本系列文章分为两部分，第一部分为墨西哥湾深水平台采出水系统的系统设计和运行提供了指导。文章重点介绍了六个要素：流体特性、化学处理、设备、工艺配置、操作和流出物质量。流入采出水的特性以及目标流出物质量决定了水处理的挑战。

由于深水环境中空间和重量成本高昂，水处理必须具有高强度，并且必须高度集成。高强度是指设备能够处理高体积流速、短停留时间，并且占用空间小。高度集成系统的概念是指将水处理设备、工艺配置和化学处理集成到一个高性能单一系统中的系统工艺设计，该系统可以去除大部分污染物。

充分利用一切机会，确保采出水得到高标准的处理。调峰、界面排污和作为澄清器的断流罐只是改善采出水质量的几个机会。

第 1 部分包括采出水特性和化学处理。第 2 部分将涵盖设备、工艺配置、操作和流出物质量。它还将包括与第 1 部分和第 2 部分相关的结论部分。

介绍

在墨西哥湾 (GOM) 深水作业中，分离系统的设计旨在最大程度地减少重量和空间，同时去除大量污染物。典型的油水分离系统设计包括一到两个阶段的两相气液分离，然后是三相分离器（如自由水分离器）。然后根据监管要求对来自三相分离器的采出水进行处理，以便排放到船外。在这种生产情况下，由于节流阀和阀门之间的压力下降很大，油滴和水滴经历了显著的剪切。此外，这种生产情况限制了在设计阶段对减少液滴剪切和放置水处理设备产生任何有意义的影响的能力。

本文讨论的采出水处理系统适用于高强度深水处理。目前，有许多经验法则和临时方法指导海上采出水系统的设计和管理。

然而，对于许多设计决策，该行业缺乏系统和方法。采出水系统的设计往往基于当地经验，以及导致之前设计成功的参数（Walsh 2019）。这种方法的问题在于，附近的系统可能具有流体特性，需要采用截然不同的设计方法。

塑造深水设计创新的六个关键主题

从概念上讲，设计项目或操作优化的组织可以如图 1所示。其中涉及六个要素或主题。流体特性是第一个主题，它定义了采出水的挑战。

在进入的生产水和排出的废水之间有四个元素，它们为水处理系统提供了设计细节（图 1）。这些元素有助于将特定给水转化为非常干净的所需废水质量。这些元素就像拼图一样紧密贴合，形成完整而连贯的设计。

本文包括采出水特性和化学处理。第 2 部分将包括设备设计和操作、工艺配置、操作和流出采出水质量。

每个主题都以简短的描述进行定义和讨论，然后会对整个系统进行更详细的解释。

需要注意的是，在设计阶段通常无法获得采出水样本和特性数据。因此，也无法获得分散油、悬浮固体浓度和颗粒大小分布数据。

为了进行表征，需要采集采出水样本。通常在采出水被分成含油水流和湿油时进行采样。湿油通常被脱水，然后对采出水进行分析。在设计阶段必须适当解决这种信息缺乏的问题。本文后面将进一步讨论。

图 1 中所示的化学处理通常包括凝结剂、絮凝剂、阻垢剂、除氧剂、腐蚀抑制剂和杀菌剂。我们还将简要介绍油田中通常用于其他目的的化学品，例如油脱水（破乳剂）、硫化氢清除剂和流动保证化学品，这些化学品已知会影响采出水处理设备的性能和最终处理后的水质。油脱水系统在水处理系统的性能中起着非常重要的作用。如果该系统出现瓶颈或限制，水质可能会因操作员为实现干油而进行的调整而受到影响。本文后面将详细介绍化学处理。

从设备和工艺配置的角度来看，初级设备（如撇渣罐）根据采出水和水中污染物之间的密度差异进行分离。二级设备包括水力旋流器和浮选槽等设备。三级设备是任何依赖深层过滤、膜和筒式过滤器等介质的设备，GOM 深水作业的操作员通常不会使用三级设备。本文将不讨论三级设备（精滤、微滤和超滤）。

工艺配置（也称为工艺排列、工艺路线和系统集成），指的是工艺流程图，即罐和容器的顺序、工艺流程的连接和路线，以及最重要的废料或回收料流的路线）​​。这将在本系列的第 2 部分中介绍。

在项目的设计阶段，流程配置的开发称为流程集成。通常，在确定设备对整个流程的影响之前，不应选择任何设备。操作员通常会根据现场经验进一步修改这些做法。给定系统是否有效运行取决于操作员的技能。该系统的目标是提高生产水流出物的质量。在这个主题下，我们将描述操作员在墨西哥湾处理生产水时需要满足的监管准则。

以下讨论提供了设计深水应用水处理系统的详细信息和策略。

进料采出水的表征

进入的采出水的特性以及目标流出物的质量决定了水处理的挑战（SPE 36587、SPE 56847）。每条采出水流都是独一无二的，其特性由多种变量决定，例如水源、加工操作以及直接或间接添加的化学品。

对于进行注水作业的场地，由于向地层注入额外的水以增加碳氢化合物产量，采出水的性质和体积可能会发生巨大变化。在油田寿命期间，各种井流的混合也是采出水化学的一个重要因素。简而言之，在油田寿命期间，采出水的化学性质从来都不是均匀的。处理系统设计应考虑到这一点。

