Sunday, January 11, 2009

T2. What causes Oil Field Production Decline and Depletion?

This is one of a series of somewhat technical articles that I plan on putting up on Sundays, in which I describe, in somewhat simple terms, some of the technical aspects of fossil fuel production. Because they are simplified some aspects may be glossed over, but for those that would like to explain some of the aspects in more detail, please feel free to do so (or to correct any apparent mis-statements).

After the algal remains from which crude oil would come first became trapped in the sands and mud of the time, those beds were successively covered over the eons, until the heat and pressure generated by rocks that had been deposited above the original sediments, transformed the original fluid into oil. The original bed, which we call a source rock, also contained some water and, as geological time passes, the oil would rise above the water, and start to migrate up through the overlying rock towards the surface. The oil and water could do this, because rock is not a complete solid, but rather has many small cracks and passages in it, that we define as the rock permeability. As a general rule rocks that are made up of larger grains (such as a sandstone) are more permeable than those with finer grains (such as a limestone). I’ll come back and talk about that more in a later post. But some rocks have virtually no permeability and so, as the oil moves upwards and it reaches one of these, which we will call cap rocks, then the oil movement stops, and the oil is trapped in the underlying rock, which is normally called the reservoir. It helps, in this case, if the movement of the rocks over time has caused the rocks to bend into a dome, or fold, which geologists call an anticline.

Simple slice through the ground showing, from the side, how a fold can capture oil that has been pushed up, by water, from a source rock and trapped by the overlying cap rock.

Now, when we find this pool of oil, which is really not a big open space filled with oil, but rather a rock with lots of small holes or pores in it, (the porosity) that are each filled with oil, we would like to get the oil out. To do this we drill a hole, which we call a well, down to the oil bearing rock, and through it to the top of the underlying water. The well might be somewhere around 6 inches in diameter, for the sake of illustration, and the rock might be some 6,000 ft below the current surface of the ground. And for the sake of making the arithmetic that follows a little easier, we will assume that the rock that has the oil in it starts out with the oil layer (or column as it is called) being some 100 ft thick.
Slice through the side of the rock showing the well in the oil bearing rock (the reservoir)

The pressure of the overlying rock on the reservoir rock can be simplified to around 1 pound per square inch (psi) for every foot the rock is below the surface. So that at 6,000 ft the rock pressure is 6,000 psi. (which you might think of as the weight of a full sized pickup or two resting on a single square inch – not something you want your hand under).

The pressure squeezes on the oil in the rock, so that when the well appears, with a lower (and controlled pressure) in the well, the difference between the two pressures causes the oil to flow to the well. The amount of oil that flows into the well depends on the difference in pressure between the oil pressure in the rock and in the well, the permeability of the rock and the length of the well that is actually in the oil bearing rock (the 100 ft shown above).

Time passes and the oil flows into the well, often for a number of years, as it flows the pressure in the oil drops a little, but also the water level below the oil rises, as the oil above it disappears. (And in many cases that I will talk about in a later post the company will pump water through other wells under the oil, so that it will keep the pressure in the oil high enough to make it flow out of the well faster).

But the amount of oil that comes out of the well is controlled by the length of the oil column in the well, and after a certain amount of time the water level has risen.

Later stage of well production only 20 ft of oil left

Now as the water level has risen, and the amount left has been reduced (depleted) the amount of oil produced a day will reduce (decline) because the rate is controlled by the length of the column. In this example, very crudely we will be getting 20% (20/100) of the production that we got from the well when it was first started. (Actually because of other factors that I will cover in later posts it will likely be a bit less than this.) As a very rough rule this decline for the overall field starts in at about the point that half the oil in the reservoir that the well is tapping has been removed, and the decline rate increases from that point on. There is not a lot that can be done with a conventional vertical well to stop the decline in production once the well starts to pump out the oil, and when there are a lot of wells successively being developed and then pulling oil out of a reservoir, then the overall production rate from the field very often will follow a bell-shaped curve.

In other words, as the wells first start to produce the field (which is what one of these large underground pools of oil is called) the production of oil will rise, it will have a relatively steady increase until as it approached full production, the increase tails off and there is a short period of steady production. Then as the oil columns get shorter in more and more wells the decline in production sets in and the remaining time that oil can be economically recovered from the field becomes more finite. (Though in some fields residual production of a barrel or two a day can continue for years).

The average decline in production rate from wells in a field is a matter of some debate, but for conventional quasi-vertical wells which have been the main method of getting oil out of the ground until about ten years ago, the rate, during the steady decline phase, has been assumed to be 4%. That rate is now thought to be increasing, but there are other factors that control that, and we’ll leave that discussion until another day.

17 comments:

  1. Aleklett's recent studies on oil field declines shows that declines typically start at 30% of URR.

    ReplyDelete
  2. Girl, I’ll treat you right
    And I’ll never lie
    For all that it’s worth
    I give good love (good love, good love, good love, good love)
    web cam to camvegas strippers

    ReplyDelete
  3. thanks fpr reading.
    Brisbane EscortsEscort Accompagnatrici
    How to make money online doing freelance jobs
    You can learn how to make money online as a freelancer if you have the right skills to bid and win the thousands of jobs that are available in online freelance websites.

