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Waterflood monitoring with tracers: Difference between revisions

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Waterflood monitoring is one of the main aspects of Enhanced Oil Recovery as it needs quality and control requirements of injecting water and accurate chemical, physical analysis of fluids. Tracers help to assess the optimum flowing path of an existing pattern. Therefore, this monitoring program follows the approaches and methods described based upon the developed design solution and experience obtained over operation time.
Waterflood monitoring is one of the main aspects of Enhanced Oil Recovery as it needs quality and control requirements of injecting water and accurate chemical, physical analysis of fluids. Tracers help to assess the optimum flowing path of an existing pattern. Therefore, this monitoring program follows the approaches and methods described based upon the developed design solution and experience obtained over operation time.


== Injection Water Quality Requirements ==
==Injection water quality requirements==
The water quality required for an oil reservoir is primarily a function of reservoir permeability. Tight, low-permeability zones generally require better-quality water than higher-permeability zones.
The water quality required for an oil reservoir is primarily a function of reservoir permeability. Tight, low-permeability zones generally require better-quality water than higher-permeability zones.


When the injection water quality is inadequate, the consequence can be inferior a reservoir plugging. The result of plugging is reduced sweep efficiency, which results in decreased recovery and, ultimately, loss of revenue. Also, operational costs are increased because of workovers and system repairs required to restore injectivity.
When the injection water quality is inadequate, the consequence can be inferior a reservoir plugging. The result of plugging is reduced sweep efficiency, which results in decreased recovery and, ultimately, loss of revenue. Also, operational costs are increased because of workovers and system repairs required to restore injectivity.


Generally, the following analyses and calculations are required to determine Injection water quality:
*Injection water physical and chemical characteristics;
*Sulphate reducing bacteria (SRB) count;
*The suspended solids content and particle size distribution;
*Scaling tendencies;
*Injection and formation water compatibility (total produced formation water and produced formation water from different oil wells);
==Injection water quality control==
One of the most critical factors affecting the success of waterflood performance is the maintenance of water quality. In oilfield waterflooding, water quality is usually defined in terms of the plugging tendency of the water. Ideally, the water quality should be such that there is no reservoir plugging, and hence no loss of injectivity during the life of the flood.
The injection system must be protected against corrosion to preserve its physical integrity and prevent the generation of insoluble corrosion products.
Any insoluble material in water, either solid or liquid, can contribute to plugging. This includes formation solids (sand, silt, or clay), corrosion products, water-formed scales, bacterial growths and algae, oil (both crude and lubricating), and undissolved treating chemicals.
The contaminants primarily responsible for plugging fall into three categories:
#Present contaminates at the source. Some of the primary contaminants commonly present at the source water include:
#*in produced water - oil, corrosion products from the production system, bacteria, H2S, dissolved oxygen;
#*in groundwater supply wells - formation solids, corrosion products, bacteria;
#Generated contaminates within the injection system.  Contaminants generated within the system may include corrosion products, bacterial masses, biogenic hydrogen sulphide, and scale <ref>Generated contaminates within the injection system.  Contaminants generated within the system may include corrosion products, bacterial masses, biogenic hydrogen sulphide, and scale</ref>.
#Added contaminates to the injection system. Sometimes, intentionally added materials ultimately contribute to plugging. For example, contaminants such as dissolved oxygen, bacteria, suspended solids, and usually oil are the inevitable results of pumping trucked water or pit water into an injection system. Improperly selected corrosion inhibitors that are not sufficiently soluble in the injection water can contribute to plugging <sup>[1]</sup>.
Any water-injection operation's objective is to inject water into the reservoir rock without plugging or permeability reduction from particulates, dispersed oil, scale formation, bacterial growth, or clay swelling. Ideally, injection water should enter the reservoir free of suspended solids or oil. It should also be compatible with the reservoir rock and fluids and would be sterile and non-scaling. Besides, the souring of sweet reservoirs by sulphate-reducing bacteria should be prevented if possible <sup>[2]</sup>.
==Reservoir souring control==
An unwanted side effect of water injection can be reservoir souring, which refers to the generation of hydrogen sulphide, H<sub>2</sub>S. This usually occurs sometime after the breakthrough of the injected water at the producing wells <sup>[3]</sup>. Thus a reservoir that initially produces oil and gas with negligible concentrations of H<sub>2</sub>S can later produce fluids containing significant H<sub>2</sub>S concentrations. This situation results from water flooding should be distinguished from reservoirs that already contain substantial H<sub>2</sub>S when discovered. In many cases, the reservoir at the primary production stage is a hostile environment to microbial activities. As a result of water injection, a more favourable environment develops. This could be because of more suitable temperature distribution or availability of sulphate and nutrients.
Hydrogen sulphide, generated by SRB in the water phase, is partitioned into the other phases, moving in all present phases towards the production wells. Part of H2S remains in the residual oil and the free gas. It also can be consumed in the reservoir by reacting with iron minerals, such as siderite.


