Optimum Water

8/13/2019 Optimum Water

1/6

OPTIMISING STANDARD AND PRODUCED WATER INJECTION

SYSTEMS

Extracted and adapted by Bob Eden of CAPCIS Ltd. from the forthcoming IndustryGuidelines: Disposal of Produced Water by Injection

BACKGROUNDIn 1996, the Society of Petroleum Engineers established a Task Force Group toexamine Produced Water Injection (PWI) issues. The paper presented todaycomprises edited extracts from the Disposal of Produced Water by Injectiondocument prepared by the Task Force Group. Earlier this year the full document waspresented to the E + P Forum and to OSPARCOM authorities as an E + P IndustryGuideline for produced water disposal operations. Publication of the full document isexpected next year.

1. INTRODUCTION

The production of hydrocarbons is usually associated with the generation of aproduced water stream. The ever increasing volumes of produced water, which now

constitutes the largest single fluids stream in Exploration & Production operations,warrant a structural and integral approach towards its management. The single mostimportant aspect of produced water disposal is its fate and effect in the receivingenvironment. Prior to the produced water disposal, the quality should be upgradedthrough treatment to the locally required standards. It is therefore incumbent upon theindustry that its produced water management practices should be of a standardaccepted by the host government and the public at large.

There are many methods of produced water disposal of which injection is an option.Confinement of the injected produced water within the identified strata is critical to theenvironmental acceptability of the disposal process.

The purpose of the injection programme must be defined. In general, the injection ofproduced water is carried out under one of two scenarios:

Waterflooding operation. The purpose here is to inject the produced water into theoil-bearing layers or pressure-supporting aquifer of the reservoir to sweep the oilout of the pore space and into the production wells

Disposal operation. The objective of this process is to dispose of the producedwater in an environmentally safe manner within an underground formation wherelong term confinement is assured.

In both cases prediction and control of the water movement into the matrix isparamount. This approach both ensures economic recovery and minimises thechance of break-out of produced water to surface or potable water sources. Oneaspect crucial to this success is control of the injection water quality.

2. PRODUCED WATER QUALITY & TREATMENT

2.1 PRODUCED WATER QUALITYIn assessing the water quality requirements in a PWI scheme, the concentration andparticle size distribution of dispersed hydrocarbons and suspended solids are themost crucial characteristics covering both water treatment and water injectivity.However, produced water streams typically contain a wide range of contaminants.The relative importance of these contaminants is strongly dependent upon the

receiving environment, particularly the differential between the produced water to bere-injected and the receiving environment. Notwithstanding, in line with the industry's


2/6

2

commitment to identification, management and minimisation of all waste streams, allthe constituents of the produced water stream(s) should be identified and accountedfor, specifically:

Dispersed hydrocarbons

Dissolved hydrocarbons Dissolved inorganic chemicals (e.g. toxic heavy metals)

The presence of naturally occurring radioactive materials (NORM)

Dissolved treating (production) chemicals

Suspended solids

Water salinity

Note that produced waters for disposal/re-injection may also include secondaryaqueous waste streams (e.g. crude desalting wash waters, miscellaneous drainagewaters, etc.) and marine waters only suspended solids and marine life.

2. 2 QUALITY CHARACTERISATION

In assessing the differential between the injected water and the receiving environment,the potential problems arising therefrom and the resulting impact on the PWI schemedesign, the following data requirements need to be considered.

Priority measured parameters. The target areas for consideration include theassessment of scaling potential, rock/ fluid and fluid/fluid compatibility, injectivityimpairment potential and the potential for inducing and/or sustaining bacterialactivity. The following data set might be typically collected:

3Major cations (Na+, K+, NH4+, Ca2+, Mg2+, Ba2+, Sr2+, Fe2+)3Major anions (Cl-, Br-, SO4

2-, HCO3 -, CO32-, BO2

-, NO3-, OH-, PO4

3-)3

Fatty acids (formic, acetic, propionic, butyric, valeric)3Dissolved gases (CO2, H2S, O2)3Temperature, pH3Free and dissolved hydrocarbons3Total and oil-free suspended solids3Particle size distribution3Filterability or injectivity (membrane and/or coreflood tests)

