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PVT and Flow course - Lab PVT Tests DLE

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Lab PVT Tests DLE

The differential liberation experiment is conducted only for oils. It's the same process if you will as the SEP test BUT the temperature at each stage remains equal to the reservoir temperature. The whole experiment is going on at reservoir temperature.
The number of stages probably going to be five to ten.

The purposes of this experiment:

  1. to study how the oil density varies as a function of pressure below the bubble point. There's a parallel experiment that's run, it's also a differential liberation and we might as well say it's done at the same time, it's just that it's not) but it's essentially the same experience, we're also interested in how the viscosity varies at pressures at and below the bubble point.
  2. how much the oil shrinks.
  3. the composition of the oil.

These are the main things that we want to know: oil density, oil viscosity and how the oil shrinks.
Likewise you want to know the same for the gas.
Effectively because Sg is basically 1 - So (forgetting the water) it's basically if you know the oil shrinkage you know the gas volume. But what you would like to know is how much gas is released as we go below the bubble point. And then finally we might be interested and probably more so in the composition of the gas that pressures below the bubble point. That gas starts larger, saturation the gas starts flowing more and more and more because the gas saturation is increasing and the composition of that gas is important because it's carrying with it some of the oils that we're going to make turn into a liquid at the surface. It's the gas released in the reservoir, you want to know the gas released in the reservoir.

Basically we will know PVT properties at reservoir conditions: TR and P ≤ bubble point. Nothing at all about the surface, nothing at all about how much oil we're going to sell.

We use a blind cell, it's a simple experiment, the gas that's removed do you basically take this through a flash process to standard conditions you go through a gas meter to estimate basically the equivalent surface gas volume that's been removed, typically we don't worry too much about the liquid that condenses from the gas in in most experiments.

Differential Liberation

Watch the full video

Lab PVT Tests DLE (20160223 Part 1)

In this video, Curtis Whitson talks about fluid properties - what is measured, what is calculated, and what is reported.
He also talks about the importance of the final stage: difference between bleed and flash processes.

Watch the full video

Lab PVT Tests DLE (20160223 Part 2)

The last topic on oil PVT is using DLE data together with the multistage separator test data with the purpose of generating tables of black oil PVT properties for engineering calculations.
All petroleum engineering calculations are based on these black oil tables: reservoir (material balance, rate equations, reservoir simulation), production and pipe flow (flow assurance, pressure drop calculations).

We need for oil phase:

  • Bo = Oil FVF
  • Rs = Solution GOR
  • Oil viscosity

The lab step experiment is of the original oil right, but here we're talking about any oil at or below the bubble point. So from the Sep test what we get is what we call Bob (oil volume at the bubble point over the surface volume) and Rsb.
We need Bo(P<Pb) and Rs(P<Pb) for saturated oils. The question is as we deplete below the bubble point how does the Bo and RS change? That's what we want to estimate.

The standard method to do this (not necessarily the best method but it's the standard method) is that we take the differential Bo from the DLE experiment times the bubble point from the separator test divided by differential at the bubble point:
Bo(P) ≈ Bod (P) * Bob / Bod (Pb)

  • Bo(P) - Bo as a function of pressure
  • Bod (P) - Bo from the DLE at the pressure below bubble point
  • Bob - Bo from the SEP test
  • Bod (Pb) - Bo from the DLE at the bubble point

You can't use the differential because it is not how we process, we don't keep reservroir temperature at the proccess separator.

For Rs the equations looks like this:
Rs(P) ≈ Rsb - (Rsd,b - Rsd(P))*Bob/Bod,b

  • Rsb - Rs from the SEP test
  • Rsd,b - Rs from the DLE at the bubble point
  • Rsd(P) - Rs from the DLE at pressure P
  • Bob - Bo from the SEP test
  • Bod,b - Bo from the DLE at the bubble point

The method is OK for Rsb < 1000 scf/bbl. But almost nobody nobody does this anymore, it is an old school.

The oil viscosity you just use straight because it's measured at pressure and temperature. There's no surface process involved.

Watch the full video

Other lectures from the PVT and Flow course

Class notes developed during lectures are available as PDF files, named with the format yyyymmdd.pdf located on:

See also

PVT tests
Curtis Whitson Petroleum Engineering Videos

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