Generally, the following analyses and calculations are required to determine Injection water quality:
SRB are most likely to be found in stagnant or low-velocity areas, and beneath scales or sludges. Familiar places for bacterial activity in injection systems are tanks, filters and the rat hole in the injection and water source wells.
 
The main problems associated with H'''<sub>2</sub>'''S production are very aggressive corrosion of the metallic materials in the production and processing facilities and the generation of iron sulphide (black powder) that clogs filters and other equipment.
 
===Souring prevention===
Souring prevention strategies typically relate to the water cycle injection site: injection water quality control (limiting nutrient and bacteria introduction in the reservoir, biocide injection and nitrate injection) <sup>[3]</sup>. Otherwise, referred to as the water cycle's production side, souring migration strategies comprise practices like applying effective hydrogen sulphide scavengers and/or nitrite squeezes in production wells. It is necessary to conduct several analyses to detect the source of the problem.
 
*'''''Source well'''.'' Take water samples from source wells for SRB count and H<sub>2</sub>S content. If SRBs are not present, it means that H<sub>2</sub>S is not generated by microbiological activity.
*'''''Production wells.''''' Take water samples from production wells for SRB count and H<sub>2</sub>S content. Suppose SRBs are not present in the source water and are detected in water sampled from the production wells water. In that case, they originate from the reservoir, which means that cure should start before injection wells or inject appropriate chemicals into the production wells. Determine H<sub>2</sub>S content in the gas phase. Try to make a correlation between total production, water cut and H<sub>2</sub>S content in the gas phase.
 
==Tracer application==
Tracers material can be introduced into injection water to establish an inter-well flow pattern. Application of tracers entitles the introduction of reservoir compatible spaces into injection water and monitors its presence in the production fluid at the target producing wells throughout the field. Analysis of the resulting tracer concentration versus time curves from the individual producing wells enables inter-well flow characteristics to be determined so that improvements can be made to increase injection fluid sweep efficiency of the hydrocarbon reserve.
 
The information gained from the tracers study includes:
 
*Fault block & channel communication testing
*Exact determination of the source of produced water at all target production wells from the injection well
*First tracer breakthrough time
*Percentage of tracer and hence the percentage of injection water flowing from one injector to a specific production well
*Average tracer transit time from each injector to production wells allowing the injector to producer relationship volumetric sweep efficiency to be calculated
 
Tracer must possess properties that will ensure behavior as the bulk carrying medium - water. The deviation may generate misleading information. Therefore, the tracer must have the following properties:
 
*No reactivity with substances present in the reservoir and stable at reservoir conditions of temperature and pressure
*Will not undergo adsorption/absorption or exchange with the formation in the reservoir system.
*Produces a clear, unambiguous response during the analysis
 
The most proven reliable tracer for waterflooding studies is tritium in the form of tritiated water. This tracer is chemically identical to the medium that is being traced and would be expected to show similar reservoir characteristics. A further advantage in the use is that it poses relatively few radiological problems since it emits only low-energy Beta radiation. Considerable quantities of this tracer can be injected without the necessity to shield personnel from the radiation.
 
Besides tritium, fluorinated benzoic acids are strongly recommended. A number of these materials are stable to the harsh environments within oil reservoirs with negligible partitioning into the oil phase, resistant to both aerobic and anaerobic bacteria, and temperatures up to 150<sup>o</sup>C, as well as a range of reservoir types<sup>[4]</sup>.
 
Application of tracers in inter-well monitoring requires unequivocal identification of fluid breakthrough in some wells in a field. This requires the field to be simultaneously traced. In most cases, this can only be achieved by using one unique tracer species per well.
 
==Water sampling==
Sampling is the most important since unless the liquids tested are flowing through the equipment, and the results will be meaningless. The water sample must be representative of the water of interest. When water is being injected to a flowline from Pumping Station (PS) facilities, samples should be obtained at both ends of the line. Transfer lines and containers must be corrosion resistant and clean. The transfer should be with the line inserted to the bottom of the container with the flow at a low rate that prevents splashing or agitation that will aerate the water. Some waters tend to precipitate solids on the release of pressure, and where this may occur, acid can be added to the sample container before sampling.
 