Parameters for environmental risk considerations . The data set below would notnormally be collected for sub-surface injection purposes but may need to beconsidered if discharge to a surface environment constitutes a fall-back option inthe event of PWI scheme failure(s):

3Aromatic compounds3Aliphatic compounds (mainly dispersed oil)3Naphtalenes3Poly-aromatic hydrocarbons3Phenols3Polar organics (mainly organic acids)3Heavy metals3Production chemicals3Toxicity (e.g. Microtox)3BOD, COD

Additional parameters. To address issues related to facilities design, collection ofany or all of the following data may be required depending upon the objectives of the


3/6

3

PWI programme. Target areas for consideration include treatment processselection, corrosion management and future injection well interventions (e.g.stimulation):

3 Specific gravity of produced water3 Specific gravity of produced hydrocarbon fluids3 Total dissolved solids3 Water corrosivity and conductivity3 Rheological properties of separable (from produced water) oil3 Microscopic analysis of separable solids3 Chemical analysis of separable solids3 Water treatability characteristics (e.g. settling, flotation, hydrocyclones, etc.)

2. 3 TREATMENT AND INJECTION FACILITIESGenerally, produced water may be injected with minimal treatment when injectionunder fracturing conditions or into (thermally) fractured formations. Occasionally (e.g.under conditions of matrix injection) the quality of the produced water may require

modifications prior to its injection, and so consideration should be given to thetreatment and injection facilities. A step by step approach to facilities design is givenbelow. Note that conventional and raw seawater injection scheme designs follow asimilar path to PWI facilities rationale, feeding into their equivalent of box 2:

1. Identify the source(s) andmagnitude of the producedwater stream

3. Identity the quality requirementsfor subsurface disposal., taking intoaccount regulatory constrains

5. Identify upstream processes which

could complicate the water treatmentand what steps can be taken to makethe water easier to treat

7. Select suitable water treatment andinjection equipment considering droplet/particle size and other constraints.\

9. Review design to ensure schemeis optimised and integrated into theoverall production process scheme

8. Identify the methods for the

treatment or disposal of secondary

waste streams

6. Select the number of watertreatment stages required to achievethe required water quality

4. Select a suitable water

treatment system

2. Identify the concentrationand nature of contaminants inthe produced water


4/6

4

3. PREDICTION TECHNOLOGY

Successful PWI operations achieve economic disposal or re-use of produced waterfor pressure maintenance without causing negative consequences to the undergroundenvironments. Such success can be achieved by ensuring the confinement of theinjected water within acceptable disposal / injection zones and away from anyunderground source of usable water for drinking or irrigation. The certainty with whichre-injected produced water will be contained within the receiving environment is ofparamount importance in preserving the injection option for the disposal and/or re-useof produced water in reservoir waterflood operations. This section will address therecommended efforts needed to ensure that the operator of any PWI scheme hasavailable the necessary information that provides the basis for predicting the outcomeof the injection operations.

Predictive models:

must be verified and validated;

shall be appropriate for the injection site;

shall be appropriate for the mode of operation; shall have been calibrated for other existing sites where sufficient data are available

or shall have been (are) used extensively by the industry to perform similarpredictions.

3. 1 INJECTION SCHEME PREDICTIONThe requirement for adequate prediction of the fate of the injected produced water andhow it impacts upon the subsurface strata is tied closely to the objectives of the PWIscheme. This is especially important as most PWI operations require fracturing of theinjection zone(s) to maintain long term adequate injectivity and thus reduce the needfor multiple injectors. The fracturing process will facilitate the use of higher injectionrates. However, the conditions under which confinement of the injected producedwater and the created fracture within the intended horizon can be achieved must bedefined. In this context we define here (see figure below):

Confinement as the process by which the produced water is kept within specifiedhorizons.

The injected water may be allowed to enter into a rock layer above or below theinjection horizon, i.e. containment layers (which are not directly accessible from thewell bore). The water, however, would not be allowed to get out of the containmentlayers.

The two layers immediately surrounding the containment layers are known asconfinement zones. The confinement zone is an impermeable rock layer into whichfracture propagation is not allowed. Hence the purpose of the confinement layers isto prevent the encroachment of the produced water or fracture into non-permittedareas of the subsurface strata.

