ty le ca cuoc 188bet QUALITY ASSESSMENT OF HYDROCARBON PVT DATA

Summary

PVT data are essential for proper field development plan, accurate reserve estimation, reservoir simulation, material balance calculation, and optimum fluid recovery plans. High quality and accurate PVT data can, therefore, reduce uncertainty in reservoir fluid properties and set ty le ca cuoc 188bet stage for reservoir engineering modelling whilst improving ty le ca cuoc 188bet technical work on which investment decisions are made. Poor quality data will, however, result in lost time due to rework and additional studies, poor development plans, and inefficient investment.

This paper focuses on ty le ca cuoc 188bet testing of ty le ca cuoc 188bet quality of ty le ca cuoc 188bet PVT data from DH-8X-DST#4 performed in ty le ca cuoc 188bet lab based on Hoffman plot, material balance, and Buckley plot.

Key words: PVT, material balance, Hoffman, Buckley, Dai Hung field.

1. Introduction

Quality control (QC) of PVT data is very important. Improper quality control or any error in ty le ca cuoc 188bet recorded data can be misleading in ty le ca cuoc 188bet next steps of fluid modelling,  fluid properties measurement, and construction of thermodynamic model for representative reservoir fluid.  It is, therefore, important to conduct fluid sampling and recombination using standard protocols to minimise  errors and uncertainties. In this paper, we introduce quality control of PVT measurements on collected gas and liquid samples within a reservoir to select ty le ca cuoc 188bet representative reservoir fluid. After that, ty le ca cuoc 188bet representative reservoir fluid will be used for simulation.

2. Methodology

a) Graphical  technique

Graphical plots [1] of ty le ca cuoc 188bet non-compositional data as a function of pressure were made. ty le ca cuoc 188bet following properties were plotted against pressure to check ty le ca cuoc 188bet quality of ty le ca cuoc 188bet PVT data: relative volume, Y-function, density, solution GOR and oil FVF, ty le ca cuoc 188bet graphs are expected to show a smooth trend.

b) Hoffman plot

ty le ca cuoc 188bet Hoffman plot [1] is another method of evaluating ty le ca cuoc 188bet quality of compositional data. This method utilises a log-linear plot of K-value versus Hoffman factor, F. F (psi/ oR) is defined as follows:

A plot of Log (K) versus F for each component of ty le ca cuoc 188bet separator liquid and reservoir fluid compositions will   be expected to yield a linear graph which implies high quality data where all points plotted are expected to fall close to ty le ca cuoc 188bet line of best-fit.

Equilibrium ratios trend (K): ty le ca cuoc 188bet  equilibrium  ratio  [2], Ki, of a given component in a hydrocarbon mixture is defined as ty le ca cuoc 188bet ratio of ty le ca cuoc 188bet mole fraction of ty le ca cuoc 188bet component in ty le ca cuoc 188bet gas phase, yi, to that of ty le ca cuoc 188bet mole fraction of ty le ca cuoc 188bet component in ty le ca cuoc 188bet liquid phase, xi. Mathematically, ty le ca cuoc 188bet relationship is expressed as:

ty le ca cuoc 188bet K-value measures ty le ca cuoc 188bet tendency of ty le ca cuoc 188bet component to remain or escape to ty le ca cuoc 188bet gas phase. It is essentially a property that measures ty le ca cuoc 188bet volatility of ty le ca cuoc 188bet component at a pressure and temperature. This tendency suggests that in a multicomponent system, components’ Ki-value should follow ty le ca cuoc 188bet trend of their decreasing K-values with ty le ca cuoc 188bet increasing order of their normal boiling point; that is

c) Material balance

ty le ca cuoc 188bet material balance test [1] was used to evaluate compositional consistency. It was applied on ty le ca cuoc 188bet separator liquid and reservoir fluid compositions. ty le ca cuoc 188bet material balance equation is expressed  below.

A plot of yi/zi against xi/zi should give  a  straight  line  with  a  slope  of  –L/V and an  intercept  of  F/V.  Any  deviations  from  a straight line reflect material balance discrepancies. Reservoir fluid compositions are typically calculated from a mathematical recombination of flashed vapour and liquid compositions.  When  feed   compositions are determined from a mathematical recombination of ty le ca cuoc 188bet separator vapour and separator liquid compositions, a perfectly straight line should be observed  in  ty le ca cuoc 188bet  plot. In this case, ty le ca cuoc 188bet plot is sometimes useful for identifying discrepancies in ty le ca cuoc 188bet reported compositions. ty le ca cuoc 188bet reciprocal of  ty le ca cuoc 188bet slope of ty le ca cuoc 188bet line can then be used to calculate ty le ca cuoc 188bet GOR and compare that  with ty le ca cuoc 188bet  reported  GOR.  ty le ca cuoc 188bet  liquid  density  and molecular  weight  must  be  provided so that  ty le ca cuoc 188bet  conversion  from  moles  to barrels can be made. When ty le ca cuoc 188bet feed composition   is truly independent from ty le ca cuoc 188bet vapour and liquid compositions, ty le ca cuoc 188bet material balance test provides a good check on ty le ca cuoc 188bet overall consistency of ty le ca cuoc 188bet analyses [3].

d) Buckley plot

ty le ca cuoc 188bet Buckley plot (Figure 1) is expressed by ty le ca cuoc 188bet semi logarithmic plot of K-value versus ty le ca cuoc 188bet square of ty le ca cuoc 188bet critical temperature Tc of ty le ca cuoc 188bet reservoir fluid [4]. It gives a straight line with negative slope for ty le ca cuoc 188bet light hydrocarbon components and any significant deviation indicates possible non-equilibrium separation or numerical errors in ty le ca cuoc 188bet data reporting. As a component becomes less paraffinic then ty le ca cuoc 188bet deviation from ty le ca cuoc 188bet linearity becomes greater.

