Water efficiency

Site-wide implications of water disposal and recycling for thermal in-situ oil sands facilities

AER issued Directive 081 “Water Disposal Limits and Reporting Requirements for Thermal In Situ Oil Sands Schemes” in 2012. The regulation sets water disposal limits and reporting requirements. The disposal limits are defined to recycle produced water efficiently and ensure that make-up water is effectively used. The Directive’s intent is to promote “Efficient water treatment, recycle, and disposal at thermal operations”.

The objective of this article is to evaluate how the integration of new technologies as well as changes in operating conditions in a SAGD CPF impact boiler feedwater quality and compliance with regulations (AER Directive 081). This work was performed using InsituSIM v 2.1.1. InsituSIM is a cloud-based simulation software that implements an efficient methodology to perform techno-economic analyses of process options in a SAGD central processing facility.

For this evaluation, the Base case template available in InsituSIM was used. In the base case, produced water is treated by Warm Lime Softening (WLS) and Ion Exchange before being sent to steam generation (OTSG). The following table shows the parameters used in the base case.


Parameter Name
Unit
Value
Oil Production
bpd
33,000
SOR

3
Blowdown Recycle Rate
%
30
Retention Rate
%
10
Blowdown flashing pressure
kPa
1,500
OTSG Steam quality
%
77
Make up water TDS
ppmw
17,147
Produced water TDS
ppmw
2,870


Directive 81 regulates how much water can be disposed of in a SAGD CPF. Actual disposal and disposal limit of a SAGD CPF process are calculated as per the following equations:


Actual disposal = Total disposal / (Fresh water + Brackish water + Produced water)

Disposal limit=  (Fresh water × 0.03 + Brackish water × 0.35 + Produced water × 0.1) / (Fresh water + Brackish water + Produced water)

Brackish water is defined as water that has Total Dissolved Solids (TDS) exceeding 4,000 mg/L.

The following parameters were evaluated for their impact on boiler feedwater (BFW) quality and D81 compliance:

  • Reservoir retention rate
  • Blowdown recycle rate (BDRR)
  • Make-up water quality

The following chart shows the impact of retention rate on D81 performance.

Here are few takeaways from the results in this chart:

  • Disposal limit increases with increasing retention rate due to increased makeup water use (brackish).
  • High negative retention rates lead to non-compliance with D81 and hence blowdown recycle rate needs to be increased.
  • High retention rate leads to more makeup water use which leads to poor BFW quality, hence a lower BDRR might be required.

The following charts show the impact of Blowdown recycle rate on the Directive 81 performance and BFW quality.


There is a negative retention rate threshold at which increasing blowdown recycle rate would not improve D81 performance. In these cases, the produced water is more than enough to satisfy the steam demand. In these cases, lowering blowdown flashing pressure could be an option to reduce the disposal volume.

If fresh water is used as make-up water, the disposal limit is more stringent and higher blowdown recycle rates are required to meet D81 disposal.

This article demonstrates impact of various parameters on Directive 81 compliance and illustrates how InsituSIM enables quick and accurate evaluation of scenarios for various SAGD CPF configurations.

References:


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