View source for Downhole oil–water separation technology

'''Downhole oil–water separation''' ('''DOWS''') '''technologies''' are apparatuses and methods that separate [[production fluid]]s into a [[Petroleum#Composition|petroleum]]-rich stream and [[water]]-rich stream within an [[oil well]]. A DOWS system installed in a [[borehole]] will receive the fluids from an oil-producing zone in an [[oil reservoir]] and separate the mixture into a stream that is mostly water and a stream that is primarily  [[petroleum|crude oil]] and [[natural gas]] and direct the streams to different destinations. After the separation in the borehole, DOWS systems pump the petroleum-rich stream to the surface and inject the water-rich stream into a different zone or formation accessible to the same wellbore.

==Economics==
An oil producing well is usually [[Completion (oil and gas wells)|completed]] to pump all produced fluids to the surface where the fluids will be  [[separator (oil production)|separated]] into their constituent components. In the early life of most oil producing wells, more petroleum will usually be produced than [[produced water]]. The produced water is considered to be [[wastewater]] that will need to be  [[wastewater treatment|treated]].

Over time, as the petroleum products are being [[Extraction of petroleum#Secondary recovery|depleted]] from the reservoir, the water cut (the ratio of water produced compared to the volume of total liquids produced) may increase. Or, when the field is undergoing [[Extraction of petroleum#Secondary recovery|secondary recovery]] with a [[Water injection (oil production)|waterflood]], the water cut may become very high. Alternatively, the well may develop other problems requiring a [[workover]]. When the water cut becomes high, or when a workover is considered, the economics will be evaluated<ref>{{Cite journal |last=Rike |first=L. |date=1972-01-01 |title=Workover Economics-Complete but Simple |url=https://onepetro.org/JPT/article-abstract/24/01/67/164028/Workover-Economics-Complete-but-Simple?redirectedFrom=fulltext |journal=Journal of Petroleum Technology |publisher=OnePetro |doi=10.2118/3588-PA}}</ref> and may favor the installation of DOWS technology so that the waste water does not need to be treated at the surface. In those cases, the economics may favor [[Water injection (oil production)|injection]] of the produced water into a different [[permeability (Earth sciences)|permeable]] zone accessible to the borehole. <ref name=AppGuide>{{Cite journal |last1=Blanco |first1=A.E. |last2=Davies |first2=D.R. |date=2001-03-24 |title=Technical & Economic Application Guidelines for Downhole Oil-Water Separation Technology |url=https://onepetro.org/SPEOKOG/proceedings/01POS/All-01POS/Oklahoma%20City,%20Oklahoma/134982 |journal=SPE Production and Operations Symposium|location=Oklahoma City, Oklahoma |publisher=SPE |pages=SPE–67182–MS |doi=10.2118/67182-MS}}</ref> <ref>{{Cite journal |last1=Shaw |first1=C. |last2=Fox |first2=M. |date=1998-10-20 |title=Economics of Downhole Oil-Water Separation: A Case History and Implications for the North Sea |url=https://onepetro.org/SPEEURO/proceedings-abstract/98EUROPEC/All-98EUROPEC/SPE-50618-MS/191100 |journal=European Petroleum Conference |location=The Hague, Netherlands |publisher=SPE |pages=SPE-50618-MS |doi=10.2118/50618-MS}} </ref> However, not all oil wells are good candidates for deploying DOWS.<ref>{{cite web |last1=Gao| first1=C. |last2=Rivero |first2=M. | last3=Nakagawa |first3=E. | last4=Sanchez | first4=G. |title= Downhole Separation Technology —Past, Present and Future |url=https://espace.curtin.edu.au/bitstream/handle/20.500.11937/33521/20530_downloaded_stream_518.pdf?sequence=2&isAllowed=y}}</ref>

==Operation==
The producing zone that a DOWS system is located in must be [[Seal (mechanical)|sealed]] from the other zones of the borehole by [[production packer|packers]]. This allows the [[intake]] of a DOWS system to receive the petroleum and water mixture. In some types of DOWS systems, a [[Submersible pump|pump]] will force the received mixture through the oil/water separation system. In some DOWS systems, the pressure of the fluids in the borehole will be enough to force the water-rich stream into a permeable zone below the DOWS system. In other DOWS systems, a separate pump is needed to inject the water-rich stream into a permeable zone.<ref>{{patent|US|5,296,153}}</ref>

