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[a] Southwest Petroleum University， Sichuan， China； b） Geological Exploration and Development Research Institute in Sichuan-Changqing Drilling and Exploration Engineering Corporation， CNPC， Sichuan， China.
[b] Geological Exploration and Development Research Institute in Sichuan-Changqing Drilling and Exploration Engineering Corporation， CNPC， Sichuan， China.
* Corresponding author.
Received 3 January 2013； accepted 5 March 2013
Abstract
It’s a great challenge in identifying gas bearing formation from conventional logs in tight gas sandstones due to the low resistivity contrast caused by high irreducible water saturation. Based on the difference of the principles of three kinds of porosity logs （density， neutron and acoustic logs）， three porosities difference method， three porosities ratio method， correlation of neutron and density logs and the overlap method of water-filled porosity and total porosity are introduced to identify tight gas bearing reservoirs. In gas bearing formations， the difference of three porosities is higher than 0.0， the ratio of three porosities is higher than 1.0， the correlation between density and neutron logs is negative， and the water filled porosities are lower than total porosities. On the contrary， in water saturated formations， the difference of three porosities is lower than 0.0， the ratio of three porosities is lower than 1.0， the correlation between density and neutron logs is positive， and the water filled porosities are overlapped with total porosities. Considering the complexity of in-suit formation， when the proposed identification criterion are mainly meet， the pore fluid should be determined， field examples show that the proposed techniques are applicable in tight gas formation identification.
Key words： Low resistivity contrast gas bearing formation； Tight gas sandstones； Identification； Difference of three porosities； Ratio of three porosities； Correlation of neutron and density logs
Liu， X. P.， Hu， X. X.， & Zhang， X. L. （2013）. Low Resistivity Contrast Gas Bearing Formation Identification from Conventional Logs in Tight Gas Sandstones. Advances in Petroleum Exploration and Development， 5（1），-0. Available from： URL： http：//www.cscanada.net/index.php/aped/article/view/j.aped.1925543820130501.1088
DOI： http：//dx.doi.org/10.3968/j.aped.1925543820130501.1088
INTRODUCTION
It’s a great challenge for petrophysicists and well log analysts in identifying tight gas reservoirs by using conventional logs due to the low resistivity contrast， which is caused by high irreducible water saturation （Tang et al.， 2013）. The resistivity difference between gas bearing formations and water saturated reservoirs is lower than 2.0， this is quite different with the conventional reservoirs （Ouyang et al.， 2009； Xiao et al.， 2012）. From resistivity log response， tight gas sands cannot be identified accurately. Between gas bearing formations and water saturated layers， three different kinds of porosities are different. In gas bearing reservoirs， porosity obtained from acoustic log is higher than the true value， the density derived porosity is also higher， while the neutron porosity is lower than the true porosity due to the effect of Hydrogen index. In water saturated layers， these three derived porosities are equivalent， and they all equal to true formation porosity. Hence， these three porosities， and the derivative methods can be used to identifying gas bearing formations with low resistivity contrast. In this study， based on these three porosities， three methods， which are three porosities difference method， three porosities ratio and the neutron and density logs correlation method， are proposed to identifying low resistivity contrast gas bearing formations. In the meanwhile， two porosities overlap method， which is established based on the water filled porosity and total porosity， is also used. The results illustrate that these methods are all effective in tight gas sands. 1. METHODS OF IDENTIFYING LOW RESISTIVITY CONTRAST GAS BEARING FORMATIONS IN TIGHT GAS SANDS
1.1 Three Porosities Difference Method
In gas bearing formations， three porosity logs （density， neutron and acoustic log） all cannot be used to reflect true formation porosity due to the effect of natural gas. Porosities obtained from acoustic and density logs are higher than true formation porosity， while porosity derived from neutron log is lower. In water saturated layers， porosities calculated from acoustic， density and neutron logs are equivalent （Gao et al.， 1999）. In this paper， a parameter is established to reflect the difference of these three kinds of porosities， gas bearing zones can be identifying， and this parameter is named as the difference of three porosities， which is calculated by using Equation 1.
