You must log in to edit PetroWiki. Help with editing

Content of PetroWiki is intended for personal use only and to supplement, not replace, engineering judgment. SPE disclaims any and all liability for your use of such content. More information


Log analyses in tight gas reservoirs: Difference between revisions

PetroWiki
Jump to navigation Jump to search
 
No edit summary
Line 1: Line 1:
Logs provide the most economical and complete source of data for evaluating layered, complex, low porosity, tight gas reservoirs. The recommended logging suite for a tight gas reservoir consist of:
Logs provide the most economical and complete source of data for evaluating layered, complex, low porosity, tight gas reservoirs. The recommended logging suite for a tight gas reservoir consist of:
* [[Spontaneous (SP) log|Spontaneous potential]] (SP)
* Gamma ray (GR)
* Density (FDC)
* Neutron (CNL)
* Sonic (SON)
* Dual (or array) [[Induction logging|induction logs]] (DIL)


==Preprocessing data==
*[[Spontaneous_(SP)_log|Spontaneous potential]] (SP)
*Gamma ray (GR)
*Density (FDC)
*Neutron (CNL)
*Sonic (SON)
*Dual (or array) [[Induction_logging|induction logs]] (DIL)
 
== Preprocessing data ==
 
All openhole logging data should be preprocessed before the data are used in any detailed computations. The steps required to preprocess the logs are:
All openhole logging data should be preprocessed before the data are used in any detailed computations. The steps required to preprocess the logs are:
* Digitize all log data
 
* Depth shift the data as required
*Digitize all log data
* Perform all environmental corrections
*Depth shift the data as required
* Normalize data so that all logs from different wells are reading the same in zones, such as thick marine shales in which one expects the log readings to be consistent from well to well.<ref name="r1" /><ref name="r2" />
*Perform all environmental corrections
*Normalize data so that all logs from different wells are reading the same in zones, such as thick marine shales in which one expects the log readings to be consistent from well to well.<ref name="r1">_</ref><ref name="r2">_</ref>


Once the data have been preprocessed and stored in a digital database, a series of statistical analyses must be conducted to quantify certain evaluation parameters. These statistical analyses consist of a Picket plot to determine estimates of:
Once the data have been preprocessed and stored in a digital database, a series of statistical analyses must be conducted to quantify certain evaluation parameters. These statistical analyses consist of a Picket plot to determine estimates of:
* Water resistivity (''R<sub>w</sub>''), cementation factor (''m''), and saturation exponent (''n'')
* Shale histograms to find the shale endpoints on all logs
* Sand and/or limestone histograms to determine the clean zone endpoints on all the logs
* Linear regressions between each porosity log and any core data to establish correlation constants
* Linear regressions among the porosity logs to develop correlations that can be used to correct for bad hole effects on one or more of the logs


The series of articles by Hunt ''et al.''<ref name="r3" /> clearly describes the steps required to:
*Water resistivity (''R<sub>w</sub>''), cementation factor (''m''), and saturation exponent (''n'')
* Preprocess the logs
*Shale histograms to find the shale endpoints on all logs
* Develop the correlation parameters
*Sand and/or limestone histograms to determine the clean zone endpoints on all the logs
* Analyze logs in shaly, low porosity formations
*Linear regressions between each porosity log and any core data to establish correlation constants
*Linear regressions among the porosity logs to develop correlations that can be used to correct for bad hole effects on one or more of the logs
 
The series of articles by Hunt ''et al.''<ref name="r3">_</ref> clearly describes the steps required to:
 
*Preprocess the logs
*Develop the correlation parameters
*Analyze logs in shaly, low porosity formations
 
== Computing porosity ==
 
To correctly compute porosity in tight, shaly (clay-rich) reservoirs, one of the first values to compute is the volume of clay in the rock. The clay volume is normally computed using either the self-potential (SP) or the GR log readings. The following equations are commonly used to compute the clay volume in a formation.