采出水化学性质在一定程度上取决于工艺条件（如温度、压力）和动态条件（剪切、聚结、沉降时间）以及生产化学品的使用。在地层中，水与碳氢化合物和地层岩石物质处于平衡状态。随着水的产生，压力和温度下降，导致平衡发生变化。这可能导致各种成分以固体颗粒形式沉淀。其他成分可能会迁移到油/水界面。换句话说，采出水化学性质既复杂又高度动态。系统设计需要考虑到这一点。

采出水含有悬浮固体、分散油、溶解矿物质和盐、有机化合物以及溶解气体、悬浮矿物质、聚合物和生产化学品。这些污染物的化学成分对于选择、设计和操作水处理设备非常重要。

采出水污染物的重要成分可能包括

• 涣散油。
  
• 溶解油（油、BETX、酚、PAH）。
  
• 溶解的有机酸和其他有机物。
  
• 溶解或沉淀的矿物质（NaCl、CaCO3 _、 FeCO3 _、 FeSx _、 BaSO4 _、 SrSO4 _）。
  
• 溶解金属（Fe、Zn、Cr、Mn）。
  
• 工艺和生产化学品（腐蚀抑制剂、水净化剂、甲醇、乙二醇）
  
• 生产地层固体（粘土、沙子、碳酸盐）。
  
• 溶解和沉淀的腐蚀产物（溶解的金属、固体金属氢氧化物和氧化物）。
  
• 溶解气体（O2 _、 H2S _、 CO2 _）。
  
• 上述的组合（例如，当采出水中的固体被油和化学物质覆盖时，就会形成浮渣，使其具有中性浮力。这些颗粒随后聚集并吸引其他物质，如石蜡、沥青质和细菌）。
  
• 各种细菌和副产物（例如硫酸盐还原细菌、一般异养细菌）。

以上列表是从化学实体的角度给出的，而不是从分析类型的角度给出的。其中许多实体是在典型或标准分析中确定的。
采出水的化学成分有助于

• 油滴对剪切的敏感性。
  
• 油滴聚结的趋势。
  
• 通过化学处理使污染物凝结和絮凝。
  
• 溶解矿物和有机化合物的沉淀和结垢趋势。
  
• 采出水的腐蚀性。
  
• 生产水的毒性。
  
• 处理设备的选择和设计（设备类型、停留时间要求）。
  
• 水排放和处置选项。

采出水设计特征

设计采出水特征包括

• 油滴尺寸分布。
  
• 油包水浓度。
  
• 石油流动保证特性（SARA——饱和物、芳香烃、树脂、沥青质）、蜡稳定性、沥青质）。
  
• 气体组成和石油特性（H ₂ S；CO ₂；C1—7；BTEX—苯、甲苯、乙苯、二甲苯；TAN—总酸值）。
  
• 水分析（阴离子、阳离子、pH 值、硬度、溶解气体和有机物）。
  
• 悬浮固体浓度、粒度分布、成分和矿物学。

重要的是要认识到，对于新项目，由于在项目的勘探和开发阶段缺乏具有良好代表性的采出水和固体样品，因此通常无法获得采出水成分和固体数据。分散油和悬浮固体浓度和粒度分布数据也不可用。相比之下，气体成分和石油的特征相当好。在这种情况下， 必须通过对周围油藏的了解来识别模拟系统。

然后根据现有油田中其他学科的信息调整模拟油田/油藏的采出水特性，以便了解采出水特性，从而用于设计目的。这些包括：

• 油藏工程（原油特性、API重力、TAN、含水率和地质背景）
  
• 工艺工程（气体成分，如 H2S _、 CO2 _、流速）
  
• 流动保障（沥青质、蜡、结垢趋势、水合物生产化学品需求、水垢、沥青质）

此类信息可以洞悉溶解矿物的结垢趋势、沥青质的存在、可用于原油脱水的化学品类型、腐蚀控制和流动保障。这些都在系统设计中发挥着非常重要的作用。

采出水取样

良好的取样和分析方法的重要性怎么强调也不为过。要对采出水处理设备进行有效的监控和故障排除，就需要在分离设备的所有阶段的上游和下游都设置高质量的取样点。

在选择采样点的位置时，建议遵循以下指导原则（Walsh 2019），即采样点应

• 位于流体向上流动的垂直管道部分。
  
• 距离任何流动干扰（如弯头、泵、歧管）的下游至少为 5 个管道直径，上游至少为 3 个管道直径（ASTM D4177-22茞1）。
  
• 位于脱油设备的上游和下游。
  
• 位于控制阀和泵的上游和下游，以便测量由此类设备引起的液滴和颗粒直径分布的变化。取样位置应位于此类设备下游至少 20 个管径处。该设备上游的取样位置可以近至几个管径。
  
• 处于稳定、均匀湍流的区域。
  
• 提供有效、环保且安全的方法来收集和处理冲洗采样点和采样管线所需的液体流。

对于用于法规遵从性监测目的的采样点，可以考虑以下指导。

• 取样点应紧接在湍流区域内的最后一项处理设备之后或下游，且无论如何应在任何后续稀释之前。取样点还应位于任何已安装的采出水体积计的下游。
  
• 采出水取样点的位置必须符合监管指南的规定，未经书面许可不得更改。

采样点

• 应始终以适当的方式安装取样探头，以确保采集到具有代表性的样本。取样探头最好延伸到管道中心，或距离管壁至少为管道内径的 0.25 倍。在某些情况下，探头的长度可能会受到限制，以承受管道中流动产生的机械力。
  