An [[artificial lift|artificial lift pump]] is used to lift the petroleum-rich stream to the surface.<ref>{{Cite journal |last1=Jiang |first1=Minzheng |last2=Cheng |first2=Tiancai |last3=Dong |first3=Kangxing |last4=Liu |first4=Jintang |last5=Zhang |first5=Huaiyu |date=2020-08-13 |title=An Efficient Downhole Oil/Water-Separation System with Sucker-Rod Pump |url=https://onepetro.org/PO/article/35/03/522/450768/An-Efficient-Downhole-Oil-Water-Separation-System |journal=SPE Production & Operations |language=en |volume=35 |issue=3 |pages=522–536 |doi=10.2118/201234-PA }}</ref>

DOWS systems do not entirely separate petroleum from water in the borehole. Instead, DOWS systems will decrease the amount of water brought to the surface. Some [[Entrainment (engineering)|entrained]] droplets of water will be retained in the petroleum-rich stream, and some droplets of petroleum and [[Solvation|dissolved]] gas will be entrained within the water-rich stream.

Trials have shown that a majority of properly operating DOWS systems may reduce the amount of water brought to the surface by around 75%.<ref name=feasibility>{{Cite report |last1=Veil |first1=J. A. |last2=Langhus |first2=B. G. |last3=Belieu |first3=S. |date=1999-01-31 |title=Feasibility evaluation of downhole oil/water separator (DOWS) technology. |doi=10.2172/917614 |url=http://dx.doi.org/10.2172/917614|doi-access=free }}</ref><ref>{{Cite journal |last1=Ogunsina |first1=Oluseye Olugbenga |last2=Wiggins |first2=Michael Lloyd |date=2005-04-16 |title=A Review of Downhole Separation Technology |url=https://onepetro.org/SPEOKOG/proceedings/05POS/All-05POS/Oklahoma%20City,%20Oklahoma/88825 |journal=All Days |location=Oklahoma City, Oklahoma |publisher=SPE |pages=SPE–94276–MS |doi=10.2118/94276-MS}}</ref> Research into further reducing the amount of water brought to the surface continues.<ref>{{Cite journal |last1=Zhao |first1=C. |last2=Sun |first2=H. |last3=Li |first3=Z.  |date=2017-01-31 |title=Structural optimization of downhole oil-water separator |url=https://www.sciencedirect.com/science/article/pii/S0920410516304223 |journal=Journal of Petroleum Science and Engineering |language=en |volume=148 |pages=115-126 |doi=10.1016/j.petrol.2016.09.033 |doi-access=free }}</ref>

==Types==
Two traditional types of DOWS systems have been developed. Initially, a [[gravity separation]] mechanism was developed.<ref>{{patent|US|4,766,957}}</ref> However, gravity separation requires time downhole for the components to separate, which results in lower production rates.<ref name=feasibility />

[[Hydrocyclone]] DOWS systems<ref>{{patent|US|5,456,837}}</ref> were developed to speed up the separation and production rate.<ref name=feasibility /> Hydrocyclone type DOWS systems usually use electric submersible pumps to overcome any [[pressure head]] differential needed to inject water into another zone. Additionally, the supplemental draw of fluids into the wellbore by the pump has been known to increase the production of oil from some wells.<ref name=feasibility />

A new line of research is being pursued in which permeable membranes are deployed in the wellbore to separate water from the oil and gas.<ref>{{Cite journal |last1=Patel |first1=Mohit |last2=Patel |first2=Jigarkumar |last3=Pawar |first3=Yogendrasingh |last4=Patel |first4=Nehal |last5=Shah |first5=Manan |date=2020-02-24 |title=Membrane-based downhole oil-water separation (DOWS) technology: an alternative to hydrocyclone-based DOWS |url=http://dx.doi.org/10.1007/s13202-020-00848-x |journal=Journal of Petroleum Exploration and Production Technology |volume=10 |issue=5 |pages=2079–2088 |doi=10.1007/s13202-020-00848-x |s2cid=211254157 |issn=2190-0558 |doi-access=free }}</ref>

==References==
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[[Category:Petroleum technology]]

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