Application kèo nhà cái 188bet geochemical technique to reduce allocation cost for commingled production wells from multiple reservoirs

Summary

This paper presents a new technique which has been developed to achieve production allocation at a reduced cost by the Vietnam Petroleum Institute (VPI). A case study kèo nhà cái 188bet using geochemical fingerprint technology to back allocate commingled well stream from multiple reservoirs in an offshore basin, southeast Vietnam has been carried out. A large number kèo nhà cái 188bet oil samples were collected over time and analysed by gas chromatography for their fingerprint and production allocation. Quantitative results were attained using a proprietary computer program which mathematically calculates the relative contributions kèo nhà cái 188bet end member oils based on chromatographic data. The geochemical allocation results compare very favourably with the metering data. The success kèo nhà cái 188bet the geochemical allocation allows the cost kèo nhà cái 188bet field operation to be reduced and effective support to be given to the production management.

Key words: Back allocation, gas chromatography, geochemical finger-printing.

1.    Introduction

Crude oils originating from different producing zones, wells, or fields are mixed through commingling operation for reducing operation cost, improving production efficiency and field management. For technical reasons, it is sometimes necessary to quantitatively deconvolute these mixtures to facilitate monitoring kèo nhà cái 188bet individual zones, wells, or fields. For example, where multiple pipelines commingle, an accurate assessment kèo nhà cái 188bet individual field contributions may be essential for establishing sales value or tax liability because oil quality or tax rates can be different between neighbouring fields that share a transport pipeline to terminal facilities. The ability to back allocate helps in effectively managing reservoirs and production operations.

Normally, field/reservoir allocation is done by monitoring individual field production with flow meters  or by MPLT (Memory Production Logging Test) running. However,inthiscase,therewasalackofdedicatedmetering for production from some fields/reservoirs during a period kèo nhà cái 188bet time. Moreover, the tax liability was different for those neighbouring fields that shared a pipeline to transport oil to terminal facilities. Thus, it was necessary to investigate alternative methods for reallocating production from these fields/reservoirs in order to determine an accurate field/reservoir contribution. The use kèo nhà cái 188bet geochemical fingerprinting technique presented in this report is one method to re-assess and crosscheck individual field/ reservoir contribution to commingled pipeline oils.

Geochemical-based oil allocation has been applied for allocation kèo nhà cái 188bet commingled production from multiple zones and production from multiple fields to a single stream [1, 2]. This method uses whole-oil chromatography distinctive peak height ratios to quantify the contribution kèo nhà cái 188bet each individual production stream to a commingled production stream. The distinctive compounds are natural fingerprints for oil classification or oil grouping and used for proration kèo nhà cái 188bet each end-member oil in the commingled oil [3].

2.    Methodology

The methods are based on the assumption that oils from separated reservoirs or different parts kèo nhà cái 188bet a reservoir bear different chemical signatures or distinctive chemical fingerprints.  At  the  reservoir   scale,   steep   gradients   in  oil  composition  and   associated   fluid   properties   are understood to be the product kèo nhà cái 188bet preferential biodegradation kèo nhà cái 188bet different hydrocarbons, which gives  oils a distinct molecular signature or “fingerprint” related to the level kèo nhà cái 188bet degradation. This natural variability in oil composition can be used to allocate oil production along horizontal wells or to assess the contribution kèo nhà cái 188bet different production streams in commingled wells by mapping the original oil composition distribution.

Each oil type is  identified  and  clarified  through  a  data  set kèo nhà cái 188bet physicochemical  properties  named  fingerprints.  Depending  on different targets, oil fingerprints are determined by gas chromatography, biomarker analysis, IR, UV-VIS instruments, trace element, and isotope analysis which can lead to high capacity kèo nhà cái 188bet oil type identification. For example, the original oil type kèo nhà cái 188bet gasoline and oil spill can be determined by fingerprint analysis.

