Data Quality Checks (QC)

Once all sample data has been filled in from the PVT report, and all experiments have been added, whitsonPVT provides quality checks (QCs), to assess the quality and feasibility of data and experiments. While checks are not mandatory and you can move on to EOS modeling without them, quality checks give a good indication of and provide a way to quickly evaluate whether you should go back and check over your input values, or if there may have been a lab error. It will also show the QC status in the Quality Checks column in the PVT Samples Overview, letting you know whether you should be weary or more attentive when looking at a specific sample.

The Quality Check tab is located next to the PVT Experimental Data tab in the overhead ribbon. Once you have navigated to the Quality Check tab, running quality checks is as easy as pressing the blue Run Quality Checks button. After the quality checks have run, three sections will pop up. These sections are split into the user defined status, a section for the status evaluation and QC Name, as well as a graph section.

User Defined Status

The user-defined status section allows you as the user to evaluate and override whether quality checks are of Good, Intermediate, or Bad quality and requires a message to be attached. If the software evaluates QCs to be of bad quality but you as the user think they are acceptable, you can select the good quality option from the dropdown, which will show a green checkmark in the Quality Checks column for the sample in the PVT Samples Overview.

Status and QC Name

Directly below the User Defined Status section, you will see a box with the Status and QC Name. This section shows the QC status from bad to good in the status column and next to it, the associated experiment name. QCs are run for compositional data, and any added experiments. This section also provides a drop-down arrow, to show the breakdown of each QC, allowing you to see which parts were good or which ones were bad. Additionally, if a QC is not evaluated as good, you can hover over either the yellow or red exclamation mark to see why it has not gotten a green check. If part of a QC does not get an evaluation and does not run, there will be a blue information circle in place of the usual evaluation symbols. Using your cursor, you can hover over this QC in the drop-down menu, and it will show why it did not run.

Graphs

The last Section under the Quality Checks tab is the graphs section. This section shows graphs corresponding to each QC. While starting with the Hoffman graph for the QC of the Recombination of Flashed Fluids, there is a graph corresponding to each part of every QC. Each one can be selected by clicking the corresponding QC part from the drop-down menus. Each graph will show the experiment and what either went wrong or what is correct. For example, if the backward material balance was incorrect and one component was in excess mol percent, the graph shows this in the Total Composition plot type, showing which component was outside the tolerance, which by default is set to 1 percent for backward material balance. The graphs reflect the feedback that is given when hovering above the QC evaluation symbols.

Data QC Tests

Hoffman Check

The Hoffman Check is used to validate your flashed fluid recombination. It checks if the fluid composition reflects the actual well stream conditions. The Hoffman KpF plot, plots log(K_i*p) against F_i. The equations used to find K_i and F_i are given below with the equation for b_i which is used in the formula for F_i also being given:

$$\begin{equation}\label{eq:component_characterization_factor} F_i = b_i(\frac{1}{T_{bi}}-\frac{1}{T}) \end{equation}$$

Where b_i is given by the equation below, T_bi is the normal boiling point temperature of component i, and T is absolute temperature.

$$\begin{equation}\label{eq:vapor_pressure_constant} b_i = \frac {log\frac{p_{ci}}{p_{sc}}}{\frac{1}{T_{bi}}-\frac{1}{T_{ci}}} \end{equation}$$

Where p_ci is the critical pressure of component i, p_sc is the pressure at standard (atmospheric) conditions, T_bi is the normal boiling point of component i, and T_ci is the critical temperature of component i.

$$\begin{equation}\label{eq:k_value} K_i = \frac{y_i}{x_i} \end{equation}$$

Where y_i is the mole fraction of component i in the vapor phase and x_i the mole fraction in the liquid phase.

The value that should be used for p in log(K_i \(\cdot\) p) depends on whether the sample is a recombined sample or a separator recombined sample. In the case of a recombined sample, p should be 1 atm while for a separator sample it is the separator pressure. The result of the Hoffman plot should be a straight line, indicating that the components are indeed in equilibrium.

Recombination Factor

The recombination factor is used to check the consistency of your data by making sure the lab measurements obtained match the compositions which should mathematically result if the data is chemically consistent. whitsonPVT graphs the mole fraction of the vapor phase on the y-axis against that of the liquid phase on the x-axis, comparing the corrected (mathematically calculated) data to the lab obtained data. When looking at the graph to see if successful, look for a perfectly linear line, with the corrected and lab data overlapping as closely as possible. The equations used in the graph are given below.

Bulk Molar Recombination Factor:

$$\begin{equation}\label{eq:molar_recombination_factor} F_g = (1+\frac{2130 \cdot \rho_{\bar{o}}}{M_{\bar{o}} \cdot GOR})^{-1} \end{equation}$$

Where \(\rho_{ \bar{o}}\) is stock tank oil density, \(M_{ \bar{o}}\) is stock-tank oil molecular weight, and GOR is the total Gas-Oil ratio. F_g is the bulk molar recombination factor, representing the overall mole fraction of gas in the reservoir fluid.