• 取样探针的内径大小必须允许等速流体通过探针，同时确保体积流速在取样设备的容量范围内。
  
• 如果需要，采样点应该可以方便地进入和检索。
  
• 应策略性地提供备用采样地点以供将来使用。

取样探头

• 所有采样地点都应配备采样探头（也称为羽毛管）来收集代表性样本。
  
• 取样探头有多种设计，例如皮托管式、圆形端口式和 45° 开口式。一种廉价且相对简单的探头设计是将取样探头的末端斜切为 45°（45° 开口探头）。这种探头很容易买到。

采样技术、样品处理、保存、储存和分析应符合完善的程序。

化学处理

在本节中，我们将简要讨论化学处理方案，特别强调设计阶段的重要事项。然而，重点应该是确保尽可能在操作过程中使用最少的化学品。采出水量和化学性质会随着油田寿命而变化。系统设计应提供灵活性，以处理油田寿命期间不同的化学品需求。例如，如果有必要，提供备用注入位置以及化学品储存和注入设备的空间以备将来使用可能很重要。

我们将简要描述整体化学处理，特别强调采出水处理化学品、化学品选择、化学兼容性以及化学注入系统的设计。

优化注入化学品的剂量非常重要，因为它们可能会导致生产水中的毒性、泡沫和可溶油水平增加，从而不利于满足海上作业中监管的舷外处置水质规范。

油田生产和加工设施中的化学处理涵盖三个领域。

• 产品/流出物质量——油、水和气体质量。用于清洁采出水的化学品包括凝结剂、絮凝剂、杀菌剂、腐蚀抑制剂和阻垢剂。为了满足油的质量要求，在油脱水工艺中使用的破乳剂可以在水相中形成稳定的乳状液。
  
• 流动保障 — 注入的化学品，用于防止蜡沉积、结垢、水合物控制、泡沫预防。流动保障化学品具有多种特性和机制，可用于管理设施中的流动保障问题。其中一些化学品包括水合物抑制剂，例如低剂量水合物抑制剂 (LDHI)，它们会对水处理设备的性能产生重大影响。
  
• 资产完整性——例如腐蚀控制、硫化氢清除剂和杀生物剂。油田腐蚀抑制剂通常具有表面活性，对水处理有巨大影响。表面活性化学品往往能稳定油包水和水包油乳剂，并极大地影响液滴聚结。

为了为设施制定最佳、全面的化学处理方案，应考虑上述所有应用。需要测试各种化学品的兼容性，因为一种化学品可能与另一种化学品发生反应，导致化学品失效或导致沉淀或结垢。

一个由工艺工程师、生产化学家和化学品供应商组成的跨学科团队必须共同努力，优先考虑有关气体质量、石油质量、流动保障、资产完整性和水质的相互竞争的目标。水质尤其重要，因为它必须满足海上作业中舷外处置的监管要求和陆上作业中注入水的质量规范。

化学处理：选择、兼容性和优化

在大多数情况下，在设计阶段无法获得代表性采出水样本。因此，所有化学选择测试都将在实验室中使用模拟流体进行。模拟油田的流体样本成分以及正在设计的油田的原油和天然气成分可以作为选择模拟流体成分的基础。实验室测试最好尽可能模拟工艺设计温度和压力进行。

为现有设施中的特定工艺选择最佳化学品是一个多阶段过程，包括

1. 采出水特性分析及兼容候选物的选择。

2. 对产出的液体进行取样，用于实验室和现场瓶内测试。

3. 对候选化学品进行瓶内/台面测试（如果可行，包括实验室内的流动回路测试）。

4. 根据标准实验室和/或现场瓶子测试选择最佳化学品进行现场测试。

5. 重复步骤3和4，选择最佳化学品和剂量。

在设计阶段，将使用模拟流体和实验室测试来选择化学品。实验室测试包括瓶/罐测试、台架测试（例如使用台架模型浮选装置）和流动回路测试。当设施投入使用时，需要对它们进行优化。实验室测试还用于确定所选的各种化学品之间是否存在兼容性问题。化学品供应商通常对其产品的化学性质和兼容性有深入的了解，在化学品选择过程中必须依靠它们。设施启动后，应使用代表性流体在实验室和现场重复这些测试，以优化所选化学品及其剂量。

化学药剂注入

为了使处理化学品有效，它们必须充分混合到生产水中，并且必须在机械分离之前给予它们一定的反应时间并且不进行高剪切。

化学注入系统设计可能非常复杂，以适应以不同的速率和压力注入设施中的多个位置。典型的海上设施通常会注入几种化学品。这些包括井下、海底和顶部应用。

流动保证化学品、水合物抑制剂（LDHI 和热力学水合物抑制剂，如甲醇和单乙二醇）以及蜡、水垢和腐蚀抑制剂被注入海底。

• 将沥青质抑制剂注入井下。
  
• 有时会在顶部设施以及井下注入阻垢剂，以防止生产油管结垢。
  
• 破乳剂被注入三相分离器的上游。
  
• 泡沫抑制剂被注入两相分离器的上游。
  
• 凝结剂和絮凝剂被注入到初级采出水处理系统（如水力旋流器）的下游或二级水处理设备（如浮选装置）的上游。
  
• 杀菌剂有时用于污水舱的微生物控制。化学品也被注入气体处理和海水注入系统中。
  

由于化学品使用量大，确保化学兼容性变得非常重要。还要注意的是，在设计阶段应提供备用注入配件，以确保化学品注入能力，因为在现场使用寿命期间改变流体化学成分可能需要注入额外的和/或新的化学品。
化学品注入系统包括储罐、计量泵、仪表、吸入/排出管道和阀门。大多数化学品注入系统都是滑轨安装的，以便安全运输和操作，同时在日常操作期间为系统提供结构保护。化学品以所需的速率泵送到安装的注入位置，并使用注入管或雾化器注入工艺流，以确保与流体流正确混合。

