Case Study: Ultrafiltration

The evaluation process used to determine the best solution for the Client’s water filtration needs focused on reservoir parameters and reservoir protection.  The chosen solution was designed to ensure all potentially damaging components were removed from the injection stream and that the water was of suitable quality to provide the enhanced recovery sought by the Client.

A review of historical water analysis reports, fluid compatibility reports, operational data and forward looking fluid production data was completed.

Currently 2000m³/d source water is produced from an aquifer and injected into the shale oil formations to provide pressure support. Produced water is separated at the oil battery, processed and then sent to a disposal well for deep well injection. The field is producing 32° API oil at 95% water cut. The produced emulsion is sour with 0.5% Hydrogen Sulphide in solution.

Source water is treated with biocide and then passed through bag filters prior to field distribution and injection. Injection water bag filters are changed when the filter vessels reach a predetermined differential pressure.

The reservoirs were characterised as having a permeability range from < 1mD up to 3mD at 15% porosity. Using the Flow Zone method (Amaefule and Altunbay) Pi-Cubed was able to ascertain a bulk average pore throat diameter of approximately 1 µm and from that, determine that the minimum size particle required to be removed from the injection path in order to prevent bridging to be 0.12 µm diameter.

Figure 1: Reservoir Data

The client wished to increase water injection capacity beyond the current capability of the aquifer supply to 2500 m³/d water for injection while at the same time reducing or eliminating costs associated with produced water disposal. In order to implement this efficiently, water blending of produced and source water was considered.

The main concerns with water injection in tight reservoir applications are bridging, well bore plugging, precipitation due to water blending and scaling components.  Analysis of the water sample data and predictive scale modelling suggested that many of these concerns could be addressed using ceramic membrane ultrafiltration.

Ultrafiltration was specified with membrane pores sized for 0.1µm absolute filtration to remove entrained oil and grease, the majority of bacterial contaminants, solids of a diameter greater than 0.1 µm and a portion of the salt precipitates present in the fluid stream.

A sample of produced water from the Client’s skim tank was sent to Pi-Cubed’s test facility for processing through a laboratory scale ceramic membrane filtration unit.

Water was processed through a 0.1 µm ceramic membrane, in order to have a sufficient volume for concentrate sampling a 75%-25% permeate-concentrate ratio was achieved.  During field operations this ratio would be 98%-2%.  Results and interpretation of the data are below.

The following data is a compilation of the results from the laboratory analysis of the samples taken during the fluid filtration test performed using a 0.1µm ceramic membrane filtration unit.

Figure 3: Laboratory Data

It can be seen from the data that pH, Conductivity, Resistivity and Total Dissolved Solids remained approximately constant when comparing Source water and Permeate (clean water) as should be expected. The small variations are considered to be within the standard error of the measurement method.

Total Suspended Solids (TSS) were reduced by 59%. It is believed that residual solids are significantly smaller than the minimum measurement window of the analyser used. This is borne out by the fact that the analyser was unable to detect any particulate in the Permeate sample and so no data was collected. Most importantly Oil and Grease content was reduced by 98%, bacterial (APT) count by 99.7% and turbidity (optical measurement of clarity of the water sample) was reduced by 80% resulting in a turbidity count of 8.3 NTU only slightly higher than that considered normal for drinking water. (Drinking water typically classified as between 0.5 NTU and 5.0 NTU.)

The results of this test are consistent with previous produced water filtration tests and field trials.

Ceramic membrane ultrafiltration was proven during laboratory testing to effectively process the Client’s produced water for the removal of the majority of oil and grease, bacteria, TSS and salt precipitates.  The permeate water met all client specifications for injection in the tight oil reservoir for Enhanced Oil Recovery.

Upon completion of the data analysis, results were presented to the Client’s multidisciplinary team.  The recommendation from the team to management was that field scale on-stream processing should be completed to confirm technology scalability in this application.  Once favourable results from this test are received, the goal is to then install a 2,500 m3/d ceramic membrane ultrafiltration unit to process injection water for enhanced oil recovery in this tight oil reservoir.