Figure 1
Field Example of Identifying Tight Gas Bearing Formation by Using the Proposed Methods
Figure 2 is another field example of identifying water saturated layer by using the proposed technique. From the processed result， it can be observed that the value of B is lower than 0.0， C is lower than 1.0， PHIW is close to POR， and the correlation factor of density and neutron porosities is positive， all these results indicate that in the interval of xx36 to xx88 m， only water can be produced. This is verified by the drill stem testing data.
Figure 2
Field Example of Identifying Water Bearing Formation by Using the Proposed Method
CONCLUSIONS
（1） It is difficult in identifying tight gas bearing formations from resistivity response due to the low contrast， the difference of resistivity between gas bearing formation and water saturated layer is lower than 2.0.
（2） In gas bearing formation， porosities derived from acoustic and density logs are overestimated， and neutron log will underestimate reservoir porosity， based on this difference， three porosities difference and three porosities ratio methods are proposed to identifying tight gas bearing formations. The correlation factor method of density and neutron logs can also be usable.
（3） The overlap method of water filled porosity and total porosity is usable in identifying gas bearing reservoirs from pure water layers.
REFERENCES
[1] Tang， W.， Liu， X. P.， Hu， X. X.， Zhang， X. L.， & Wang， S. X. （2013）. The Correlation of Neutron and Density Logs and Its Application in Tight Gas Reservoirs Identification. SPE163951.
[2] Ouyang， J.， Mao， Z. Q.， Xiu， L. J.， Shi， Y. J.， & Li， C. X. （2009）. Genetic Mechanism for Low Contrast Hydrocarbon Bearing Reservoirs and Well Log Evaluation Method. Beijing： Petroleum Industry Press.
[3] Xiao， L.， Mao， Z. Q.， Dou， W. T.， Hou， Y. T.， & Jin， Y. （2012）. Types and Genesis of Low Contrast Hydrocarbon Reservoirs in Low Permeability Sandstones. Special Oil & Gas Reservoirs， 19（1）， 42-46.
[4] Gao， C. Q.， Zhong， X. S.， Yuan， X. D.， Yu， S. Y.， & Jiang， W. （1999）. Identifying Gas Reservoir with Lower Resistivity from Logging Information. Journal of Jianghan Petroleum Institute， 21（4）， 15-17.
[5] Mao，Z. Q. （2012）. The Physical Dependence and the Correlation Characteristics of Density and Neutron Logs. Petrophysics， 42（5）， 438-443.
[b] Geological Exploration and Development Research Institute in Sichuan-Changqing Drilling and Exploration Engineering Corporation， CNPC， Sichuan， China.
* Corresponding author.
Received 3 January 2013； accepted 5 March 2013
Abstract
It’s a great challenge in identifying gas bearing formation from conventional logs in tight gas sandstones due to the low resistivity contrast caused by high irreducible water saturation. Based on the difference of the principles of three kinds of porosity logs （density， neutron and acoustic logs）， three porosities difference method， three porosities ratio method， correlation of neutron and density logs and the overlap method of water-filled porosity and total porosity are introduced to identify tight gas bearing reservoirs. In gas bearing formations， the difference of three porosities is higher than 0.0， the ratio of three porosities is higher than 1.0， the correlation between density and neutron logs is negative， and the water filled porosities are lower than total porosities. On the contrary， in water saturated formations， the difference of three porosities is lower than 0.0， the ratio of three porosities is lower than 1.0， the correlation between density and neutron logs is positive， and the water filled porosities are overlapped with total porosities. Considering the complexity of in-suit formation， when the proposed identification criterion are mainly meet， the pore fluid should be determined， field examples show that the proposed techniques are applicable in tight gas formation identification.