==Computing porosity==
[[File:Vol6 page 0307 eq 001.png|RTENOTITLE]]....................(1)
To correctly compute porosity in tight, shaly (clay-rich) reservoirs, one of the first values to compute is the volume of clay in the rock. The clay volume is normally computed using either the self-potential (SP) or the GR log readings. The following equations are commonly used to compute the clay volume in a formation.


[[File:Vol6 page 0307 eq 001.png]]....................(1)
[[File:Vol6 page 0307 eq 002.png|RTENOTITLE]]....................(2)


[[File:Vol6 page 0307 eq 002.png]]....................(2)
[[File:Vol6 page 0307 eq 003.png|RTENOTITLE]]....................(3)


[[File:Vol6 page 0307 eq 003.png]]....................(3)
[[File:Vol6 page 0307 eq 004.png|RTENOTITLE]]....................(4)


[[File:Vol6 page 0307 eq 004.png]]....................(4)
The SP provides reasonable estimates of ''V<sub>SH</sub>'' if the formation water and the mud filtrate do not have the same salinities. The GR log provides reasonable estimates of ''V<sub>SH</sub>'' as long as all the radioactive materials in the formation are part of the clays and not part of the sandstone, such as potassium feldspar.


The SP provides reasonable estimates of ''V<sub>SH</sub>'' if the formation water and the mud filtrate do not have the same salinities. The GR log provides reasonable estimates of ''V<sub>SH</sub>'' as long as all the radioactive materials in the formation are part of the clays and not part of the sandstone, such as potassium feldspar.  
Once the values of ''V<sub>SH</sub>'' are known as a function of depth, then the petrophysicist can compute values of clay-corrected porosity from the density, neutron, and sonic logs with '''Eqs. 5''', '''6''', or '''7'''.


Once the values of ''V<sub>SH</sub>'' are known as a function of depth, then the petrophysicist can compute values of clay-corrected porosity from the density, neutron, and sonic logs with '''Eqs. 5''', '''6''', or '''7'''.  
If the petrophysicist only has a density, sonic, or neutron log, the clay-corrected estimates of porosity from '''Eqs. 5, 6,''' or '''7''' should be used to determine the porosity. However, if two or all three logs are available, crossplots should be used to determine the best estimate of porosity.<ref name="r3">_</ref>


If the petrophysicist only has a density, sonic, or neutron log, the clay-corrected estimates of porosity from '''Eqs. 5, 6,''' or '''7''' should be used to determine the porosity. However, if two or all three logs are available, crossplots should be used to determine the best estimate of porosity.<ref name="r3" />
[[File:Vol6 page 0307 eq 005.png|RTENOTITLE]]....................(5)


[[File:Vol6 page 0307 eq 005.png]]....................(5)
[[File:Vol6 page 0308 eq 001.png|RTENOTITLE]]....................(6)


[[File:Vol6 page 0308 eq 001.png]]....................(6)
[[File:Vol6 page 0308 eq 002.png|RTENOTITLE]]....................(7)


[[File:Vol6 page 0308 eq 002.png]]....................(7)
=== Computing water saturation ===


===Computing water saturation===
There have been numerous [[Water_saturation_determination|water saturation]] equations published in the petroleum engineering and petrophysical literature. Worthington<ref name="r4">_</ref> published a complete review of all the commonly used water-saturation equations. For tight gas sandstones, the best method to compute the value of water saturation is normally the dual-water model.<ref name="r5">_</ref> '''Eq. 8''' and '''Fig. 1''' illustrate the dual-water model.
There have been numerous [[Water saturation determination|water saturation]] equations published in the petroleum engineering and petrophysical literature. Worthington<ref name="r4" /> published a complete review of all the commonly used water-saturation equations. For tight gas sandstones, the best method to compute the value of water saturation is normally the dual-water model.<ref name="r5" /> '''Eq. 8''' and '''Fig. 1''' illustrate the dual-water model.  


