Produced Water Treatment

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<ul><li><p>POLPetroleum Open Learning</p><p>OPITO</p><p>THE OIL &amp; GAS ACADEMY</p><p>Produced WaterTreatment</p><p>Part of thePetroleum Processing Technology Series</p></li><li><p>Petroleum Open Learning</p><p>Designed, Produced and Published by OPITO Ltd., Petroleum Open Learning, Minerva House, Bruntland Road, Portlethen, Aberdeen AB12 4QL</p><p>Printed by Astute Print &amp; Design, 44-46 Brechin Road, Forfar, Angus DD8 3JX</p><p> OPITO 1993 (rev.2002) ISBN 1 872041 85 X</p><p>All rights reserved. No part of this publication may be reproduced, stored in a retrieval or information storage system, transmitted in any form or by any means, mechanical, photocopying, recording or otherwise without the prior permission in writing of the publishers.</p></li><li><p>Petroleum Open Learning</p><p>Visual Cues training targets for you to </p><p>achieve by the end of the unit </p><p> test yourself questions to see how much you understand </p><p> check yourself answers to let you see if you have been thinking along the right lines </p><p> activities for you to apply your new knowledge </p><p> summaries for you to recap on the major steps in your progress</p><p>Produced Water Treatment(Part of the Petroleum Processing Technology Series)</p><p>Petroleum Open Learning</p><p>Contents Page* Training Targets 4</p><p>* Introduction 5</p><p>* Section 1 - The Problems Associated with Produced Water 6 TheMechanicsofWaterProduction CorrosionProblems ScaleProblems TransportationProblems DisposalProblems</p><p>* Section 2 - The Basics of Produced Water Treatment 15 PrimarySeparation GravitySeparation Coalescence ShortDistanceGravitySeparation GasFlotation CentrifugalForceSeparation ChemicalTreatment</p><p>1</p></li><li><p>Petroleum Open Learning</p></li><li><p>Petroleum Open Learning</p><p>Visual Cues training targets for you to </p><p>achieve by the end of the unit </p><p> test yourself questions to see how much you understand </p><p> check yourself answers to let you see if you have been thinking along the right lines </p><p> activities for you to apply your new knowledge </p><p> summaries for you to recap on the major steps in your progress</p><p>Produced Water Treatment(Part of the Petroleum Processing Technology Series)</p><p>Petroleum Open Learning</p><p>Contents (contd) Page</p><p>* Section 3 - Produced Water Cleaning Equipment 23 APISeparators PlateInterceptors(orSeparators) Oil/WaterFiltersCoalescers GasFlotationUnits Hydrocyclones UseofChemicalAdditives</p><p>* Section 4 - A Typical Produced Water System 36 TiltingPlateSeparators TheFlotationUnit ChemicalDosingPackage ProducedWaterCaisson</p><p>* Test Yourself - Answers 47</p><p>3</p></li><li><p>Petroleum Open Learning</p><p>Training Targets</p><p>When you have completed this unit on Produced Water Treatment you will be able to :</p><p> List the sources of produced water </p><p> Describe the mechanics of water production </p><p> Explain what problems can arise from the production of water </p><p> Explain the basic principles which govern the separation of oil from produced water </p><p> Describe the construction and operation of 5 types of oily water clean up facility </p><p> Explain the requirement for chemical injection in a produced water treatment system </p><p> Describe the flow of water and separated oil through a typical produced water treatment facility </p><p>Tick the box when you have met each target.</p><p>4</p></li><li><p>Petroleum Open Learning</p><p>In the vast majority of oil fields, water productionbecomes a problem as the field gets older. Towardsthe end of their useful lives some oil wells may beproducing 95% of their total liquid as water. Thisproduced water may be extremely salty and likely tobe of little value to the operator. It is removed fromthe oil stream during primary separation and byother facilities, and has then to be disposed of.However, we are talking of a great deal of water insome cases. How do we dispose of it, and where dowe put it?</p><p>Offshore, the obvious place would be into the sea.Dumping this produced water directly fromseparators into the sea, would however, soon havethe operator in trouble with the authorities. Evenafter initial separation the water still contains oil insmall amounts. Serious environmental pollutionwould build up if oil contaminated water were to bedumped directly to the sea.</p><p>Onshore, disposal wells may have to be drilled, intowhich the produced water can be injected fordisposal. This also may have its problems. Oil inthe water, or fine solids, could plug the injectionwells in a very short time.</p><p>So the water which is produced with, and separatedfrom, the oil in an oilfield must be cleaned prior todisposal. This is what this unit is all about. In the</p><p>unit, we will be looking at the produced waterhandling system of an oil production facility. Beforewe examine a typical system, however, I think weshould look at where the water comes from and theproblems it poses in a little more detail. So, I havesplit the unit into four sections as follows:</p><p> In Section 1 we will look at the sources of produced water and the problems which may be encountered if we fail to treat it.</p><p> Section 2 will cover the basic principles involved in the treatment and clean-up of produced water.