Component Molar Recombination Factor:

$$\begin{equation}\label{eq:component_molar_recombination_factor} F_{g,i} = \frac{z_i - x_i}{y_i - x_i} \end{equation}$$

Where z_i is the total composition fraction, y_i is the vapor phase composition fraction, and x_i is the liquid phase composition fraction.

Gas Composition Check

The purpose of the gas composition check is to make sure the composition of the gas stream is accurate and correct. It ensures the vapor phase data is correct, to make sure error is not carried over into the recombined stream. The equation used to get the mole ratio of the gas phase, including the Lever Rule, which the equation is derived from are shown below:

Lever Rule:

$$\begin{equation}\label{eq:lever_rule} z_i = F_g \cdot y_i + (1-F_g) \cdot x_i \end{equation}$$

Where F_g is the fraction of the total system that is in the vapor phase, y_i is the vapor phase composition of component i, with x_i being the liquid phase composition. (1 - F_g) give the remaining fraction of the system in the liquid phase, and this is all used to calculate z_i, the total composition of component i in the fluid.

Gas Mole Equation:

$$\begin{equation}\label{eq:gas_mole_equation} \frac{y_i}{z_i} = (\frac{F_g - 1}{F_g}) \cdot \frac{x_i}{z_i} + \frac{1}{F_g} \end{equation}$$

Where F_g is again the fraction of the total system that is in the vapor phase, x_i is the liquid phase composition, z_i is the total composition of component i in the fluid and y_i is the vapor phase composition of component i. y_i/z_i represents the ratio of the gas composition to the total composition of the component in the fluid. When checking the QC plot within whitsonPVT, look for there to be no more than a magnitude of 1 mole% difference. If there is a difference greater than a magnitude of 1 mole%, you may need to check your data to ensure it is correct.

Oil Composition Check

The oil composition check, make sure the composition of the liquid stream is correct. Ensuring the liquid phase data is correct, similarly to the gas composition check, it makes sure there is no error carried over into the recombined stream. The equation used to get the mole ratio of the liquid phase is shown below. The liquid mole equation below is based off of the lever rule given in equation \eqref{eq:lever_rule}.

Liquid Mole Equation:

$$\begin{equation}\label{eq:liquid_mole_equation} \frac{x_i}{z_i} = (\frac{F_g}{F_g - 1}) \cdot \frac{y_i}{z_i} + \frac{1}{F_g - 1} \end{equation}$$

x_i/z_i represents the ratio of liquid composition to the fluid's total composition. y_i/z_i is the ratio of gas composition to total composition, and F_g is the fraction of the total system in the vapor phase. Identical to the gas composition check, there should be no greater difference than 1 mole%, with a larger difference indicating you may need to check your inputted data.

Total Composition Check

The purpose of the total composition check is to confirm whether the composition of the overall reservoir fluid is mathematically and physically consistent with the obtained lab data. To do this, the lab-measured liquid and gas compositions are mathematically recombined, resulting in a calculated total composition which is then compared directly to the lab measured total composition of the reservoir fluid. This is calculated by multiplying the total mole fraction of the gas phase by the mole fraction of component i in the gas, then adding the total mole fraction of the liquid phase multiplied by the mole fraction of component i in the liquid phase. This is done as shown below:

Equation for Delta Z_i

$$\begin{equation}\label{eq:delta_z} \Delta z_i = z_i^{calc} - z_i^{lab} \end{equation}$$

Where z_i^calc is the mathematically calculated recombined total mole fraction of component i and z_i^lab is the lab reported value.

Total Recombination Formula

$$\begin{equation}\label{eq:total_recombination} z_i^{calc} = F_g \cdot y_i + F_o \cdot x_i \end{equation}$$

Where y_i is the lab reported mole fraction of component i in the gas phase and x_i the lab reported mole fraction of component i in the liquid phase. F_g and F_o are the total mole fraction of the gas phase and the total mole fraction of the liquid phase respectively. Their equations are given below.

Total Gas Mole Fraction Equation

$$\begin{equation}\label{eq:Total_Gas_Mole_Fraction_Equation} F_g = \frac {n_g}{n_{total}} \end{equation}$$

Where n_g is the moles of gas and n_total is the total moles.

Total Liquid Mole Fraction Equations

$$\begin{equation}\label{eq:Total_Liquid_Mole_Fraction_Equation} F_o = \frac {n_o}{n_{total}} \end{equation}$$

$$\begin{equation}\label{eq:Total_Liquid_Mole_Fraction_Equation_b} F_o = 1 - F_g \end{equation}$$

Where n_o is the moles of liquid and n_total is again the total moles. F_g is the total gas mole Fraction.

Similarly to the gas and oil composition checks, look for the graph of the total composition check to show no more than a 1 mol% difference for any component. If there is a difference greater than 1 mol%, there may be an issue with the data.