NGL Calculation

The NGL Calculation requires 3 inputs:

1. Reservoir fluid
2. Surface process
3. NGL efficiencies

The main objective of this feature is to calculate NGL yields and wellhead gas shrinkage. These are the two values required to split oil and gas rate forecasts (type curves) into three sales products (sales gas, sales NGL and sales oil).

NGL Yield

NGL yield is defined as volume of NGL per volume of "feed gas". The volume of "feed gas" is typically assumed to be the total wellhead gas produced.

$$\begin{equation} NGL_{yield} = \frac{q_{NGL}}{q_{gas,wellhead}} \end{equation}$$

Wellhead gas shrinkage

Wellhead gas shrinkage quantifies how much the wellhead gas shrinks after NGLs are separated from it.

$$\begin{equation} SF_{gas,wellhead} = \frac{q_{g,sales}}{q_{gas,wellhead}} \end{equation}$$

Relevant Equations

The following inputs are given:

1. amount of surface gas (\(n_{gas}\))
2. composition of surface gas (\(y_{i}\))
3. NGL efficiency (e)

From this, NGL yield is calculated

1. \(n_{gas, sales, i}\) = \(n_{gas}\) * \(y_{i}\) * (1-\(e_{i}\))
2. \(n_{ngl, sales, i}\) = \(n_{gas}\)* \(y_{i}\) * \(e_{i}\)
3. \(n_{gas, sales, tot}\) = sum(\(n_{gas, sales, i}\))
4. \(n_{ngl, sales, tot}\) = sum(\(n_{ngl, sales, i}\))
5. \(y_{gas, sales, i}\) = \(n_{gas, sales, i}\) / \(n_{gas, sales, tot}\)
6. \(x_{ngl, sales, i}\) = \(n_{ngl, sales, i}\) / \(n_{ngl, sales, tot}\)
7. \(q_{gas, sales}\) = \(n_{gas, sales, tot}\) * \(v_{ideal gas molar volume}\)
8. \(MW_{NGL}\) = sum(\(MW_{i}\) * \(x_{ngl, sales, i}\)) / sum(\(x_{ngl, sales, i}\))
9. \(SG_{NGL}\)= sum(\(MW_{i}\)* \(x_{ngl, sales, i}\)) / sum(\(MW_{i}\) * \(x_{ngl, sales, i}\) / \(SG_{i}\))
10. \(q_{ngl, sales}\) = (\(n_{ngl, sales, tot}\) * \(MW_{NGL}\)) / (\(SG_{NGL}\) * \(\rho_{water}\))

Ethane Rejection vs. Ethane Recovery

When demand for ethane, and therefore ethane prices, are low relative to dry natural gas prices, natural gas processing plant operators may choose to leave ethane in the processed natural gas (provided the processed natural gas meets pipeline specifications) and sell it into the natural gas market at its heat value. This process is known as ethane rejection.

When ethane prices rise higher than natural gas prices on a heat-value equivalent basis, natural gas processing plant operators may choose to recover the ethane along with other NGPLs and sell it at their market value into the petrochemical sector. This process is known as ethane recovery.

Most recovery units are built for both ethane recovery and ethane rejection, and when operated in ethane rejection mode the ethane recovery (NGL efficiency) is low (e.g. 40%), while when operated in ethane recovery mode the ethane recovery (NGL efficiency) is high (e.g. 90%).

References

[1] U.S. Energy Information Administration 2021, accessed November 18, 2021.

[2] Paradowski, H., Le-Gall, A., and B. Laflotte. "Compare the Different Options for NGL Recovery from Natural Gas." 2005.