喷射喷嘴通常用于将液体注入气流，以快速分散注入的气流。当需要用喷射剂将化学物质涂满/润湿管壁时，这一点至关重要，例如在天然气管道中注入腐蚀抑制剂的情况。

使用在线可伸缩注射套筒，无需执行完整的工艺减压即可缩回和插入套筒。这些套筒最好插入工艺管道的中心线或距离管壁至少 d/3 的距离，以便将化学品正确分散到溶液中。化学品注入系统设计应确保注入位置易于接近，周围有足够的空间，便于取回。

结论

主要结论总结如下。

• 六个要素或主题是墨西哥湾海上平台采出水处理系统设计的基础：流体特性、化学处理、设备、工艺配置、操作和流出物质量。
  
• 流入流体的特性以及流出物的质量决定了水处理的挑战。
  
• 在设计阶段，大多数情况下无法获得有关生产水化学、分散油和悬浮固体浓度以及粒度分布的数据。
  
• 必须通过对周围油藏的了解来识别模拟系统。应根据油藏工程、工艺工程和设计油田的流动保障信息来调整模拟油田/油藏的采出水特性，以加深对采出水特性的理解，从而达到设计目的。
  
• 采出水的质量和产量在油田寿命期间会发生变化。系统设计应确保充分处理这些变化。
  
• 应在分离设备各级的上游和下游提供高质量的采样点，以便在现场使用寿命期间对设备进行有效的监测和故障排除。
  
• 应按照最佳实践安装取样探头，以确保以环保的方式采集有代表性的样品。取样探头应易于取用和回收。
  
• 应安装化学注入点来注入化学药品，并根据最佳实践关注产品/流出物质量、流量保证和资产完整性。
  
• 注射点应配备注射管或喷嘴，以确保与流体流正确混合。这些应易于接近。注射管或喷嘴应易于在线取回。
  
• 化学品选择应基于模拟生产水化学（用于系统设计）的实验室测试，最好模拟工艺设计的温度和压力。

John Walsh博士、SPE，在水行业工作了近 30 年。他在壳牌工作了 20 多年，在加入壳牌之前曾在 Westvaco 造纸公司工作，后来又在 CETCo 能源服务公司工作，最近又加入了 Worley Consultants。在壳牌，他是上游水处理的全球主题专家。在担任该职位期间，他曾在数十个国家工作，负责监督这些国家的研发、故障排除和技术项目。他参与了各种水处理挑战，包括页岩、水底、提高采收率以及常规陆上和深水海上项目。

他有幸与杰出的生产水专家合作，这些专家给予他源源不断的鼓励，使他撰写了一本两卷的书，名为《生产水》。书的封面材料中列出了贡献者名单。

他曾担任生产水协会主席兼董事总经理。他曾任 SPE 董事会成员，并担任两门 SPE 水处理课程的指定讲师。他拥有约翰霍普金斯大学化学工程博士学位。

Kris M. Bansal博士、SPE 擅长开发最佳工程解决方案，这些解决方案需要系统集成并与上游石油和天然气作业中从油藏到顶部的多个学科进行交互。他最近以工程研究员的身份从康菲石油公司退休，在该公司工作了 30 年，是上游水管理领域的全球主题专家。他教授注水课程，并举办解决问题的研讨会，以将技术转移到运营和工程领域。在加入康菲石油公司之前，Bansal 在沙特阿美公司从事运营工程工作 6 年，在 Calgon 从事活性炭研究工作 3 年，从事学术研究工作 8 年。他教授过页岩作业水管理方面的 SPE 课程。他拥有物理化学硕士和博士学位以及化学工程硕士学位。他的联系方式是Krismbansal@yahoo.com。

进一步阅读

生产水：第 1 卷：基础知识、水化学、乳液、化学处理 （作者：J. Walsh）。

SPE 36587 原油和采出水稳定分散体的形成：油类型、蜡和沥青质含量的影响， 作者：曼彻斯特科技学院 G. Davies；Christian Michelsen 研究中心 F. Nilsen；Hydro 波什格伦研究中心 P. Gramme。

SPE 56847 MTU——用于研究海上分离问题和优化气/油/水分离过程的多测试装置， 作者：P. Gramme、Norsk Hydro；B. Dybdahl、Petrotech；以及×. Holt、Statoil；等人。

This first installment of a two-part series provides guidelines for the systematic design and operation of produced-water systems specifically for deepwater Gulf of Mexico (GOM) platforms. Six elements are highlighted: fluid characterization, chemical treatment, equipment, process configuration, operations, and effluent quality. The characteristics of the incoming produced water, together with the target effluent quality, define the water-treating challenge.

Due to the high cost of space and weight in the deepwater environment, water treatment must have high intensity and must be highly integrated. High intensity refers to equipment that can handle high volumetric flow rates, over short residence times, and which occupies a small footprint. The concept of highly integrated systems refers to system process design that integrates water-treatment equipment, process configuration, and chemical treatment into a high-performing single system that removes a very high fraction of the contaminants.

