwei Wang is with Shell Exploration and Production Co. This abbreviated paper is reproduced with permission from NACE International, Houston, Texas. All rights reserved.