Microbially Induced Formation Damage in Oilfield Reservoirs
Authors: Wood Jonathan M. , Spark Iain S.C. 
Published: December 2007

Abstract
Formation damage in terms of physio-chemical reactions such as asphaltene deposition, scale precipitation and fines migration for example, is relatively well understood. In contrast to this the effects of indigenous and introduced microbes and their role in oilfield formation damage is less well understood. Oilfield microbes, especially the S.R.B. (sulphate reducing bacteria) have been identified from many oilfields world wide (indigenous) and have been isolated from drilling mud (particularly water based muds) and other down hole chemicals and injection water (introduced microbes). The combination of the various microbial populations often results in the precipitation of insoluble metal sulphides, biopolymer and/or hydrogen sulphide production, with loss in production or injection rates. By utilising bottle tests performed at a range of temperatures up to the actual oilfield reservoir temperature, it is possible to model the microbial formation damage mechanisms using the thermal gradients present in the field. Using this thermal model it is possible to predict where the various microbial formation damage mechanisms, such as hydrogen sulphide production, biopolymer and cell pore blockage, occur in the field area and to predict from the injection water model when the various damage mechanisms will occur in the field. This novel approach using thermal models coupled with microbial bottle and flood tests, and geological interpretative techniques, is the initial step in predicting microbial formation damage in oilfield reservoirs.

Introduction
Formation damage in terms of physio-chemical reactions such as asphaltene deposition, scale precipitation and fines migration for example is relatively well understood. In contrast to this the effects of indigenous and introduced microbes and their role in oilfield formation damage is less well understood. The presence of microbes in oilfield reservoirs is widely accepted, and have been isolated from oilfield reservoirs world wide (1 & 2). These microbes while often native to the reservoirs can also be introduced as a result of drilling operations and/or water injection (3). The indigenous microbial reservoir populations can be activated by water injection and well chemical squeeze programs resulting in both near wellbore and deep reservoir damage due to microbial/polymer plugging (4) and insoluble metal sulphide precipitation (5). Introduced bacteria from drilling muds, clean up chemicals and completion chemicals can also cause microbial formation damage, which can result in loss of well productivity or injection (6). The production of hydrogen sulphide gas by sulphate reducing bacteria (S.R.B.) both in well and in the reservoir, can cause significant problems due to enhanced corrosion of topside and down hole tubulars, metal sulphide precipitation, health and safety hazards, and increases the refining cost of the oil (7).
Temperature is one of the major controls on the growth of bacteria and their by-products. To evaluate the microbial formation damage mechanisms the incubation of preserved core samples from an oilfield reservoir, live drilling mud and injection seawater, in various growth media at selected temperatures was performed. The live reservoir mud and injection seawater was inoculated into growth media at 30°C, 60°C and 90°C. The inoculated core samples, drilling mud and injection seawater subcultures were analysed using dry scanning electron microscopy (S.E.M.) to observe the presence of bacteria, biopolymer and insoluble metal sulphides.