Every opportunity is utilized to ensure that the produced water is treated to a high standard. Peak shaving, interface bleed, and break tanks that act as clarifiers are just a few of the opportunities that can be utilized to improve produced-water quality.

Part 1 includes produced water characterization and chemical treatment. Part 2 will cover equipment, process configuration, operations, and effluent quality. It will also include a conclusions section relevant to both Parts 1 and 2.

Introduction

In deepwater Gulf of Mexico (GOM) operations, the separation systems are designed to minimize weight and space while removing a very high fraction of contaminant. A typical oil/water separation system design consists of one or two stages of two-phase gas/liquid separation, followed by a three-phase separator such as a freewater knockout vessel. Produced water from the three-phase separator is then treated for overboard discharge as per the regulatory requirements. In this production scenario, the oil and water droplets have undergone significant shearing because of large pressure drops across chokes and valves. Further, this production scenario limits the ability to make any meaningful impact on reducing the droplet shearing and placing of the water-treatment equipment during the design phase.

The produced-water treatment systems discussed in this article are intended for high intensity deepwater treatment. Currently, there are many rules of thumb and ad hoc methods that guide the design and management of offshore produced-water systems.

However, for many design decisions the industry is lacking systematic and methodical approaches. Very often a produced-water system is designed based on local experience, and on parameters that have led to a successful previous design (Walsh 2019). The problem with this approach is that nearby systems may have fluid properties that warrant a significantly different design approach.

Six Critical Themes Shaping Deepwater Design Innovation

Conceptually, the organization of a design project or an optimization of operations can be thought of as shown in Fig. 1. Six elements or themes are involved. Fluid characterization, which defines the produced-water challenge, is the first theme.

Between the incoming produced water and the discharge effluent are the four elements that provide design specifics for the water-treating system (Fig. 1). These are the elements that contribute to the transformation of a particular feedwater to a very clean desired effluent quality. The elements fit together like a jigsaw puzzle, fitting together closely to form the complete and coherent design.

This article includes produced-water characterization and chemical treatment. Part 2 will include equipment design and operation, process configuration, operations, and effluent produced-water quality.

Each theme is defined and discussed in a short description and will be followed by a more detailed explanation of the entire system.

It is important to recognize that produced-water samples and characterization data are generally not available during the design phase. Hence, dispersed oil, suspended solids concentration, and particle-size distribution data are also not available.

For characterization, produced-water samples are needed. Sampling is usually carried out where the produced water is split into an oily water stream and wet oil. The wet oil is generally dehydrated, and the produced water is then analyzed. This lack of information must be appropriately addressed during the design phase. Further discussion is presented later in this article.

The chemical treatment indicated in Fig. 1 generally includes coagulants, flocculants, scale inhibitors, oxygen scavengers, corrosion inhibitors, and biocides. We will also include a brief mention of the chemicals typically used in the oil field for other purposes such as oil dehydration (demulsifiers), hydrogen sulfide scavenging, and flow assurance chemicals that are known to have an impact on the performance of produced-water treatment equipment and resulting treated-water quality. An oil-dehydration system plays a very important role in the performance of the water-treatment system. If this system is bottlenecked or constrained, it is likely that water quality will suffer because of adjustments made by operators to achieve dry oil. Further details on chemical treatment will be presented later in this article.

From the standpoint of equipment and process configuration, primary equipment (such as skim tank) provides separation based on density differences between the produced water and contaminants in water. Secondary equipment will include equipment such as hydrocyclones and flotation cells. Tertiary equipment is any equipment that relies on media such as deep bed filtration, membranes, and cartridge filters, and is generally not used by the operators in GOM deepwater. Tertiary equipment (fine filtration, micro-, and ultra-filtration) will not be discussed in this article.

Process configuration (also known as the process line-up, routing, and system integration) and refers to the process flow diagram, i.e., the sequence of tanks and vessels, the connections and routing of process flows, and the all-important routing of reject or recycle streams). This will be presented in Part 2 of the series.

In the design phase of a project, the development of process configuration is known as process integration. Generally, no piece of equipment should be selected until the impact of the equipment on the overall process has been determined. Operators normally further modify these practices based on their field experiences. A given system can perform effectively or not depending on the skill of the operators. The objective of the system is to improve the produced-water effluent quality. Under this theme, we will describe the regulatory guidelines that operators will need to meet for overboard disposal of treated produced water in the GOM.

The following discussion provides detailed information and strategy for designing water-treatment systems for deepwater application.

Characterization of Feed Produced Water

The characteristics of the incoming produced water, together with the target effluent quality define the water treating challenge (SPE 36587, SPE 56847). Each produced-water stream is unique, with characteristics defined by a wide range of variables such as the water source, processing operations, and directly or indirectly added chemicals.

For those sites where waterflooding is conducted, the properties and volumes of the produced water may vary dramatically due to the injection of additional water into the formation to increase hydrocarbon production. Blending of various well streams during the field life is also an important factor in produced-water chemistry. In short, produced water chemistry is never uniform during the field life. Treatment-system design should take this into account.

