Prevention of Oil-field Souring

The key to avoiding oil-field souring is to prevent the contamination of the field by sulfate reducing bacteria (SRB’s). These anaerobic bacteria grow on organic compounds found in water contaminated with hydrocarbons and convert sulfate into hydrogen sulfide (rotten egg gas). The presence of hydrogen sulfide not only reduces the commerical value of natural gas, but also rapidily corrodes pipes, tanks and other iron and steel structures.
The approach we have taked to reduce this process is to reduce the presence of SRB’s in produced water ponds, frac storage ponds and to completely eliminate SRB’s from any water that goes back into the formation. We test and treat all water at workover rigs, fracing operations and injection wells.
The following is the abstract and introduction from a paper we published on this subject. You can find a complete copy of the paper at the following reference.

Tischler, A., Woodworth, T.R., Burton, S.D., and Richards, R.D. 2010. Controlling Bacteria in Recycled Production Water for Completion and Workover Operations. SPE Prod & Oper 25 (2): 232-240. SPE-123450-PA. doi: 10.2118/123450-PA.

Copyright 2009, Society of Petroleum Engineers

This paper was prepared for presentation at the 2009 SPE Rocky Mountain Petroleum Technology Conference held in Denver, Colorado, USA, 14–16 April 2009.
This paper was selected for presentation by an SPE program committee following review of information contained in an abstract submitted by the author(s). Contents of the paper have not been reviewed by the Society of Petroleum Engineers and are subject to correction by the author(s). The material does not necessarily reflect any position of the Society of Petroleum Engineers, its officers, or members. Electronic reproduction, distribution, or storage of any part of this paper without the written consent of the Society of Petroleum Engineers is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of SPE copyright.

Abstract

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 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 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 (certain types of emulsion like consistency).
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, 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 applied was chosen and discuss future improvements and testing.

Introduction

The Piceance Basin is located in western part of Colorado. It is classified as an elongated structural depression trending northwest – southeast. The basin is more than 100 miles long and has an average width of over 60 miles, encompassing an area of approximately 7,110 square miles. The Piceance structural basin runs through portions of Moffat, Rio Blanco, Garfield, Mesa, Pitkin, Delta, Gunnison, and Montrose counties (Figure 1).
The basin has come to increasing public attention in recent years because of widespread drilling to extract natural gas. The primary target of gas development has been the Williams Fork Formation of the Mesaverde Group, of Cretaceous age. The Williams Fork is a several-thousand-foot thick section of shale, sandstone and coal deposited in a coastal plain environment. The formation has long been known to contain natural gas. However, the sandstone reservoirs have low permeability and limited areal extent, which made gas wells uneconomic in the past. Advances in hydraulic fracturing technology within the past decade, along with higher gas prices, have made gas wells broadly economic in the area.
Williams is operating approximately 2,600 wells between Parachute and Rifle in the Piceance Basin. Operations are performed with efficiency rigs and simultaneous completions drilling and completing around 500 wells every year. Simultaneous Operations (SIMOPS) enables hydraulically fracturing wells from remote frac locations situated up to two miles away from the actual wellhead. Four to six wells are completed in the same completion group. Up to six hydraulic fracture treatments are performed per day, averaging about 100,000 gal of sand laden fluid per stage for approximately 25,000 bbl of water per day.
The water for hydraulic fracturing operations is stored on site in pits of about 80,000 bbl capacity. These pits are initially filled with produced water coming from a central waste facility where all the produced water is collected and goes through a treatment process to remove solids and hydrocarbons.
The frac treatment itself is a so called slick water frac type – meaning that the frac fluid is a linear frac fluid consisting of water, friction reducer, and a surfactant.
After the initial round of fracs the pit is filled from flow back water from the wells – the average recovery rate in two days is approximately 50% of the initial frac treatment. The remaining volume is made up if necessary from the central waste facility by truck. Reusing flow back water cuts down on trucking costs to fill the frac pit, reduces traffic to minimize H&S challenges as well as reducing the impact on the environment.
15,000 bbl of water are produced every day and are processed through two E&P waste facilities. Reutilization of water also provides a cushion in order not to overwhelm the waste facilities.
Reutilization of flowback and produced water for workover and completion operations poses the additional problems of introducing aerobic and anaerobic bacteria into the wellbore and formation. Introduction of anaerobic bacteria like sulfate reducing bacteria can cause localized sour gas (H2S) production and in the long term can start souring of the reservoir. Other anaerobic bacteria include AFB’s that are the main source for bacteria related corrosion challenges.
Most aerobic bacteria can grow under aerobic, microaerobic, and anaerobic environments. However, growth is slower in the absence of oxygen. Many anaerobic bacteria can also survive under aerobic conditions and resume growth when the aerobic bacteria have consumed the available oxygen.
The flowback and produced water is normally a microaerobic or anaerobic with < 2 ppm of oxygen (O2). During pumping and mixing operations, enough O2 is introduced to change the environment to an aerobic environment.
In order to prevent bacteria from multiplying and introducing live bacteria into the wellbore and formation (both aerobic and anaerobic), several different types of bactericidal and bacterial treatments have been investigated.

The following methods will be discussed in more detail showing the pros and cons of each system:

  • Aeration
  • Chlorination (hypochlorites, chlorine dioxide)
  • Other biocides (quaternary ammonium compounds, aldehydes)

Web design by Ontra Studios