Using whole oil gas chromatography fingerprints in order to identify oil type is the most popular method which uses a set kèo nhà cái 188bet peak height ratios to identify oil type. The height or area kèo nhà cái 188bet a GC (gas chromatography) peak reflects the concentration kèo nhà cái 188bet a hydrocarbon component kèo nhà cái 188bet the oil. Due to approximately 1000 peaks in a whole oil gas chromatogram, the distinctive peaks are usually chosen according to experience rather than their chemical nature. Therefore, many sets kèo nhà cái 188bet GC fingerprints exist from different laboratories.

Depending on the oil field, these compositional differences exist for one or more kèo nhà cái 188bet the following 3 reasons [4]:

-    Oils from different sources differ in composition since oils  from different source rocks have different times kèo nhà cái 188bet generation and/or different migration paths.

-    Oil, which a source rock generates at a given time, differs slightly both from subsequently generated oil and previously generated oil due to continuous, subtle  changes  in  the maturity kèo nhà cái 188bet source rock and changes in precisely which part kèo nhà cái 188bet the source rock is in the oil window. Since no two compartments are kèo nhà cái 188bet identical geometry, and since no two compartments have exactly the same filling history, it is difficult to achieve precisely the same homogenised composition in two separate compartments - even with oil from the same source.

-    Processes that affect oil composition after oil enters a reservoir (such as biodegradation, water washing, evaporative fractionation) do not operate to exactly the same extent in separate compartments.

Figure 1. A whole oil gas chromatogram [5]

Figure 2. Commingled production and allocation [6]

If two oil zones (1 and 2) were commingled, the respective contributions kèo nhà cái 188bet the zones to the commingled sample could be determined by identifying the chemical differences between end-member oils, with the end member being pure samples from core or cuttings samples from the production well or nearby delineation wells to those kèo nhà cái 188bet the produced oils (1 and 2). The combination kèo nhà cái 188bet mass chromatograms as molecular fingerprint data and direct measurement kèo nhà cái 188bet specific component abundances in oil samples are then made numerically. The data were then used to mathematically express the composition kèo nhà cái 188bet the commingled oil in terms kèo nhà cái 188bet contributions from respective end-member oils.

The relationship between a GC peak height ratio Y (measured in the GC kèo nhà cái 188bet a commingled oil) and the GC peak height ratio X kèo nhà cái 188bet the corresponding peaks in the “m” end- member oils being commingled is given by a linear relationship kèo nhà cái 188bet the form:

In which:

Y = β1X1 + β2X2 + ... βmXm  (1)

Y: The peak height ratio matrix kèo nhà cái 188bet commingled oil;

X1, X2... Xm: The peak height ratio matrix kèo nhà cái 188bet m end-member oils;

β1, β2... βm: The percentages kèo nhà cái 188bet m end- member oils.

Production allocation is the process kèo nhà cái 188bet determining the values kèo nhà cái 188bet β1, β2... βm for end-member oils.

In reality, there are multiple sources kèo nhà cái 188bet error, including the following:

-    Analytical error in measuring the height kèo nhà cái 188bet each GC peak;

-    Error associated with potential contamination kèo nhà cái 188bet GC peaks;

-    Error associated with the non-ideality kèo nhà cái 188bet the samples chosen as end-members.

Because kèo nhà cái 188bet the various sources kèo nhà cái 188bet error, certain GC peaks will do a better job than others at allocating each zone’s contributions to a commingled oil. It is impossible to know, in advance, which GC peaks will do the best job kèo nhà cái 188bet the potentially hundreds available in an oil allocation process.

Figure 3. An example kèo nhà cái 188bet NNMF (non-negative matrix factorisation) algorithm in least square technique [7]

The problem thus becomes one kèo nhà cái 188bet a linear regression. Specifically, given a set kèo nhà cái 188bet samples containing a value for each independent variable and the corresponding value kèo nhà cái 188bet the dependent variable, the “β” values must be computed in a relationship kèo nhà cái 188bet the form:

Y = β1X1 + β2X2 + ... βmXm + E

Where, “E” represents the error that is not captured by the linear relationship.

Linear regression can be used to converge upon a set kèo nhà cái 188bet β value that minimises the sum kèo nhà cái 188bet the square kèo nhà cái 188bet errors (min|E|2).