[[File:Vol6 page 0308 eq 003.png]]
[[File:Vol6 page 0308 eq 003.png|RTENOTITLE]]


and
and


[[File:Vol6 page 0309 eq 001.png]]....................(8)
[[File:Vol6 page 0309 eq 001.png|RTENOTITLE]]....................(8)


It is possible to use a clay-corrected Archie equation, the Simandeaux equation, the Waxman-Smits equation, or any number of other equations as described by Worthington;<ref name="r4" /> however, for many situations, the [[Log analysis in shaly formations|dual-water model]] provides accurate estimates of water saturation.  
It is possible to use a clay-corrected Archie equation, the Simandeaux equation, the Waxman-Smits equation, or any number of other equations as described by Worthington;<ref name="r4">_</ref> however, for many situations, the [[Log_analysis_in_shaly_formations|dual-water model]] provides accurate estimates of water saturation.


<gallery widths="300px" heights="200px">
<gallery widths="300px" heights="200px">
Line 64: Line 71:
</gallery>
</gallery>


In the Archie equation, all the electrical conductivity in the formation is assumed to be transmitted through the water in the pore space. The rock is assumed to be an insulator and does not conduct current. However, in clay-rich formations, the clays conduct an electric current. The Simandeaux and Waxman-Smits equations provide for a conductive rock but assume that the water associated with the pore space and the water associated with the clays have the same properties. In the dual-water model, there is free water and bound water. The free water is in the pores, and the bound water is associated with the clays. More accurate estimates of water saturation can be achieved by taking into account the current conducted by the clays using the dual-water model.  
In the Archie equation, all the electrical conductivity in the formation is assumed to be transmitted through the water in the pore space. The rock is assumed to be an insulator and does not conduct current. However, in clay-rich formations, the clays conduct an electric current. The Simandeaux and Waxman-Smits equations provide for a conductive rock but assume that the water associated with the pore space and the water associated with the clays have the same properties. In the dual-water model, there is free water and bound water. The free water is in the pores, and the bound water is associated with the clays. More accurate estimates of water saturation can be achieved by taking into account the current conducted by the clays using the dual-water model.
 
When the formation permeability in a gas reservoir is between 0.01 and 10 md, mud filtrate invasion from freshwater mud into a formation with saline interstitial water can substantially alter the resistivity profile near the wellbore during the time period before the openhole logs are normally run.<ref name="r6">_</ref> In such cases, dual-induction logs or array-induction logs should be run and used to make corrections to determine the true resistivity, ''R<sub>t</sub>'', of the formation. The log readings change with time because of mud filtrate invasion.
 
Most tight gas reservoirs are tight because they are highly cemented and have low porosity. The low porosity and cementation cause many tight gas reservoirs to become hard and abrasive, which may prevent the use of logging while drilling (LWD) equipment. In addition, the flow rates and ultimate recovery from individual wells are low, and the operator must control drilling, completion, and operating costs to improve the profitability of each well. For these reasons, LWD is not often used when drilling tight gas reservoirs. Most of the logging data come from openhole logs run after the well reaches total depth. See more discussion on logging practices in the section on petrophysics in [[Geothermal_reservoir_engineering|Geothermal reservoir engineering]]


When the formation permeability in a gas reservoir is between 0.01 and 10 md, mud filtrate invasion from freshwater mud into a formation with saline interstitial water can substantially alter the resistivity profile near the wellbore during the time period before the openhole logs are normally run.<ref name="r6" /> In such cases, dual-induction logs or array-induction logs should be run and used to make corrections to determine the true resistivity, ''R<sub>t</sub>'', of the formation. The log readings change with time because of mud filtrate invasion.
== Mud filtrate invasion ==