Analysis and prediction of fracture confinement and the evaluation of the area ofreview are critical to safe and environmentally acceptable PWI schemes. Hence, thenecessary prediction capability, as will be discussed later, requires specificinformation, data and tools to ensure confinement with a high degree of certainty. Theoperator, therefore, must strive to obtain an adequate definition of the injection sitespresent status and operating history, including locations of current and abandoned

wells.


5/6

5

3. 2 RESERVOIR AND FRACTURE PROPAGATION PREDICTIONThere are three aspects to the prediction modeling of produced water re-injection:

Flow (reservoir) simulation Injection well fracturing and fracture propagation

Area of review and the zone of endangerment influence

The first two establish the whereabouts of the injected produced water and provide theoperator with an integrated knowledge sufficient to manage the injection process in anenvironmentally safe manner. The third aspect ensures that the injected stream willnot intersect any open conduits (i.e. old wells, penetrations or future wells) that maycause upward fluid movements.

In order to provide a high level of confidence of the modelled predictions, modelcalibration, verification and sensitivity of the results must be demonstrated. Thehistorical or previous performance of any model applied, or alternatively wide industryuse, may be considered adequate measures of the credibility of model predictions.Notwithstanding, it is highly recommended that the following tasks be carried out aspart of the prediction effort:

1. Input data uncertainty and model sensitivity analysis1.1 Define input parameters uncertainties1.2 Assess sensitivity of predictions to data uncertainties

2. Modelling Results2.1 Provide predictions during the injection operational period

2.2 Provide an assessment of changes during post-operational period

The last item (2.2) may be important if the injection process leaves the formation withhigher pore pressure than existed prior to the start of PWI as a result of e.g. injectioninto shales or low permeability zones. It is also pertinent where injection takes place inareas that may encounter potential/future drilling operations.

Definition and prediction with a high degree of certainty of the extent of a fracture,especially during disposal operations, is critical. Fracture geometry simulatorsspecifically aimed at predicting the vertical and areal extent of the fracture as well asits geometry (height, length and width) must provide complete representation of thephenomena involved. This requirement has its roots in the observation that care must

be taken to ensure that both the fracture and the injected fluid are contained within theallowed zones. The disposal process must be terminated by the operator before the

CONFINEMENT LAYER

CONTAINMENT LAYER

CONFINEMENT LAYER

CONTAINMENT LAYER

INJECTION LAYER


6/6

6

predicted fracture propagation becomes detrimental to the vertical conformance or theareal sweep.

Most fracture propagation models that have been traditionally used in the prediction ofPWI have been developed as modifications of simulators that are intended todesign/predict hydraulic fracture geometry for given injection and formationparameters during well stimulation operations. More recently, however, severalsimulators have been developed with the specific application to water injection,disposal and waterflooding. The use of any of these or other verified and calibratedmodels will provide adequate prediction of the geometry and extent of the createdfracture during the PWI process.

3. CONCLUSIONS

The above extracts, though specifically addressing PWI, form a framework uponwhich other injection water sources guidelines can be built, including onshore surfacewater and marine rawwater injection. Crucial to the success of any injection strategyensuring targeted flow is the collaboration of the facilities design department and the

fracture modelling group. Water need only be conditioned sufficiently for a safeinjection operation and no more. By ensuring that the two groups itterate water qualityneeds with injectivity performance to correctly size the treatment plant not only areenergy and costs saved, the all-important environmental safety record stays clean.

Acknowledgements

The author gratefully acknowledges his fellow authors, the major contributors, of theGUIDELINES: To an operator in evaluating, designing and managing a safe andenvironmentally responsible produced water injection (PWI) operation, namely Ahmed

Abou-Sayed (BP Exploration), Rob Eylander (Shell-NAM), Ghassam Gheissary (Shell),Leo Henriquez (Dutch State Supervision of Mines), Laurence Murray (BP Exploration),Hans de Pater (Technical University, Delft), Richard Paige (BP Exploration), IanPhillips (Halliburton), Keith Robinson (Oil Plus Ltd.), and Kre Salte (Statoil).

Documents

Optimum Water