Implications of Produced Water Re-injection on Reservoir Souring

Abstract

Implications of Produced Water Re-injection on Reservoir Souring

Produced water re-injection (PWRI) has been widely practised, worldwide, for many decades, especially at on-shore oilfields in temperature climates where simply and cheaply discharging it to sea is not possible. In the Middle East, in general, little produced water has been re-injected compared to the more common practices of disposal injection (to a shallow, very high permeability, non-oil bearing aquifer or ‘lost circulation zone’), or evaporation in ponds.

Because of generally increasing produced water flowrates, and increasing environmental protection requirements, PWRI is being implemented on a wider scale in many countries, at a time when some major international oil companies are actually retracting from their earlier commitments to do this, ironically also on the grounds of environmental protection.

The key arguments for PWRI are:

1. Avoiding the pollution of seas (especially enclosed ones), rivers, aquifers and land from toxic chemicals and salts in produced water.
2. Productive use of a scarce resource – water.

The key arguments against PWRI are:

1. Increased power requirements for PWRI, compared to other cleaner and cooler waters, with consequent increased CO2 emissions.
2. Increased amount (mass and concentration) of H2S in produced fluids, by bacterial action (sulphate-reducing bacteria, SRB, convert sulphate to sulphide), and hence increased capital and operating expenditures to prevent or remove the H2S.

It is the latter which is the subject of discussion in this paper, explaining why sometimes PWRI may lead to increased reservoir souring, but not always.