The advantage kèo nhà cái 188bet geochemical allocation

Geochemical allocation kèo nhà cái 188bet commingled production from multiple zones in  a  single  well typically costs less than 5% as much as conventional e-line production logging. The cost saving is even more dramatic when compared to those for coiled-tubing-conveyed or tractor- conveyed MPLTs.

The low cost kèo nhà cái 188bet geochemical production allocation allows field engineers to monitor output frequently over long time periods (e.g., weekly, monthly, and quarterly), allowing early identification kèo nhà cái 188bet zone performance problems. The much higher cost kèo nhà cái 188bet production logging limits that technique to infrequent use. Therefore, production logging typically provides only a snapshot kèo nhà cái 188bet the production origin at the time the log was run, rather than a continuous performance history [2].

Geochemical fingerprinting techniques are applicable to highly deviated and horizontal wells, in addition to vertical wells. By contrast, production logging interpretation is problematic in highly deviated wells. Geochemical fingerprinting techniques can be applied to all types kèo nhà cái 188bet pumping wells, including those with tubing-deployed electrical submersible pumps, and progressive cavity pumps. Other than  those with unusual completion style, such as Y-block completion, most pumping well cannot accommodate a production logging tool, because the pumping apparatus prevents logging tool’s access to the underlying perforated interval.

The geochemical allocation approach can also be used to assess the contribution kèo nhà cái 188bet multiple fields to commingled pipeline production streams. There are some additional advantages including the ability to allocate in the absence kèo nhà cái 188bet flow meter data, and the ability to identify problems with flow meter data.

Procedure

Oil samples collected from wellhead were taken to the laboratory for geochemical analysis. GC processing was carried out by Chemstation software, chemometric tools and  polynomial  statistic to group oils and identify distinctive pairs kèo nhà cái 188bet peaks for allocation calculation.

3.    Case study kèo nhà cái 188bet geochemical production allocation

Geochemical-based allocation method was developed by the Vietnam Petroleum Institute and has been applied for hundreds kèo nhà cái 188bet oil samples collected from different fields. The paper introduces the application kèo nhà cái 188bet this method for commingled well P01 in A field, offshore Vietnam.

Figure 4. The principal diagram kèo nhà cái 188bet a GC-FID system [5]

Figure 5. Agilent 6890N gas chromatography

Table 1. Oil samples collected from A field

Sampling and analysis

Miocene reservoir, A field, includes 2 pay zones: clastic and carbonate. P01 well started producing in 2011 and has been perforated at both clastic and carbonate reservoirs. In order to determine the oil contribution kèo nhà cái 188bet these two reservoirs to P01 well, clastic and carbonate oil samples from P02 and P03 respectively were collected to be the end-members.

The whole oil samples (Table 1) were fully analysed on Agilent 6890N. Gas chromatograms are input data for Geochemical Allocation software to identify oil group and allocate commingled production.

Qualitative results

Oil - oil correlation is usually evaluated according to [1, 8]:

-    light hydrocarbon distribution (based on C7 isomers);

-    hierarchical cluster analysis (based on whole oil gas chromatograms).

Light hydrocarbon distribution (B-F diagram) was used to determine the differences between Miocene oils produced from the clastic and carbonate reservoirs kèo nhà cái 188bet Afield. Figure

6 shows the correlation between aromatic index (toluene/n-heptane, B) and saturated index   (n-heptane/methylcyclohexane,   F). The  differences  between  oils  produced   from two pay zones are displayed through alkane, aromatic and naphthene hydrocarbon correlation. The negative slope illustrates the secondary alternation such as evaporative fractionation, thermal cracking, water washing, and biodegradation [8]. When this slope translates to the right, the maturity kèo nhà cái 188bet source rock generating these oils increases.