The produced-water chemistry depends to some degree on process conditions (such as temperature, pressure) and dynamic conditions (shearing, coalescence, settling time), and the use of production chemicals. In the formation, water is in equilibrium with hydrocarbon, and the formation rock material. As the water is produced, the pressure and temperature decrease causing a shift in the equilibria. This may cause various components to precipitate as solid particles. Other components may migrate to oil/water interface. In other words, produced-water chemistry is both a complex and a highly dynamic variable. The system design needs to take this into consideration.

Produced water contains suspended solids, dispersed oil, dissolved minerals and salts, and organic compounds along with dissolved gases, suspended minerals, polymers, and production chemicals. The chemical composition of these contaminants is important in selecting, designing, and operating water treatment equipment.

Important components contaminants of produced water may include

• Dispersed oil.
  
• Dissolved oil (oils, BETX, phenols, PAH).
  
• Dissolved organic acids and other organics.
  
• Dissolved or precipitated minerals (NaCl, CaCO₃, FeCO₃, FeS_x, BaSO₄, SrSO₄).
  
• Dissolved metals (Fe, Zn, Cr, Mn).
  
• Process and production chemicals (corrosion inhibitor, water clarifier, methanol, glycols)
  
• Produced formation solids (clay, sand, carbonate).
  
• Dissolved and precipitated corrosion products (dissolved metals, solid metal hydroxides, and oxides).
  
• Dissolved gases (O₂, H₂S, CO₂).
  
• Combinations of the above (e.g., schmoo, which forms when solids in produced water become coated with oil and chemicals, making them neutrally buoyant. These particles then agglomerate and attract other substances like paraffins, asphaltenes, and bacteria).
  
• Various bacteria and byproducts (e.g., sulfate-reducing bacteria, general heterotrophic bacteria).

The above list is given from the perspective of chemical entities, rather than types of analyses. Many of these entities are determined in typical or standard analyses.
The chemical composition of produced water contributes to the

• Sensitivity of oil droplets to shear.
  
• Tendency of oil droplets to coalesce.
  
• Chemical treatment to coagulate and flocculate the contaminants.
  
• Precipitation and scaling tendency of dissolved minerals and organic compounds.
  
• Corrosivity of produced water.
  
• Toxicity of produced water.
  
• Selection and design of treatment equipment (equipment type, residence time requirements).
  
• Water discharge and disposal options.

Produced-Water Characteristics for Design

Produced-water characteristics for design include

• Oil-droplet size distribution.
  
• Oil-in-water concentration.
  
• Oil flow assurance characterization (SARA—saturates, aromatics, resins, asphaltenes), stability of wax, asphaltenes).
  
• Gas composition and oil characterization (H₂S; CO₂; C1–C7; BTEX–benzene, toluene, ethyl benzene, xylene; TAN–total acid number).
  
• Water analyses (anions, cations, pH, hardness, dissolved gases, and organics).
  
• Suspended solids concentration, particle-size distribution, composition, and mineralogy.

It is important to recognize that for new projects, produced-water composition and solids data is generally not available because of the lack of good representative produced-water and solids samples during the exploration and developmental phase of the project. Dispersed oil and suspended solids concentration and particle-size distribution data are also not available. By comparison, gas composition and oil are reasonably well characterized. In such a case, analog systems must be identified through knowledge of surrounding reservoirs.

Produced-water characteristics from analog fields/reservoirs are then adjusted based on the information from other disciplines in the existing field for developing an understanding of produced-water characterization for design purposes. These include:

• Reservoir engineering (crude oil properties, API gravity, TAN, water cut, and geological setting)
  
• Process engineering (gas composition such as H₂S, CO_2, flow rates)
  
• Flow assurance (asphaltenes, wax, scaling tendency, need for production chemicals for hydrates, scales, asphaltenes)

This type of information can provide insights into the scaling tendency of dissolved minerals, presence of asphaltenes, type of chemicals that may be used for crude oil dehydration, corrosion control, and flow assurance. These all play a very important role in the system design.

Produced-Water Sampling

The importance of using good sampling and analysis practices cannot be overstated. Effective monitoring and troubleshooting of produced-water treatment equipment requires good-quality sampling points both upstream and downstream of all stages of separation equipment.

In selecting the location of the sampling points, the following guidelines are recommended (Walsh 2019), i.e., the sample point should

• Be in a vertical section of piping where the fluid flow is upwards.
  
• Be at least five pipe diameters downstream and three pipe diameters upstream of any flow disturbance such as a bend, pump, manifold (ASTM D4177-22Ɛ1).
  
• Be located both upstream and downstream of the deoiling equipment.
  
• Be located both upstream and downstream of control valves and pumps so that the change in drop and particle diameter distribution caused by such equipment can be measured. The sample location should be at least 20 pipe diameters downstream of such equipment. The sample location upstream of this equipment can be as close as a few pipe diameters.
  
• Be in a region of steady, uniform turbulent flow.
  
• Provide an effective, environmentally sound, and safe means for the collection and disposal of the fluid flow required to flush the sample point and sample line.

For sampling points that are used for regulatory compliance monitoring purposes, the following guidance may be considered.

• The sampling point should be immediately after the last item of treatment equipment in, or downstream of, a turbulent region, and in any case before any subsequent dilution. The sample point should also be downstream of any installed produced-water volume meter.
  
• The location of the produced-water sample point must be in accordance with the regulatory guidelines and must not be changed without written permission.

Sample points

• Should always have a sample probe installed in a manner to ensure a representative sample is taken. The sampling probe should preferably extend to the center of the pipe or at least a distance of 0.25 of the internal diameters of the line away from the pipe walls. In some cases, the length of the probe may be restricted to withstand the mechanical forces exerted by the flow in the pipe.
  