Therefore, the following is recommended:
 
*Water samples shall be collected under the producing conditions of the source water.
*Samples shall be appropriately labelled so that the sample can be identified, and with all necessary data that will be stated in the report.
*Take care during the sample packing and shipping
*Samples for microbiological analyses – Sterile bottles or water samples are placed in the bottles with the appropriate culture medium.
*Samples for suspended solids analyses - can be taken: directly from the system through the membrane (the most accurate) or collected by flowing a water sample from the system into a clear plastic cylinder.
 
===Sampling Points===
The proposed ways to spot problems start at the water source and go entirely through the system to the injection wells, marking appropriate measurements at selected sample points. Sampling points are selected throughout the system to allow observation of any changes in the quality.
{| class="wikitable"
|'''''No'''''
|'''''Sample'''''
 
'''''Point'''''
|'''''Description'''''
|-
|1.
|'''#1'''
|Water at the wellhead of water source wells
|-
|2.
|'''#2'''
|Water at the PS inlet manifold
|-
|3.
|'''#3'''
|Inlet of the PS filtration unit
|-
|4.
|'''#4'''
|Outlet of the PS filtration unit
|-
|5.
|'''#5'''
|Outlet of the Process/Storage water tanks
|-
|6.
|'''#6'''
|PS Water Distribution Manifold
|-
|7.
|'''#7'''
|Injection Wells Site
|-
|8.
|'''#8'''
|Oil Treatment Plant (OTP) Produced Water at the outlet of degassing separators
|-
|9.
|'''#9'''
|Sour Water Tank
|-
|10.
|'''#10'''
|Water at the outlet of Produced Water Tank
|-
|11.
|'''#11'''
|OTP Water at the outlet of Coalescing Separator
|-
|12.
|'''#12'''
|Oil Producing Wells site
|}
Sample points are marked in the block diagram
[[File:Water Flooding System -Block Diagram.png|alt=Sample points are marked in the block diagram|frameless|626x626px]]


Injection water physical and chemical characteristics;
==Tracer monitoring – produced water sampling==
The tracer should be injected rapidly and safely. To monitor tracer progress, samples of produced water will be required to be taken on a routine basis. It is necessary to ensure that the samples are taken at recommended frequency and procedure. Sampling must be undertaken in a manner that eliminates the potential for cross-contamination of produced water samples between wells. It is necessary to rinse the sampling bottles with production fluids from the well to be sampled. If using a test separator, it is required to purge it with produced fluids from the well to be sampled for a period of time to purge fluids from other wells out from the test separator.


Sulphate reducing bacteria (SRB) count;
Sampling frequency from the target producing wells depends on conditions, and it can start once every two weeks. To reduce the analytical cost to a minimum, one in every four samples taken from the individual target well should be analyzed with the remaining samples stored. If the tracer is found, then all stored samples can be analyzed to obtain a detailed tracer production curve. Conversely, if no tracer is found, stored samples can be discarded.


The suspended solids content and particle size distribution;
As small quantities of tracer are injected, it is necessary to develop a sensitive analytical technique to measure low-level tracer species in the presence of naturally occurring radioactive isotopes.


Scaling tendencies;
==References==
<br />


Injection and formation water compatibility (total produced formation water and produced formation water from different oil wells);
<references responsive="0" />

Revision as of 01:18, 25 March 2021

Waterflood monitoring is one of the main aspects of Enhanced Oil Recovery as it needs quality and control requirements of injecting water and accurate chemical, physical analysis of fluids. Tracers help to assess the optimum flowing path of an existing pattern. Therefore, this monitoring program follows the approaches and methods described based upon the developed design solution and experience obtained over operation time.

Injection water quality requirements

The water quality required for an oil reservoir is primarily a function of reservoir permeability. Tight, low-permeability zones generally require better-quality water than higher-permeability zones.

When the injection water quality is inadequate, the consequence can be inferior a reservoir plugging. The result of plugging is reduced sweep efficiency, which results in decreased recovery and, ultimately, loss of revenue. Also, operational costs are increased because of workovers and system repairs required to restore injectivity.

Generally, the following analyses and calculations are required to determine Injection water quality:

  • Injection water physical and chemical characteristics;
  • Sulphate reducing bacteria (SRB) count;
  • The suspended solids content and particle size distribution;
  • Scaling tendencies;
  • Injection and formation water compatibility (total produced formation water and produced formation water from different oil wells);

Injection water quality control

One of the most critical factors affecting the success of waterflood performance is the maintenance of water quality. In oilfield waterflooding, water quality is usually defined in terms of the plugging tendency of the water. Ideally, the water quality should be such that there is no reservoir plugging, and hence no loss of injectivity during the life of the flood.