Most tight gas reservoirs are tight because they are highly cemented and have low porosity. The low porosity and cementation cause many tight gas reservoirs to become hard and abrasive, which may prevent the use of logging while drilling (LWD) equipment. In addition, the flow rates and ultimate recovery from individual wells are low, and the operator must control drilling, completion, and operating costs to improve the profitability of each well. For these reasons, LWD is not often used when drilling tight gas reservoirs. Most of the logging data come from openhole logs run after the well reaches total depth. See more discussion on logging practices in the section on petrophysics in [[Geothermal reservoir engineering]]
In many tight gas formations, drilling mud mixed with fresh water is used to drill. Commonly, the formation water is more saline than the water in the drilling mud. When the drill bit penetrates a permeable formation, filtrate from the drilling mud invades the formation.<ref name="r7">_</ref> The factors that affect mud filtrate invasion are:


==Mud filtrate invasion==
*Mud cake properties
In many tight gas formations, drilling mud mixed with fresh water is used to drill. Commonly, the formation water is more saline than the water in the drilling mud. When the drill bit penetrates a permeable formation, filtrate from the drilling mud invades the formation.<ref name="r7" /> The factors that affect mud filtrate invasion are:
*Reservoir pressure
* Mud cake properties
*Mud weight
* Reservoir pressure
*Formation permeability
* Mud weight
*Formation porosity
* Formation permeability
*Relative permeability
* Formation porosity
*Capillary pressure
* Relative permeability
* Capillary pressure


The factors that affect the resistivity profile around the well, in addition to the above factors, are:
The factors that affect the resistivity profile around the well, in addition to the above factors, are:
* The formation water salinity
* The mud filtrate salinity
* The initial water saturation in the formation


In low permeability gas reservoirs, mud filtrate invasion during drilling can affect the results from both drillstem tests and from openhole logs.<ref name="r8" /><ref name="r9" /> The mud filtrate invades the permeable zones, and the mud filtrate invasion profile changes with time. Therefore, the values recorded by logging tools are a function of when those logs are run. In addition, values such as mud weight, mud filtrate salinity, and mud circulation rate can change hourly or daily. As such, it is important to measure the mud properties daily and to keep accurate records during drilling operations.  
*The formation water salinity
*The mud filtrate salinity
*The initial water saturation in the formation
 
In low permeability gas reservoirs, mud filtrate invasion during drilling can affect the results from both drillstem tests and from openhole logs.<ref name="r8">_</ref><ref name="r9">_</ref> The mud filtrate invades the permeable zones, and the mud filtrate invasion profile changes with time. Therefore, the values recorded by logging tools are a function of when those logs are run. In addition, values such as mud weight, mud filtrate salinity, and mud circulation rate can change hourly or daily. As such, it is important to measure the mud properties daily and to keep accurate records during drilling operations.


The fact that mud filtrate invasion in low porosity rocks does affect openhole logs can be used to the advantage of the log analyst. Semmelbeck ''et al.''<ref name="r6" /> explained how mud filtrate invasion in low permeability formations affect the deep induction (''R<sub>ild</sub>'') and the medium induction log (''R<sub>ilm</sub>'') differently as a function of time. Thus, if one has multiple logging runs, one can evaluate how the ratio of ''R<sub>ild</sub>'' /''R<sub>ilm</sub>'' varies and can correlate that ratio with formation permeability. '''Fig. 2''' shows simulated data that describes how the ratio of ''R<sub>ild</sub>'' /''R<sub>ilm</sub>'' for one set of reservoir and drilling mud parameters varies over time as a function of reservoir permeability. Notice that the resistivity ratio changes with time as the mud filtrate continues to invade the formation. It is clear that the mud filtrate invasion affects the different resistivity logs more in high permeability formations than in low permeability formations. As such, evidence of mud filtrate invasion from log analyses can be used to estimate values of formation permeability.<ref name="r6" /><ref name="r9" /><ref name="r10" />
The fact that mud filtrate invasion in low porosity rocks does affect openhole logs can be used to the advantage of the log analyst. Semmelbeck ''et al.''<ref name="r6">_</ref> explained how mud filtrate invasion in low permeability formations affect the deep induction (''R<sub>ild</sub>'') and the medium induction log (''R<sub>ilm</sub>'') differently as a function of time. Thus, if one has multiple logging runs, one can evaluate how the ratio of ''R<sub>ild</sub>'' /''R<sub>ilm</sub>'' varies and can correlate that ratio with formation permeability. '''Fig. 2''' shows simulated data that describes how the ratio of ''R<sub>ild</sub>'' /''R<sub>ilm</sub>'' for one set of reservoir and drilling mud parameters varies over time as a function of reservoir permeability. Notice that the resistivity ratio changes with time as the mud filtrate continues to invade the formation. It is clear that the mud filtrate invasion affects the different resistivity logs more in high permeability formations than in low permeability formations. As such, evidence of mud filtrate invasion from log analyses can be used to estimate values of formation permeability.<ref name="r6">_</ref><ref name="r9">_</ref><ref name="r10">_</ref>