Figure 6. B-F diagram kèo nhà cái 188bet wells P01, P02 and P03

Figure 7. Hierarchical cluster analysis diagram kèo nhà cái 188bet VPI-P01, VPI-P02 and VPI-P03

As shown in the B-F diagram,  the  samples kèo nhà cái 188bet VPI-P02 and VPI-P03  are  located at two separated areas, it means that the oil geochemical properties from these two wells are highly different (VPI-P02 represents oil from clastic reservoir and VPI-P03 represents oil from carbonate reservoir). Moreover, the oil samples from these VPI-P02 and VPI-P03 are located along two parallel straight lines which therefore can be considered as the trends kèo nhà cái 188bet clastic and carbonate oils respectively.

The oil samples from VPI-P01 were found very near the location kèo nhà cái 188bet carbonate trend. These results illustrate that oil from carbonate reservoir mostly contributes to the oil production stream kèo nhà cái 188bet VPI-P01.

Using all GC whole oil data kèo nhà cái 188bet VPI-P01, VPI-P02 and VPI-P03 as an input data to run the cluster analysis, the dendrogram is shown in Figure 7 with two distinct groups. Accordingly, the samples kèo nhà cái 188bet VPI-P01 and VPI-P03 are grouped in one cluster and the other cluster is VPI-P02 with very large distance. Hence, the oils from VPI-P01 and VPI-P03 have a distinct carbonate reservoir signature, whereas the oil from VPI-P02 has clastic signature. The distances between the groups (along the x-axis) are a relative measure kèo nhà cái 188bet the magnitude kèo nhà cái 188bet differences among the oils. Therefore, carbonate and clastic oils are clearly separable with large compositional discrepancy. Once again, this result proves that the oil flow from carbonate reservoir mainly contributes to the production stream kèo nhà cái 188bet VPI-P01.
In addition, from over 500 chromatographic peaks (corresponding to more than 500 hydrocarbon compounds in a GC chromatogram from C6 to C14), through the adaptive algorithm and visual inspection, 26 pairs kèo nhà cái 188bet distinctive chemical compounds (or 26 distinctive peak height ratios) were screened and chosen to display on the stardiagram. According to the star diagram, the difference between the geochemical properties kèo nhà cái 188bet VPI-P01/VPI-P03 and VPI-P02 can be easily observed (Figure 8).
 

Figure 8. Polar (star) plot kèo nhà cái 188bet selected peak ratios for well P01, P02, and P03

Quantitative results

Assuming oil from well VPI-P02  is  an  end‐member  oil kèo nhà cái 188bet the clastic reservoir and oil from well VPI-P03 is an end-member oil kèo nhà cái 188bet carbonate reservoir, the oil from well VPI-P01 is now considered as commingled oil from two reservoirs and therefore need to be allocated. Distinctive peak height ratios are chosen from approximately 1,000 gas chromatography peaks from C6 to C35 as input data kèo nhà cái 188bet the Geochemical Allocation Software for allocation calculation.

The allocation results kèo nhà cái 188bet commingled well VPI-P01 show the contribution kèo nhà cái 188bet both clastic and carbonate reservoirs to oil stream kèo nhà cái 188bet P01 and vary from time to time. However, oil from carbonate reservoir mainly contributes tocommingled oil stream kèo nhà cái 188bet well P01 during the observed period. It illustrates that oil produced from P01 is characterised  by  oil produced from carbonate reservoir. These results are consistent with the qualitative results in 3.2.

In order to evaluate the accuracy and validate the technique, the geochemical allocation results were compared with field production data based on MPLT running for well VPI-P01. Allocation kèo nhà cái 188bet well VPI-P01 oils based on logging result compares very favourably with the geochemical result on the same date. The high agreement (in this case less than 1%) observed indicates the validity kèo nhà cái 188bet the geochemical fingerprinting method.

4.    Conclusion

Geochemical fingerprinting technique with gas chromatography is a valuable method to accurately allocate commingled production stream and production pipeline. The results kèo nhà cái 188bet allocation calculation kèo nhà cái 188bet commingled wells based on their geochemical fingerprints are in excellent agreement (generally within 3%) with the actual production logging data. This method can be used to replace other methods (MPLT or metering) to determine the oil allocation from commingled wells/ pipelines more frequently because kèo nhà cái 188bet their cost saving and high accuracy.

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