</p><p> In Section 3 we will examine the construction and operation of produced water clean-up equipment.</p><p> Finally, in Section 4, I will take you through a typical produced water handling facility which may be found on an offshore production platform.</p><p>Although produced water treatment applies to bothonshore and offshore locations, I will beconcentrating on the offshore situation in this unit.However, most of what I have to say would apply toboth.</p><p>Produced Water Treatment Introduction</p><p>5</p></li><li><p>Petroleum Open Learning</p><p>Water is present in some form, in most oil reservoirsbefore any production takes place.</p><p>There are however, many different types ofreservoir. In one very common one, the oilaccumulates above large volumes of water, which isusually salty. This water is what remains of ancientseas from an earlier period of Earths history.</p><p>This body of water is called an aquifer, and thereservoir is known as a water drive reservoir.</p><p>In addition, a considerable amount of water may alsobe found as small droplets distributed throughout theoil (and gas) in a reservoir. For reservoirengineering purposes this water is called connatewater or interstitial water. We will just call itformation water.</p><p>During production, further injection water may be pumped into the reservoir to assist in pressure maintenance.</p><p>Any of these types of water may eventually find theirway into the oil wells and be produced to the surfacealong with the oil. It is all then called producedwater.</p><p>Before we look at the problems which can be causedby this produced water, let us first consider how thewater gets into the producing wells, </p><p>The Mechanics of Water ProductionLook at Figure 1 which shows a cross section through a typical water drive reservoir.</p><p>Produced Water Treatment Section 1 - The Problems Associated with Produced Water</p><p>6</p></li><li><p>Petroleum Open Learning</p><p>PERMEABILITYPermeability is a measure of the ability of a fluid to flow through the rock from one pore to another. In order for it to be able to do this, the pores must be interconnected.</p><p>Permeability is measured in darcys- named after a French engineer who studied the flow of liquids through filters. He found that the flow increased in proportion to the pressure increase. However he also discovered that the flow was affected by the thickness, or viscosity, of the fluid.</p><p>Generally there is a wide spread of permeability in reservoir rocks.</p><p>So, the rock properties of porosity and permeabilityallow the oil to flow towards the producing wells. Butwhat causes the oil to flow through the reservoir?</p><p>Lets look at that now.</p><p>You are probably aware that fluids always flow fromareas of high pressure to areas of low pressure:</p><p> The oil producing wells create areas of low pressure in the surrounding reservoir rock as the well is opened at the surface and oil flows into the well</p><p> The aquifer is usually at a relatively high pressure. In addition, the injection of water into the aquifer is intended to maintain the reservoir pressure</p><p>You can see that the oil lies above the aquifer and thewell is taking oil which is not contaminated with water.The point at which the oil and water touch each other is called the oil water contact.</p><p>The oil is able to flow through the reservoir rocktowards the well because the rock is porous andpermeable. These are probably the two most important properties of reservoir rocks.</p><p>POROSITYPorostiy is the property of the rock which enables it to hold fluids within itself. The oil, gas and water are contained in tiny holes in the rock called pores.</p><p>Sandstone is a common reservoir rock. It is made up of grains of sand which are cemented together at the points where they touch. Between the sand grains are void spaces - the pores.</p><p>The ratio of the volume of the pores to total rock volume expressed as a percentage is the rocks porosity.</p><p>This means that, if you have a sandstone reservoir with a porosity of 25%, for every 4m3 of reservoir rock, 1m3 consists of holes and 3m3 solid sand grains.</p><p>Another common reservoir rock is limestone. This is a rather brittle rock which contains lots of tiny cracks and fissures. These tiny cracks give the limestone its porosity.</p><p>The high pressure water in the aquifer,therefore, will tend to displace the oiltowards the low pressure areassurrounding each well bore.</p><p>Figure 2 on the next page, shows thesituation with just one producing welland one water injection well.</p><p>7</p></li><li><p>Petroleum Open Learning</p><p>If this situation remained constant there would be noproblem. However the situation does not remainconstant. Think about this and try to answer thefollowing Test Yourself question.</p><p>Test Yourself 1 As oil is removed from the reservoir what will happen to the position of the oil water contact ?</p><p> You will find the answer to Test Yourself 1 on Page 47</p><p>8</p></li><li><p>Petroleum Open Learning</p><p>Over a period of time, as the oil water contactapproaches the well intakes, the water will start toflow preferentially to the oil wells. This occursbecause the water is much less viscous than the oiland therefore flows more easily through the rock,bypassing the oil.