<gallery widths="300px" heights="200px">
<gallery widths="300px" heights="200px">
Line 93: Line 103:
</gallery>
</gallery>


The SFE No. 3, a GRI research well in East Texas, was logged four times while drilling to measure the effects of mud filtrate invasion on the readings from openhole logs.<ref name="r11" /> '''Fig. 3''' presents some of the data for a portion of the hole in SFE No. 3. Because the resistivity measurements are changing with every logging run, it is clear that mud filtrate invasion affects the openhole resistivity logging readings in the permeable zones. However, in the shales, where minimal invasion occurred, the effects of invasion are minimal as the resistivity readings do not vary between logging runs. As a rule of thumb, if the analyst sees evidence of mud filtrate invasion on the resistivity logs and/or mud cake across a gas-bearing zone, then that zone should have enough permeability to produce gas at measurable flow rates.  
The SFE No. 3, a GRI research well in East Texas, was logged four times while drilling to measure the effects of mud filtrate invasion on the readings from openhole logs.<ref name="r11">_</ref> '''Fig. 3''' presents some of the data for a portion of the hole in SFE No. 3. Because the resistivity measurements are changing with every logging run, it is clear that mud filtrate invasion affects the openhole resistivity logging readings in the permeable zones. However, in the shales, where minimal invasion occurred, the effects of invasion are minimal as the resistivity readings do not vary between logging runs. As a rule of thumb, if the analyst sees evidence of mud filtrate invasion on the resistivity logs and/or mud cake across a gas-bearing zone, then that zone should have enough permeability to produce gas at measurable flow rates.


<gallery widths="300px" heights="200px">
<gallery widths="300px" heights="200px">
Line 99: Line 109:
</gallery>
</gallery>


Mud filtrate invasion also affects the sonic velocities, the bulk densities, and the hydrogen content of the portion of the rock near the wellbore that is invaded. As such, mud filtrate invasion also affects the sonic, density, and neutron log readings. As mud filtrate invasion proceeds, the properties change with time, and the readings from the sonic, density and neutron logs will also change with time.<ref name="r11" />
Mud filtrate invasion also affects the sonic velocities, the bulk densities, and the hydrogen content of the portion of the rock near the wellbore that is invaded. As such, mud filtrate invasion also affects the sonic, density, and neutron log readings. As mud filtrate invasion proceeds, the properties change with time, and the readings from the sonic, density and neutron logs will also change with time.<ref name="r11">_</ref>


== Nomenclature ==
== Nomenclature ==


{|border="0" cellspacing="4" width="70%"  
{| border="0" cellspacing="4" width="70%"
|''A''
|=
|surface area
|-
|-
|''C''
| ''A''
|=
| =
|conductivity, mho/m
| surface area
|-
|-
|''I''
| ''C''
|=
| =
|index
| conductivity, mho/m
|-
|-
|''ρ''
| ''I''
|=
| =
|density, g/mL
| index
|-
|-
|''t''
| ''ρ''
|=
| =
|time, hours or days
| density, g/mL
|-
|-
|Δ''t''
| ''t''
|=
| =
|travel time, μsec/ft
| time, hours or days
|-
|-
|''V''
| Δ''t''
|=
| =
|volume, fraction
| travel time, μsec/ft
|-
|-
|''φ''
| ''V''
|=
| =
|porosity, fraction  
| volume, fraction
|-
| ''φ''
| =
| porosity, fraction
|}
|}