    ReplyDelete
  4. Sex toy shopporno

    I Can't Turn
    Away From What I Believe
    I Can't Destroy
    Or Deceive
    Oh No Oh No
    I Know A Beauty
    In All That I Can See
    I Can't Hold On
    But You Can't Release

    ReplyDelete
  5. Free Gayssexshop
    She even blamed things on herself
    But no one came to her aid
    Nothing was wrong as far as we could tell
    That's what we'd like to tell ourselves
    But no! it wasn't that way
    So she fell in love

    ReplyDelete
  6. I could like to come again here for new content from your site.cape breton real estate

    ReplyDelete
  7. Thanks for sharing these info with us! this is a great site. I really like it. Thank you for the site.golf posture

    ReplyDelete
  8. It is a great article, as well as I want more information for those who have any kind of. We are attracted to this kind of pobyty pre dvoch na slovensku

    ReplyDelete
  9. We want to take this moment to remember that when collecting data it has an impact on this country location voiture agadir

    ReplyDelete
  10. Humans and cats have lived as companions for many centuries. They’ve been the subject of poems, movies, and poems. It’s clear why this is; cats believe they feel that they’re in charge.things to do in peoria il for kids

    ReplyDelete
  11. Very informative and trustworthy blog does exactly what it sets out to do. I’ll bookmark your weblog for future use.El Cangrejo

    ReplyDelete
  12. location voiture agadir ou location voiture agadir aéroport est une agence de location voitures à agadir aéroport diesel pas cher , taghazout, essaouira, marrakech, tanger, fes, meknes, rabat, casablanca , guelmim et au maroc villes et aéroports.
    chez notre agence vous pouvez bénéficiez avec des prix les plus bas au maroc avec un parc de voiture large gamme des voitures économiques essences et diesel et des petites voitures comme Hyundai I10 , Hyundai I20.
    aussi il existe des voiture rapport qualité prix comme dacia logan, sandero, duster, dokker essence ou diesel selon votre budget.
    il existe encore chez nous des autres marques comme Hyundai accent diesel automatique ou manuel , aussi Renault clio et Megane , kangoo , latitude.
    aussi les voitures de luxe comme Volkswagen golf 7 , golf 6, touareg , tiguan, range rover evoque , range rover sport, range rover voque.
    Adress : bloc H3 N 288، Agadir 80000
    Téléphone : 0667-196609
    Horaires :
    Ouvert ⋅ Ferme à 19:00
    dimanche 09:00–19:00
    lundi 09:00–19:00
    mardi 09:00–19:00
    mercredi 09:00–19:00
    jeudi 09:00–19:00
    vendredi 09:00–19:00
    samedi 09:00–19:00

    ReplyDelete
  13. location de voitures Agadir ou location voiture Agadir aéroport taghazout essaouira marrakech pas cher au Maroc. Profitez de prix attractifs, d'un service de grande qualité et d'un vaste choix de véhicules neufs .
    Hire a car in agadir with best price.
    location voiture Agadir.
    Site web: http://louer-voiture.ma/
    phone or wathsapp : +212667196609
    Address marrakech : Al massira 3A NR 530
    Marrakech 40140
    location voiture marrakech aéroport
    Address Agadir : bloc H3 N 288 cité dakhla، Agadir 80000
    location voiture agadir aéroport
    rental car agadir
    location voiture agadir aéroport.
    location voiture agadir aéroport
    location voiture essaouira aéroport
    location voiture marrakech aéroport
    hire a car in agadir airport

    ReplyDelete
  14. This comment has been removed by the author.

    ReplyDelete


  15. Looking for Best Bars in Delhi to quench the weekend thirst, or just need some of the best pubs in Delhi for enjoying evenings after long days at work. Choosing a place drink eBay celebrate should not be a difficult task. There are some things that are important while deciding about where shall you drink. The bar or the clubs should be close, economical, not congested & should also offer good food. But how can you find this information? It's actually quite simple. Just Download Boozeey Smartphone app for your smartphone.

    Boozeey will immediately give you the list of best bars in Delhi from the nearby area & show you their drinks menu & prices. So never waste time ever again by going to multiple bars or pubs, #justBoozeey. Not only does the Boozeey app helps you to find the best pubs & bars it will also give you great discounts with features like first drink free every night & happy hours anytime. Check it out on App Store & Google Play & discover for yourself the easiest & the most economical way to enjoy parties at best bars in Delhi.

    Not only your party experience is better with Boozeey but Boozeey takes care of a safe ride home for you. The Boozeey app has in-built OLA cabs integration that will allow you to book an OLA cab to reach or depart from the bar location. So if you are living in Delhi NCR download the Boozeey app & enjoy exclusive access to best bars & pubs in Delhi.

    To know more about Boozeey visit the given link - http://boozeey.com/
    Connect to Boozeey through given links:
    Facebook- https://www.facebook.com/boozeey/
    Blog-best bars in delhi

    ReplyDelete