</p><p>The water is said to finger through the oil.</p><p>Figure 3 shows water starting to follow thesepreferential routes through the reservoir rock.</p><p>9</p></li><li><p>Petroleum Open Learning</p><p>Once water starts to break through to theproducing wells, it tends to be produced in ever increasing amounts.</p><p>The ratio of water produced to total production iscalled the water cut and is expressed as apercentage. To make sure you understand this,have a go at the following Test Yourself question.</p><p>Test Yourself 2a) If a well produces 3975m3/d of oil and 795m3/d of water, what is the water cut.</p><p>b) A well produces a total of 875m3/d liquids and the water cut is 20%. What is the oil production from this well.</p><p>c) What is the water cut of a well if the total production is 556m3/d., and the oil production is 397m3/d</p><p>You will find the answers to Test Yourself 2 on Page 47</p><p>The water cut from a particular well or field dependson a large number of factors. These include:</p><p> The geology and porosity of the reservoir rock</p><p> The size and, particularly, the vertical thickness of the reservoir</p><p> The degree of fracturing of the oil field</p><p> The position and depth of the producing wells in relation to the oil water contact</p><p> How long the reservoir has been producing oil</p><p>Actual water cuts vary tremendously, of course, butcan be as much as 99%. Imagine a field whichproduces a total of 15,900m3 of liquid per day with awater cut of 60 %. This means that 9540m3 of waterare produced every day.</p><p>This can pose significant operating problems andthese are what we will look at now.</p><p>10</p></li><li><p>Petroleum Open Learning</p><p>Corrosion ProblemsWe said earlier that the aquifer water can be verysalty. Injection water, being in the main sea water, issalty as well. It follows then that the produced waterwill be salty also. In fact the saltiness, or salinity,of produced water is usually considerably more thanthat of normal sea water. To put it in perspective:</p><p> fresh water from streams, lakes etc. usually contains less than 0.2 % salt</p><p> sea water has an average salt content of 3.5%</p><p> produced water can contain up to 15% salt</p><p>Pure water in itself is not particularly corrosive.However, up to a point, the more saline it becomesthe more corrosive it is.</p><p>If the produced water is allowed to pass through allthe surface processing equipment to the oiltransportation system, it could cause considerablecorrosion damage to pipes, vessels and otherequipment.</p><p>In fact, corrosion costs the petroleum industrymillions of pounds annually. It makes sense to try toreduce this expense.</p><p>One of the ways of reducing corrosion damage is toseparate the water from the oil at the earliestopportunity and dispose of it. In fact this, togetherwith the separation of gas, is one of the firstprocesses in a production facility. This howevergives rise to another problem - one of disposal. Wewill look at this shortly.</p><p>Scale ProblemsSalts are initially dissolved in the water present in areservoir. As conditions change when this water isproduced, the salts may be precipitated as solidsand deposit as scale.</p><p>This can reduce pipe diameters, plug vessels andequipment which in turn can lead to lost production.Once again, removal and disposal of produced watercan help prevent the problems of scaling.</p><p>Transportation ProblemsThe produced oil may have to be transported from anoffshore location to a shore based refinery or tankerterminal. There are two ways of doing this. If the fieldis large and the economics justify it, the best way is bypipeline to shore. However, some fields are too smallto justify the expense of a pipeline or are too far fromshore. In this case the oil is loaded into a tanker atthe point of production via a tanker loading facility.Either way, water in the oil to be transported cancause problems:</p><p> The obvious one we have looked at already, that of corrosion. Salt water in pipelines or tanker loading units can corrode facilities rapidly. I dont think I need to elaborate on that at this time.</p><p> If the oil is going down a pipeline, excess water reduces the efficiency of the line, leaving less space for oil.</p><p> Water being sent to a refinery with the oil can cause serious upsets in the distillation process. Refinery operators usually limit the amount of salt and water which they will accept.</p><p> When loading oil to a tanker there are laid down limits of water in oil which it is permitted to take. If more than, say, 0.5 % of the cargo loaded is water, then the producing company can face severe penalties.</p><p>11</p></li><li><p>Petroleum Open Learning</p><p>It would seem from the foregoing that one of thefirst things which we must do on a production facilityis get rid of the water. This is of course whathappens. On most installations water is separatedfrom the produced oil in the first process system.The separated water has no value and has to bedisposed of. But how? This brings us to our finalproblem, that of produced water disposal.</p><p>Disposal ProblemsWhen trying to decide how to get rid of the water wemust consider first of all the location of theproduction facili...</p></li></ul>


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