The Application of Analytical Technique of Gas Chromatography and Geochemical Pyrolysis in Low Permeability Oilfields

The reservoir of low permeability oilfields is thin and its physical property is poor. The oil-water relationship is complex. It is very difficult to classify oil, gas and water layer and the coincidence rate of interpretation is very low. It can no longer meet the needs of reservoir development only dependent on logging data. Through sidewall coring logging in uncertain layers in the area with complex oil-water relationship and statistic analysis on interpretation results based on geochemical pyrolysis and gas chromatography analysis, the authors think the technique can effectively improve the coincidence rate of oil, gas and water interpretation in uncertain layers in low permeability oilfields, realize fine evaluation of reservoirs, help us formulate reasonable drilling solution and enhance the development benefit of oilfields.

Rock pyrolysis technique is a method that carries out pyrolysis chromatography analysis and gas chromatography analysis on sidewall coring sample and makes use of these analysis data to establish the means for calculating the oil saturation of reservoir and judge the reservoir properties of crude oil, establish the relationship between the viscosity and density of crude oil and geochemical pyrolysis, establish the standard for identifying oil and water layer and establish the reservoir productivity prediction method connecting with log information.

Parameters of Geochemical Pyrolysis
Quantitative assessment parameters are the basis of carrying out qualitative interpretation of oil, gas and water and preferences evaluation.Currently, the main parameters applied by rock pyrolysis technique are: , which reflects oil-abundance of the rock; 2) the ratio between light and heavy crude oil: , which reflects the properties of crude oil; 3) yield index: (Western outlying areas) 5) Reservoir resistivity Rt : For middle-shallow reservoir group, induction log is better than later log for thick sandstone, while later log parameters should be chosen for thin sandstone because induction log is affected greatly by the wall rock.6) Production index: , the number of production index Q is lower, which reflects that the lithology is purer and less mud is contained and the capacity is higher whereas, the capacity is lower.7) Effective porosity: 2) Oil and water layer: The peak shape does not distribute symmetrically.The front is full, the end of the peak declines evidently, the base line is not straightness and the end has evident vault.
3) Water layer: The front and end all have evident vault.The peak shape is not full and the main peak extrude evidently.

The Standards for Identifying Oil and Water Layer Quantitatively
According to the sidewall coring pyrolysis materials of each block, the method for predicting reservoir productivity and the standard for explaining oil and water layer in the region of Tahala-Yingtai, Longbei and Xinzhao have been established at present.
1) The qualitatively interpretation standards for Putaohua and Saertu oil and water layer in the western region.

Interpretation Plates of Each Reservoir Group in the Western Region
The dynamic interpretation and production evaluation plates of each block are established according to logging and try oil materials.
1) The interpretation plate of 1# block 2) Quantitative evaluation plate of 2# block The plate is used in the production promotion when it is established.It interprets seven new wells and accords with 15 of the 19 layers.The coincidence rate is 78.90% with higher accuracy.

The Application of Reservoir Quantitative Evaluation Method Greatly Increased the Interpretation Coincidence Rate of New Wells
At present, the application of rock pyrolysis technology explained 122 complicated wells and analyze 32 sidewall coring layer of 14 evaluation wells which have try oil materials.The results accord with 23 layers and the coincidence rate is 71.9%.The launch of perforating 29 development well have been undergone statistical analysis, in which, 11 of 15 sidewall coring layers in the 5 development wells with ring test data accord with the results and the coincidence rate is 73.3%, for the 24 wells with no ring test data, according to the statistical properties of liquid producing wells, 20 wells accord with the result and the coincidence rate is 83.3%, the specific situation is shown is Table 4: From the statistical results of table we can see that, the application effect is very good, which improves the coincidence rate of interpretation and makes up the lacks of logging data, and thus better guides the oil-water distribution laws of complex oilfields and recognition of oil reservoir geological characteristics.

Accurate Solution to the Oil Reservoir Water Recognition Using Logging Data
According to the logging, number 56 layer of 14# Fuyu oil layer is interpreted as oil layer and number 58 layer is interpreted as water layer.Sandstone can be seen in sidewall coring.According to the sidewall coring physical and geochemical pyrolysis analysis, number 56 layer has good oil-bearing and should be regarded as oil layer characteristics, while number 58 layer layer has worse oil-bearing than number 56 layer and oil abundance become low from up to down, and it is considered that connecting with the regional data analysis this layer products a small amount of oil and oil and water outflow at the same time under pressure influenced by water, which can be interpreted as oil and water layer comprehensively.The conventional tests confirmed that number 58 layer products oil of 0.025t for one day and the conclusion is it is poor oil layer, which products oil of 10.348t and water of 5.816m 3 for one day after connecting with number 56 layer and the conclusion is that it is high water and high yield industrial reservoir.The result accords with the comprehensive interpretation.

Oil-Water Layer Interpretation of 1# Block
It can be seen six palm gray oily pink sandstone on the sidewall coring of number 5 of 7#, where oil distribute uniformly and the taste is strong, which presents the oil reservoir characteristics.Geochemical pyrolysis analysis reflects oil abundance is medium, and crude oil has the characteristics of Lightweight oil.Gas chromatography results have the characteristics of oil reservoir or poor layer.Logging interprets the layer as oil reservoir.After pressing to pump, daily oil production is 6.45m 3 and the conclusion is middle-class industry oil reservoir.

Conclusions
Through application effects analysis, the following conclusions are obtained below: 1) Using quantitative identification assessment method to timely perfect and revise dynamic interpretation plot can greatly improve the comprehensive coincidence rate of oil, gas and water interpretation and realize the fine evaluation of reservoirs.
2) The application and dissemination of oil reservoirs static production potential evaluation method realizes the leap from qualitative interpretation to quantitative interpretation.
3) The design standards for trying oil and selecting fractured layer using quantitative identification assessment method provides evidence for formulating reasonable try oil and fracture solution and technical method for judging the effects of fracture.
4) The method is closely combined with development and production practice and verified in the production and dissemination.Good effects are seen and great economic benefits are created.

Suggestions
1) For the main blocks of low permeability oilfields which are intensively tapped the potential, because their developmental time is long, currently, comprehensive water content is high, it is suggested that the research in the aspect of water flooded layer interpretation should be done more and hope there will be breakthrough soon.
2) For oil reservoir whose oil-water relationship is very complex, a certain quantity of sidewall coring fluorescent images analysis should be carried out, especially for sidewall coring samples in water flooded layer.The project analysis should be carried out from the key layer in order to improve the interpretation accuracy of water flooded layer., D. S., et al. (1994).Reservoir fluid pyrolysis and gas chromatography techniques.Beijing: Petroleum industry press.

Lang
of Gas Chromatogram for Reservoirs with Different Liquid Properties 1) Oil layer: The peak shape presents evident normal distribution characteristics.The front and end are all full, the base line is straightness and the end has no evident vault.

1)
Figure 1.Characteristics of Gas Chromatogram for Reservoirs with Different Liquid Properties

Table 1 .
the standard of PS index and oil production rate rate indexOPI identifying properties of crude oil

Table 2 .
The interpretation standard of sidewall coring logging data

Table 3 .
Interpretation parameters of Putaohua reservoir in western region

Table 4 .
Try oil production and perforation verification results table

Table 5 .
The compare table between try oil results of evaluation well and comprehensive interpretation results of logging