Controlling Bacteria in Recycled Production Water for Completion and Workover Operations

Reusing flowback water and produced water from active wells becomes more and more important in today's oil and gas operations to control surface water volumes in order to keep surface water disposal costs (reinjection or trucking off the premises) to a minimum – especially in operations that do not require secondary lift support. However, reusing untreated produced and flowback water untreated in workover and completion operations can promote bacteria growth both above and below ground.

Water produced from oil and gas wells is a perfect environment for sulfate reducing bacteria (SRB) and acid forming bacteria (AFB) due to its anaerobic nature (< 2ppm O₂ content) and high nutrients content (organics, free iron, etc.). Reuse of water introduces enough oxygen through regular pumping operations to allow aerobic bacteria to grow – mostly slime forming bacteria (SFB). The oxygen content is high enough for aerobic bacteria to grow but too low to kill anaerobic bacteria. The oxygen content will cause the anaerobic bacteria to stay in a biostatic state which does not kill them but prevents them from multiplying.

As soon as the bacteria find an environment that is conducive to their growth, they will become active again and start multiplying. The anaerobic environment in the formation is ideal for growth of bacteria like SRBs and AFBs. The aerobic environment of the wellbore is conducive for SFBs. The growth of SRBs will not only lead to Health and Safety (H&S) concerns due to increased sour gas or hydrogen sulfide (H₂S) production but also to a slow souring of the formation. This also increase operation expenses due to added corrosion (H₂S pitting, stress cracking etc) in surface and subsurface tubulars and related prevention expenses. Other challenges in production can be related to AFBs (pitting) and SFBs (emulsion like materials may form).

Various different methods can be applied to prevent bacteria growth and reduce operational expenses related to corrosion prevention, remediation of corrosion effects, and remediation of emulsion like produced fluids. This paper will take a closer look at the methods of aeration, chlorine based applications (sodium hypochlorite, calcium hypochlorite, and chlorine dioxide), and biocide application.

The different methods will be compared through laboratory tests, actual field application, and a rating system. The rating system incorporates environmental, health, and safety (EH&S) concerns, operational application/considerations, effectiveness of the method, and cost.

Each method will be discussed and the pros and cons presented. The pros and cons will be supported by laboratory and field data. The conclusion portion of the paper will give reasoning on why and how the current method of application was chosen and discuss future improvements and testing.