Key words： Low resistivity contrast gas bearing formation； Tight gas sandstones； Identification； Difference of three porosities； Ratio of three porosities； Correlation of neutron and density logs
Liu， X. P.， Hu， X. X.， & Zhang， X. L. （2013）. Low Resistivity Contrast Gas Bearing Formation Identification from Conventional Logs in Tight Gas Sandstones. Advances in Petroleum Exploration and Development， 5（1），
DOI： http：//dx.doi.org/10.3968/j.aped.1925543820130501.1088
INTRODUCTION
It’s a great challenge for petrophysicists and well log analysts in identifying tight gas reservoirs by using conventional logs due to the low resistivity contrast， which is caused by high irreducible water saturation （Tang et al.， 2013）. The resistivity difference between gas bearing formations and water saturated reservoirs is lower than 2.0， this is quite different with the conventional reservoirs （Ouyang et al.， 2009； Xiao et al.， 2012）. From resistivity log response， tight gas sands cannot be identified accurately. Between gas bearing formations and water saturated layers， three different kinds of porosities are different. In gas bearing reservoirs， porosity obtained from acoustic log is higher than the true value， the density derived porosity is also higher， while the neutron porosity is lower than the true porosity due to the effect of Hydrogen index. In water saturated layers， these three derived porosities are equivalent， and they all equal to true formation porosity. Hence， these three porosities， and the derivative methods can be used to identifying gas bearing formations with low resistivity contrast. In this study， based on these three porosities， three methods， which are three porosities difference method， three porosities ratio and the neutron and density logs correlation method， are proposed to identifying low resistivity contrast gas bearing formations. In the meanwhile， two porosities overlap method， which is established based on the water filled porosity and total porosity， is also used. The results illustrate that these methods are all effective in tight gas sands. 1. METHODS OF IDENTIFYING LOW RESISTIVITY CONTRAST GAS BEARING FORMATIONS IN TIGHT GAS SANDS
1.1 Three Porosities Difference Method
In gas bearing formations， three porosity logs （density， neutron and acoustic log） all cannot be used to reflect true formation porosity due to the effect of natural gas. Porosities obtained from acoustic and density logs are higher than true formation porosity， while porosity derived from neutron log is lower. In water saturated layers， porosities calculated from acoustic， density and neutron logs are equivalent （Gao et al.， 1999）. In this paper， a parameter is established to reflect the difference of these three kinds of porosities， gas bearing zones can be identifying， and this parameter is named as the difference of three porosities， which is calculated by using Equation 1.
Figure 1
Field Example of Identifying Tight Gas Bearing Formation by Using the Proposed Methods
Figure 2 is another field example of identifying water saturated layer by using the proposed technique. From the processed result， it can be observed that the value of B is lower than 0.0， C is lower than 1.0， PHIW is close to POR， and the correlation factor of density and neutron porosities is positive， all these results indicate that in the interval of xx36 to xx88 m， only water can be produced. This is verified by the drill stem testing data.
Figure 2
Field Example of Identifying Water Bearing Formation by Using the Proposed Method
CONCLUSIONS
（1） It is difficult in identifying tight gas bearing formations from resistivity response due to the low contrast， the difference of resistivity between gas bearing formation and water saturated layer is lower than 2.0.
（2） In gas bearing formation， porosities derived from acoustic and density logs are overestimated， and neutron log will underestimate reservoir porosity， based on this difference， three porosities difference and three porosities ratio methods are proposed to identifying tight gas bearing formations. The correlation factor method of density and neutron logs can also be usable.
（3） The overlap method of water filled porosity and total porosity is usable in identifying gas bearing reservoirs from pure water layers.
REFERENCES
[1] Tang， W.， Liu， X. P.， Hu， X. X.， Zhang， X. L.， & Wang， S. X. （2013）. The Correlation of Neutron and Density Logs and Its Application in Tight Gas Reservoirs Identification. SPE163951.
[2] Ouyang， J.， Mao， Z. Q.， Xiu， L. J.， Shi， Y. J.， & Li， C. X. （2009）. Genetic Mechanism for Low Contrast Hydrocarbon Bearing Reservoirs and Well Log Evaluation Method. Beijing： Petroleum Industry Press.
[3] Xiao， L.， Mao， Z. Q.， Dou， W. T.， Hou， Y. T.， & Jin， Y. （2012）. Types and Genesis of Low Contrast Hydrocarbon Reservoirs in Low Permeability Sandstones. Special Oil & Gas Reservoirs， 19（1）， 42-46.
[4] Gao， C. Q.， Zhong， X. S.， Yuan， X. D.， Yu， S. Y.， & Jiang， W. （1999）. Identifying Gas Reservoir with Lower Resistivity from Logging Information. Journal of Jianghan Petroleum Institute， 21（4）， 15-17.
[5] Mao，Z. Q. （2012）. The Physical Dependence and the Correlation Characteristics of Density and Neutron Logs. Petrophysics， 42（5）， 438-443.