Page 457 - Fundamentals of Enhanced Oil and Gas Recovery

Microbial Enhanced Oil Recovery: Microbiology and Fundamentals 445

all around the world the data of which have been collected by National Institute for
Petroleum Energy Research (NIPER), the average and maximum depth have been
about 550 m and 800 m, respectively [856].

10.12.11 Well Spacing
It is proved that the microorganisms can grow and travel in porous media as they are
observed in wells nearby the injector well [857]. Thus, the injection well spacing is of
great importance in MEOR success. This is important because the microorganisms con-
sume all the nutrients as they move forward [23,407]. The time the microorganisms spend
within the microbial incubation zone, referred to as the residence time, should be longer
than the time needed by a bioproduct to reach to the desired concentration [407].Incases
with relatively small spacing, in order to reach the desired concentration of bioproducts,
the metabolic rate should be higher or higher concentrations of microorganisms and
nutrients should be employed. Sheng [24] proposed 40 ac for well spacing for MEOR.

10.12.12 Residual Oil Saturation
After extensive waterflooding as the secondary oil recovery, still a significant volume of
oil remains in the reservoir, which is called residual oil [6]. For applying MEOR, the
residual oil saturation should be high enough to justify the project economy. Sheng [24]
suggested residual oil saturation greater than 0.25 is suitable for MEOR processes.

10.12.13 Metals
Heavy metals can act as very toxic materials to the microorganisms in levels highly
more than what is required for nutrition, which is commonly in range of
10 23 10 24 M [25]. Generally, heavy metals’ concentration more than 10 23 M can
become toxic to many microorganisms, while high concentration of light metals
cations may induce inhibition or stimulation [25,858]. Parameters such as pH, tem-
perature, pressure, and salinity can influence the solubilization of metals; thus deter-
mining of metal toxicity in suit is complicated [25]. For example, Bubela [859]

reported that increasing temperature from 53 to 63 C makes the copper more toxic
to Bacillus stearothermophilus, which is a thermophile. Heavy metals such as copper,
zinc, ferric iron, etc. may be present at concentrations higher than the required level.
Hitzman [860] mentioned the detrimental effects of heavy metals such as arsenic and
lead on the microorganisms’ growth and the fact that their concentration should not
be in excessive quantities in the oil or the formation to be treated. It should be men-
tioned that organisms are influenced in different ways and there are some organisms
that can tolerate very high concentrations of almost any heavy metal [861]. Sheng
[24] mentioned the suitable concentration of arsenic and mercury less than 15 mg/L.