Sales product

5.1. Definition

Wellhead Gas

• The total gas produced at the wellhead before it is processed into sales gas and natural gas liquids (NGLs). It represents the gas stream entering the NGL plant and therefore includes both the gas that will ultimately remain as sales gas and the gas components that will be recovered as NGLs during processing.

• Wellhead gas is calculated from the separator oil production, separator gas-oil ratio (GOR), Separator Oil Shrinkage Factor (SF), and Flash Factor (FF). Since it accounts for the gas released from the separator oil when flashed to stock-tank conditions, the wellhead gas rate is generally slightly different from the separator gas rate ($q_{g,\mathrm{sep}}$).

The calculation is performed as follows:

$$\begin{equation} q_o = q_{o,\mathrm{sep}} \times SF \end{equation}$$

$$\begin{equation} GOR_{tot} = \frac{GOR_{sep}}{SF} + FF \end{equation}$$

$$\begin{equation} q_{g,\mathrm{wh}} = GOR_{tot} \times q_o \end{equation}$$

where:

• $q_{o,\mathrm{sep}}$ = Separator oil rate
• $SF$ = Separator Oil Shrinkage Factor
• $GOR_{sep}$ = Separator gas-oil ratio
• $FF$ = Flash Factor
• $q_{g,\mathrm{wh}}$ = Wellhead gas rate

Sales Gas

• The final gas product after the NGL plant.
• Stripped of heavier hydrocarbons (NGLs), making it drier and pipeline-ready, i.e. sales gas.

Fat Gas (a.k.a. Wet Gas)

It is an artificially reported stream in some production databases. It basically recombines:

• Gas coming out of the separator (Wellhead Gas)
• Condensate from the separator (converted to gas-equivalent)

Important: this stream does not exist as a real gas stream but is mostly used for reporting, to track total hydrocarbon content as a single gas stream.

$$\begin{equation} q_{g,fat}=\frac{q_{g,wellhead}}{F_{g}} \end{equation}$$

where $q_{g,fat}$ is the fat gas rate, $q_{g,wellhead}$ is the wellhead gas rate measured at the separator outlet, and $F_g$ is the gas recovery factor of the NGL plant, representing the fraction of the fat gas volume that remains in the sales gas stream after NGL extraction.

5.2. Natural Gas Liquids (NGL)

In this module, the daily NGL rate (\(q_{NGL}\)) and NGL yield are predicted. The required user input is NGL efficiencies (\(e\)).

5.2.1. Recovery Method

The process of removing NGLs is acknowledged as NGL recovery. There are four methods available in whitson⁺ to calculate NGL recovery:

1. Dewpoint Control: controls hydrocarbon and water dew points of natural gas through mechanical refrigeration.
2. Cascade Refrigeration: uses two refrigeration circuits thermally connected by a cascade condenser.
3. Gas Sub-cooled Process (GSP) Deep Recovery: develops a source of reflux to increase \(C_2\) recovery with the conventional design.
4. Input Efficiencies

Each method has a list of components according to the EOS model used. The default efficiencies for each method are given in Table 3. Using one of these default values is a good starting point for the NGL efficiency, and the values can be changed as desired.

Component	Dewpoint Control	Cascade Refrigeration	GSP Deep Recovery	Input Efficiencies
\(N_2\)	0	0	0	0
\(CO_2\)	0	1	1	0
\(H_2S\)	1	1	1	0
\(C_1\)	0	0	0	0
\(C_2\)	0.25	0.65	0.95	0.5
\(C_3\)	0.80	0.97	0.99	1
\(C_{4+}\)	0.97	1	1	1

Table 3. Default efficiencies for different NGL recovery methods.

5.2.2. Calculations

To predict the NGL yield and NGL rate correctly, the following inputs are fundamental:

• Daily measured oil and gas rates (\(q_o\) and \(q_g\))
• Daily gas moles (\(n_{gas}\))
• Daily composition of surface gas (\(y_i\))
• NGL efficiencies (\(e\))

Gas and oil production rates are used to calculate \(n_{gas}\) and (\(y_i\)). In whitson⁺, \(n_{gas}\) and (\(y_i\)) are calculated in the compositional tracking module.

Mole of NGL sales is written as:

$$\begin{equation} \label{eq:nngl} n_{ngl,sales,i}=n_{gas}\cdot y_i\cdot e_i \end{equation}$$

The liquid phase mole fraction of each component can be defined as:

$$\begin{equation} \label{eq:xngl} x_{ngl,sales,i}=\frac{n_{ngl,sales,i}}{\sum n_{ngl,sales,i}} \end{equation}$$

NGL molecular weight and specific gravity are expressed as follows:

$$\begin{equation} \label{eq:mwngl} MW_{ngl}=\frac{\sum \left(MW_i\cdot x_{ngl,sales,i}\right)}{\sum x_{ngl,sales,i}} \end{equation}$$

$$\begin{equation} \label{eq:sgngl} SG_{ngl}=\frac{\sum \left(MW_i\cdot x_{ngl,sales,i}\right)}{\sum \left(\frac{MW_i\cdot x_{ngl,sales,i}}{SG_i}\right)} \end{equation}$$

Finally, the NGL rate and thus NGL yield can be solved for each production day as:

$$\begin{equation} \label{eq:qngl} q_{ngl,sales}=\frac{MW_{ngl}\cdot \sum x_{ngl,sales,i}}{SG_{ngl}\cdot \rho_{water}} \end{equation}$$

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

The units of NGL rate are STB/d and NGL yield is STB/MMscf (or gallon per 1,000 cubic feet, GPM).

5.2.3. 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 it 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%).