3. QC of DH-8X-DST#4 Sand 0

ty le ca cuoc 188bet result of PVT analysis from DH-8X- DST#4 is shown in Table 1 - 3.

Based on Table  2,  ty le ca cuoc 188bet  equilibrium ratio for each component  was  calculated as shown in Figure 2, which indicates a consistency in ty le ca cuoc 188bet laboratory-reported compositional analysis.

Figure 3 is ty le ca cuoc 188bet resulting material balance plot when yi/zi is plotted against xi/zi. In this case, ty le ca cuoc 188bet plot from ty le ca cuoc 188bet material balance of separator liquid and reservoir fluid compositional data shows a linear trend. This confirms ty le ca cuoc 188bet high quality and accuracy of compositional data obtained. ty le ca cuoc 188bet slope of ty le ca cuoc 188bet line in ty le ca cuoc 188bet figure is -1.4248, which can be converted to a GOR of 1,102.5 scf/bbl by taking ty le ca cuoc 188bet negative reciprocal of ty le ca cuoc 188bet slope and converting it to ty le ca cuoc 188bet measured unit. This calculated GOR is about 0.01% lower than ty le ca cuoc 188bet reported laboratory value of 1,103.3 scf/bbl. ty le ca cuoc 188bet difference is very small, indicating that ty le ca cuoc 188bet liquid molecular weight, liquid density, and recombination ratio all appear reasonably consistent with ty le ca cuoc 188bet reported GOR.

ty le ca cuoc 188bet Hoffman plot is a standard technique for evaluating ty le ca cuoc 188bet consistency of ty le ca cuoc 188bet recombined fluid samples through a graphical technique. Usually ty le ca cuoc 188bet plus fraction  (e.g.  heptane-plus  (C7+))  is  not included in Hoffman consideration because it is generally not measured accurately [2]. To further control ty le ca cuoc 188bet quality of ty le ca cuoc 188bet recombined compositions listed in Table 2, ty le ca cuoc 188bet compositions were used to generate ty le ca cuoc 188bet Hoffman plot as shown in Figure 4, which confirms that ty le ca cuoc 188bet separator liquid and reservoir compositional data are of high quality, as ty le ca cuoc 188bet plotted points lie close to ty le ca cuoc 188bet line of best fit for components C1 through C6 for all pressure.

ty le ca cuoc 188bet plot from Buckley plot (Figure 5) is expressed by ty le ca cuoc 188bet relationship of Log K versus Tc for ty le ca cuoc 188bet reservoir fluid. A semi log plot of K value versus ty le ca cuoc 188bet square of ty le ca cuoc 188bet critical temperature gives a straight line with a negative slope for ty le ca cuoc 188bet hydrocarbon components (C1 - C6). This confirms ty le ca cuoc 188bet high quality and accuracy of ty le ca cuoc 188bet compositional data obtained.

4. Conclusions

In order to obtain trustworthy PVT fluid properties, ty le ca cuoc 188bet quality of ty le ca cuoc 188bet samples must be examined carefully. Three other techniques for evaluating ty le ca cuoc 188bet consistency of flash and composition data have also been applied for a long time, but they are still very useful today. These methods are material balance, Hoffman and Buckley plots. ty le ca cuoc 188bet material balance plot can be used as a rigorous criterion for consistency while ty le ca cuoc 188bet Hoffman plot and ty le ca cuoc 188bet Buckley plot are considered qualitative assessment of data quality.

Application of this methodology on ty le ca cuoc 188bet PVT data from DH-8X-DST#4 under study showed that ty le ca cuoc 188bet samples can be used as representative for ty le ca cuoc 188bet whole reservoir.

References

1. Nnaemeka     Nnamdi Nnabuo. Interpretation of laboratory PVT analysis result (a case study of a Niger delta field). SPE 172412-MS. SPE Nigeria Annual International Conference and Exhibition, Lagos, Nigeria. 5 - 7 August, 2014.

2. Tarek Ahmed. Equations of state and PVT analysis. Applications for Improved Reservoir Modelling. 2016.

3. John J. Lawrence. Qualityassessmentand consistency evaluation of hydrocarbon PVT data. SPE 13784-MS. SPE International Petroleum Technology Conference held in Doha, Qatar. 7- 9 December, 2009.

4. Julius Akpabio, Sunday O.Isehunwa, Oluwatoyin Akinsete. PVT fluid sampling, characterization and gas condensate reservoir modeling. Advances  in  Research.  2015;  5(5):  p. 1 - 11.

5. PVEP POC. PVT report of DH-8X-DST#4.

6. John M.Williams. Getting ty le ca cuoc 188bet best out of fluid samples. SPE 29227-PA. 1994; 46(9).

7. Alan Graham Stephen, David F.Bergman, Tim Dodd, Wayne Kriel. PVT data quality: Round Robin result. SPE 116162-MS. SPE Annual Technical Conference and Exhibition. Denver, Colorado, USA. 21 - 24 September, 2008.