• The internal diameter of the sample probe must be sized to allow for an isokinetic fluid velocity through the probe while ensuring that the volumetric flow rate is within the capacity of the sampling equipment.
  
• Sampling point should be readily accessible and retrievable, if needed.
  
• Spare sampling locations should be strategically provided for future use.

Sampling probes

• Sampling probes (also called quills) should be provided for all sampling locations to collect representative samples.
  
• Sampling probes come in a variety of designs such as pitot, circular port, and 45° opening type. An inexpensive and a relatively simple probe design has the end of the sampling probe beveled at 45° (45° opening probe). Such probes are readily available.

Sampling technique, sample handling, preservation, storage, and analysis should be in accordance with well-established procedures.

Chemical Treatment

In this section, we will briefly discuss the chemical treatment program with particular emphasis on what is important during the design phase. The focus should, however, be to ensure that the process can be operated with minimum use of chemicals, whenever possible. Produced-water volumes and chemistry change over the field life. The system design should provide the flexibility to handle varying chemical needs during field life. For example, it may be important to provide spare injection locations, and space for chemical storage and injection equipment for future use, if necessary.

We will briefly describe the overall chemical treatment with particular emphasis on produced-water treatment chemicals, chemical selection, chemical compatibility, and design of chemical injection system.

It is important to optimize the dosage of chemicals injected because they can contribute to an increase in the toxicity, foam, and soluble-oil levels in produced water that can be detrimental in meeting the regulatory overboard disposal water-quality specifications in offshore operations.

Chemical treatment in an oilfield production and processing facility encompasses three areas.

• Product/effluent quality—oil, water, and gas quality. The chemicals used to clean produced water include coagulants, flocculants, biocides, corrosion inhibitors, and scale inhibitors. Demulsifiers used in the oil-dehydration process, to meet oil quality, can result in stable emulsions in the water phase.
  
• Flow assurance—chemicals that are injected to prevent wax deposition, scale formation, hydrate control, foam prevention. Flow assurance chemicals have a wide range of properties and mechanisms to manage flow assurance issues in the facility. Some of these chemicals include hydrate inhibitors such as low-dose hydrate inhibitors (LDHI) which can have a major impact on the performance of water-treatment equipment.
  
• Asset integrity—such as corrosion control, hydrogen sulfide scavengers, and biocides. Oilfield corrosion inhibitors generally are surface active and can have an enormous effect on water treatment. Surface active chemicals tend to stabilize both the oil-in-water and water-in-oil emulsions and greatly impact droplet coalescence.

To develop an optimal and holistic chemical treatment program for the facility, all the applications listed above should be considered. Compatibility of various chemicals will need to be tested since one chemical can react with another to make the chemicals ineffective or cause a precipitate or scale to form.

An interdisciplinary team consisting of process engineers, production chemists, and chemical vendors will have to work together to prioritize competing objectives regarding gas quality, oil quality, flow assurance, asset integrity, and water quality. Water quality is especially significant because it must meet regulatory requirements for overboard disposal in offshore operations and injection water quality specifications in onshore operations.

Chemical Treatment: Selection, Compatibility, and Optimization

In most cases, representative produced-water samples are not available during the design phase. Therefore, all the testing for chemical selection will be done in the laboratory using simulated fluids. Composition of fluid samples from analog fields, along with the crude and gas composition from the field being designed, could form the basis for selecting the simulated fluid composition. Laboratory testing should preferably be done simulating process design temperature and pressure as far as practical.

Selecting optimal chemicals for a given process in an existing facility is a multistage process consisting of

1. Characterization of produced water and selection of compatible candidates.

2. Sampling of produced fluids for lab and on-site bottle testing.

3. Bottle/bench testing of candidate chemistries (includes flow-loop tests in the lab, if feasible).

4. Field testing of best chemicals selected based on standard lab and/or on-site bottle testing.

5. Repeating steps 3 and 4 to select the optimal chemical and the dosage.

During the design phase, the chemicals will be selected using simulated fluids and testing in the lab. Lab testing includes bottle/jar testing, bench testing (such as with a bench model flotation unit), and flow-loop testing. They will need to be optimized when the facilities are onstream. Lab testing is also used to determine if there are any compatibility issues between various chemicals selected. Chemical vendors generally have an excellent knowledge of their product’s chemistries and compatibility and will have to be relied upon during the chemical selection process. These tests should be repeated with representative fluids once the facility has been started, both in the lab and in the field, to optimize the chemicals selected and their dosage.

Chemical Injection

For treatment chemicals to be effective, they must be well mixed into the produced water, and they must be given a certain amount of reaction time without high shear prior to mechanical separation.

Chemical-injection system design could be quite complex to accommodate injection into multiple locations at different rates and pressures in the facility. Several chemicals are normally injected in a typical offshore facility. These include downhole, subsea, and topsides applications.

Flow assurance chemicals, hydrate inhibitors (LDHI and thermodynamic hydrate inhibitors such as methanol, and monoethylene glycols), and wax, scale, and corrosion inhibitors are injected subsea.

• Asphaltene inhibitors are injected downhole.
  
• Scale inhibitors are sometimes injected in the topside’s facility as well as well as downhole for scale prevention in production tubing.
  
• Demulsifiers are injected upstream of the three-phase separators.
  
• Foam inhibitors are injected upstream of the two-phase separators.
  