The injection system must be protected against corrosion to preserve its physical integrity and prevent the generation of insoluble corrosion products.

Any insoluble material in water, either solid or liquid, can contribute to plugging. This includes formation solids (sand, silt, or clay), corrosion products, water-formed scales, bacterial growths and algae, oil (both crude and lubricating), and undissolved treating chemicals.

The contaminants primarily responsible for plugging fall into three categories:

  1. Present contaminates at the source. Some of the primary contaminants commonly present at the source water include:
    • in produced water - oil, corrosion products from the production system, bacteria, H2S, dissolved oxygen;
    • in groundwater supply wells - formation solids, corrosion products, bacteria;
  2. Generated contaminates within the injection system.  Contaminants generated within the system may include corrosion products, bacterial masses, biogenic hydrogen sulphide, and scale [1].
  3. Added contaminates to the injection system. Sometimes, intentionally added materials ultimately contribute to plugging. For example, contaminants such as dissolved oxygen, bacteria, suspended solids, and usually oil are the inevitable results of pumping trucked water or pit water into an injection system. Improperly selected corrosion inhibitors that are not sufficiently soluble in the injection water can contribute to plugging [1].

Any water-injection operation's objective is to inject water into the reservoir rock without plugging or permeability reduction from particulates, dispersed oil, scale formation, bacterial growth, or clay swelling. Ideally, injection water should enter the reservoir free of suspended solids or oil. It should also be compatible with the reservoir rock and fluids and would be sterile and non-scaling. Besides, the souring of sweet reservoirs by sulphate-reducing bacteria should be prevented if possible [2].

Reservoir souring control

An unwanted side effect of water injection can be reservoir souring, which refers to the generation of hydrogen sulphide, H2S. This usually occurs sometime after the breakthrough of the injected water at the producing wells [3]. Thus a reservoir that initially produces oil and gas with negligible concentrations of H2S can later produce fluids containing significant H2S concentrations. This situation results from water flooding should be distinguished from reservoirs that already contain substantial H2S when discovered. In many cases, the reservoir at the primary production stage is a hostile environment to microbial activities. As a result of water injection, a more favourable environment develops. This could be because of more suitable temperature distribution or availability of sulphate and nutrients.

Hydrogen sulphide, generated by SRB in the water phase, is partitioned into the other phases, moving in all present phases towards the production wells. Part of H2S remains in the residual oil and the free gas. It also can be consumed in the reservoir by reacting with iron minerals, such as siderite.

SRB are most likely to be found in stagnant or low-velocity areas, and beneath scales or sludges. Familiar places for bacterial activity in injection systems are tanks, filters and the rat hole in the injection and water source wells.

The main problems associated with H2S production are very aggressive corrosion of the metallic materials in the production and processing facilities and the generation of iron sulphide (black powder) that clogs filters and other equipment.

Souring prevention

Souring prevention strategies typically relate to the water cycle injection site: injection water quality control (limiting nutrient and bacteria introduction in the reservoir, biocide injection and nitrate injection) [3]. Otherwise, referred to as the water cycle's production side, souring migration strategies comprise practices like applying effective hydrogen sulphide scavengers and/or nitrite squeezes in production wells. It is necessary to conduct several analyses to detect the source of the problem.

  • Source well. Take water samples from source wells for SRB count and H2S content. If SRBs are not present, it means that H2S is not generated by microbiological activity.
  • Production wells. Take water samples from production wells for SRB count and H2S content. Suppose SRBs are not present in the source water and are detected in water sampled from the production wells water. In that case, they originate from the reservoir, which means that cure should start before injection wells or inject appropriate chemicals into the production wells. Determine H2S content in the gas phase. Try to make a correlation between total production, water cut and H2S content in the gas phase.

Tracer application

Tracers material can be introduced into injection water to establish an inter-well flow pattern. Application of tracers entitles the introduction of reservoir compatible spaces into injection water and monitors its presence in the production fluid at the target producing wells throughout the field. Analysis of the resulting tracer concentration versus time curves from the individual producing wells enables inter-well flow characteristics to be determined so that improvements can be made to increase injection fluid sweep efficiency of the hydrocarbon reserve.

The information gained from the tracers study includes:

  • Fault block & channel communication testing
  • Exact determination of the source of produced water at all target production wells from the injection well
  • First tracer breakthrough time
  • Percentage of tracer and hence the percentage of injection water flowing from one injector to a specific production well
  • Average tracer transit time from each injector to production wells allowing the injector to producer relationship volumetric sweep efficiency to be calculated

Tracer must possess properties that will ensure behavior as the bulk carrying medium - water. The deviation may generate misleading information. Therefore, the tracer must have the following properties:

  • No reactivity with substances present in the reservoir and stable at reservoir conditions of temperature and pressure
  • Will not undergo adsorption/absorption or exchange with the formation in the reservoir system.
  • Produces a clear, unambiguous response during the analysis

The most proven reliable tracer for waterflooding studies is tritium in the form of tritiated water. This tracer is chemically identical to the medium that is being traced and would be expected to show similar reservoir characteristics. A further advantage in the use is that it poses relatively few radiological problems since it emits only low-energy Beta radiation. Considerable quantities of this tracer can be injected without the necessity to shield personnel from the radiation.

Besides tritium, fluorinated benzoic acids are strongly recommended. A number of these materials are stable to the harsh environments within oil reservoirs with negligible partitioning into the oil phase, resistant to both aerobic and anaerobic bacteria, and temperatures up to 150oC, as well as a range of reservoir types[4].

Application of tracers in inter-well monitoring requires unequivocal identification of fluid breakthrough in some wells in a field. This requires the field to be simultaneously traced. In most cases, this can only be achieved by using one unique tracer species per well.

Water sampling

Sampling is the most important since unless the liquids tested are flowing through the equipment, and the results will be meaningless. The water sample must be representative of the water of interest. When water is being injected to a flowline from Pumping Station (PS) facilities, samples should be obtained at both ends of the line. Transfer lines and containers must be corrosion resistant and clean. The transfer should be with the line inserted to the bottom of the container with the flow at a low rate that prevents splashing or agitation that will aerate the water. Some waters tend to precipitate solids on the release of pressure, and where this may occur, acid can be added to the sample container before sampling.

Therefore, the following is recommended:

  • Water samples shall be collected under the producing conditions of the source water.
  • Samples shall be appropriately labelled so that the sample can be identified, and with all necessary data that will be stated in the report.
  • Take care during the sample packing and shipping
  • Samples for microbiological analyses – Sterile bottles or water samples are placed in the bottles with the appropriate culture medium.
  • Samples for suspended solids analyses - can be taken: directly from the system through the membrane (the most accurate) or collected by flowing a water sample from the system into a clear plastic cylinder.

Sampling Points

The proposed ways to spot problems start at the water source and go entirely through the system to the injection wells, marking appropriate measurements at selected sample points. Sampling points are selected throughout the system to allow observation of any changes in the quality.

No Sample

Point

Description
1. #1 Water at the wellhead of water source wells
2. #2 Water at the PS inlet manifold
3. #3 Inlet of the PS filtration unit
4. #4 Outlet of the PS filtration unit
5. #5 Outlet of the Process/Storage water tanks
6. #6 PS Water Distribution Manifold
7. #7 Injection Wells Site
8. #8 Oil Treatment Plant (OTP) Produced Water at the outlet of degassing separators
9. #9 Sour Water Tank
10. #10 Water at the outlet of Produced Water Tank
11. #11 OTP Water at the outlet of Coalescing Separator
12. #12 Oil Producing Wells site

Sample points are marked in the block diagram Sample points are marked in the block diagram

Tracer monitoring – produced water sampling

The tracer should be injected rapidly and safely. To monitor tracer progress, samples of produced water will be required to be taken on a routine basis. It is necessary to ensure that the samples are taken at recommended frequency and procedure. Sampling must be undertaken in a manner that eliminates the potential for cross-contamination of produced water samples between wells. It is necessary to rinse the sampling bottles with production fluids from the well to be sampled. If using a test separator, it is required to purge it with produced fluids from the well to be sampled for a period of time to purge fluids from other wells out from the test separator.

Sampling frequency from the target producing wells depends on conditions, and it can start once every two weeks. To reduce the analytical cost to a minimum, one in every four samples taken from the individual target well should be analyzed with the remaining samples stored. If the tracer is found, then all stored samples can be analyzed to obtain a detailed tracer production curve. Conversely, if no tracer is found, stored samples can be discarded.

As small quantities of tracer are injected, it is necessary to develop a sensitive analytical technique to measure low-level tracer species in the presence of naturally occurring radioactive isotopes.

References


  1. Generated contaminates within the injection system.  Contaminants generated within the system may include corrosion products, bacterial masses, biogenic hydrogen sulphide, and scale