• Coagulants and flocculants are injected downstream of the primary produced-water treatment system such as hydrocyclones or upstream of the secondary water-treatment equipment such as flotation units.
  
• Biocides are sometimes used for microbial control in slop tanks. Chemicals are also injected in gas treatment and seawater injection system.
  

Because of extensive chemical usage, it becomes important to ensure chemical compatibility. It is also important to note that spare injection fittings should be provided during the design phase to ensure chemical injection capability when changing fluid chemistry during the field life could require injection of additional and/or new chemicals.
A chemical-injection system includes storage tank, metering pumps, instrumentation, suction/discharge piping, and valves. Most chemical injection systems are skid-mounted to allow safe transport and operation while providing structural protection to the system during routine operation. Chemicals are pumped at the desired rates to the installed injection location and injected into the process stream with an injection quill or an atomizer to ensure proper mixing with the fluid stream.

Injection nozzles are generally used to inject liquids into the gas stream to rapidly disperse the injected stream. This is critical when injection is required to coat/wet the pipe wall with the chemical as, for example, in case of corrosion-inhibitor injection in gas pipelines.

The use of online retractable injection quills allows for the quill to be retracted and inserted without having to perform a complete process depressurization. These quills are preferably inserted to the centerline of the process pipe or at least a distance of d/3 away from the pipe wall to properly disperse the chemical into the solution. The chemical injection system design should ensure that the injection locations are readily accessible and have enough space around for ease in retrieving.

Conclusion

Major conclusions are summarized below.

• Six elements or themes are the basis for the systematic design of produced-water treatment systems for offshore GOM platforms: fluid characterization, chemical treatment, equipment, process configuration, operations, and effluent quality.
  
• The characteristics of the incoming fluids, together with the effluent quality, define the water-treating challenge.
  
• In most cases during the design phase, the data on produced water chemistry, dispersed oil, and suspended solids concentration, and particle-size distribution are not available.
  
• Analog systems must be identified through knowledge of surrounding reservoirs. Produced-water characteristics from analog fields/reservoirs should be adjusted based on the information from reservoir engineering, process engineering, and flow assurance from the field under design, to develop an understanding of produced-water characterization for design purposes.
  
• Produced-water quality and volumes vary during the field life. The system design should ensure that these variations are adequately handled.
  
• Good-quality sampling points both upstream and downstream of all stages of separation equipment should be provided for effective monitoring and troubleshooting of the equipment during the field life.
  
• Sampling probes should be installed in accordance with best practices to ensure representative samples can be taken in an environmentally acceptable manner. They should be readily accessible and easily retrievable.
  
• Chemical-injection points should be installed to inject chemicals with a focus on product/effluent quality, flow assurance, and asset integrity in accordance with best practices.
  
• Injection points should have injection quills or nozzles to ensure proper mixing with the fluid stream. These should be readily accessible. The injection quills or nozzles should be easily retrievable on-line.
  
• Chemical selection should be based on laboratory testing with simulated produced-water chemistry (used for system design) and should preferably be done simulating process design temperature and pressure.

John Walsh, PhD, SPE, has worked in the water industry for close to 30 years. He worked for Shell for more than 20 years, Westvaco Paper Company prior to Shell, CETCo Energy Services, and most recently Worley Consultants. At Shell, he was the global subject-matter expert for upstream water treatment. In that role, he worked in dozens of countries, providing oversight for their R&D, troubleshooting and technical programs. He was involved in a wide range of water-treatment challenges including shale, water floor, enhanced oil recovery, and conventional onshore and deepwater offshore projects.

He had the great fortune to work with outstanding produced-water specialists who provided a steady stream of encouragement to write a two-volume book titled “Produced Water.” A list of contributors is given in the cover material of the book.

He was the president and managing director of the Produced Water Society. He has served on the SPE Board of Directors and is the designated instructor for two SPE courses on water treatment. He earned a PhD in chemical engineering from the Johns Hopkins University.

Kris M. Bansal, PhD, SPE, has expertise in developing optimum engineering solutions requiring system integration and interface with multiple disciplines from reservoir to the topsides in upstream oil and gas operations. He recently retired as an Engineering Fellow from ConocoPhillips after 30 years of service, where he was a global subject matter expert in upstream water management. He taught waterflood school and presented problem-solving seminars for technology transfer to operations and engineering. Before joining ConocoPhillips, Bansal spent 6 years at Saudi Aramco in operations engineering, 3 years in activated-carbon research at Calgon, and 8 years in academic research. He has taught SPE courses on water management in shale operations. He holds an MS and PhD in physical chemistry and an MS in chemical engineering. He can be reached at Krismbansal@yahoo.com.

For Further Reading

Produced Water: Volume 1: Fundamentals, Water Chemistry, Emulsions, Chemical Treatment by J. Walsh.

SPE 36587 The Formation of Stable Dispersions of Crude Oil and Produced Water: The Influence of Oil Type, Wax, and Asphaltene Content by G. Davies, University of Manchester Institute of Science and Technology; F. Nilsen, Christian Michelsen Research; and P. Gramme, Hydro, Research Centre Porsgrunn.

SPE 56847 MTU—The Multi Test Unit for Investigating Offshore Separation Problems and Optimizing the Gas/Oil/Water Separation Process by P. Gramme, Norsk Hydro; B. Dybdahl, Petrotech; and Ø. Holt, Statoil; et al.