'''Subscripts'''
'''Subscripts'''
{|border="0" cellspacing="4" width="70%"
 
|''b ''
{| border="0" cellspacing="4" width="70%"
|=
|-
|bulk
| ''b''
| =
| bulk
|-
|-
|''CL ''
| ''CL''
|=  
| =
|clean
| clean
|-
|-
|''f ''
| ''f''
|=  
| =
|fluid or fracture
| fluid or fracture
|-
|-
|ma  
| ma
|=  
| =
|matrix
| matrix
|-
|-
|''N ''
| ''N''
|=  
| =
|neutron log
| neutron log
|-
|-
|''NC ''
| ''NC''
|=  
| =
|neutron corrected for shale
| neutron corrected for shale
|-
|-
|''RA ''
| ''RA''
|=  
| =
|radioactive
| radioactive
|-
|-
|''SC ''
| ''SC''
|=  
| =
|sonic corrected
| sonic corrected
|-
|-
|''SH ''
| ''SH''
|=  
| =
|shale
| shale
|-
|-
|''t ''
| ''t''
|=  
| =
|true (for conductivity); total (for compressibility)
| true (for conductivity); total (for compressibility)
|-
|-
|''w ''
| ''w''
|=  
| =
|wellbore (for radius); water (for saturation)
| wellbore (for radius); water (for saturation)
|-
|-
|''wb ''
| ''wb''
|=  
| =
|bound water (for conductivity and water saturation)
| bound water (for conductivity and water saturation)
|-
|-
|''wf ''
| ''wf''
|=  
| =
|well flowing; free water (for conductivity)
| well flowing; free water (for conductivity)
|-
|-
|''wt ''
| ''wt''
|=  
| =
|total water
| total water
|}
|}


'''Superscripts'''
'''Superscripts'''
{|border="0" cellspacing="4" width="70%"
 
|''m ''
{| border="0" cellspacing="4" width="70%"
|=  
|-
|cementation factor
| ''m''
| =
| cementation factor
|-
|-
|''n ''
| ''n''
|=  
| =
|saturation exponent
| saturation exponent
|}
|}


==References==
== References ==
<references>
<ref name="r1">Aly, A.M., Hunt, E.R., Pursell, D.A. et al. 1997. Application of Multi-Well Normalization of Open Hole Logs in Integrated Reservoir Studies. Presented at the SPE Western Regional Meeting, Long Beach, California, 25-27 June 1997. SPE-38263-MS. http://dx.doi.org/10.2118/38263-MS. </ref>


<ref name="r2">Howard, W.E. and Hunt, E.R. 1986. Travis Peak: An Integrated Approach to Formation Evaluation. Presented at the SPE Unconventional Gas Technology Symposium, Louisville, Kentucky, 18-21 May 1986. SPE-15208-MS. http://dx.doi.org/10.2118/15208-MS. </ref>
<references />


<ref name="r3">Hunt, E.R. et al. 1997. Fundamentals of Log Analysis. 12-part article in ''World Oil'' (June, July, September, October, November, December 1996 and March, July, September, October, November, December 1997). </ref>
== Noteworthy papers in OnePetro ==


<ref name="r4">Worthington, P.F. 1985. The Evolution of Shaly-Sand Concepts in Reservoir Evaluation. ''The Log Analyst'' (January/February): 23.</ref>
Use this section to list papers in OnePetro that a reader who wants to learn more should definitely read


<ref name="r5">Clavier, C., Coates, G., and  Dumanoir, J. 1984. Theoretical and Experimental Bases for the Dual-Water Model for Interpretation of Shaly Sands. ''SPE J.'' '''24''' (2): 153-168. SPE-6859-PA. http://dx.doi.org/10.2118/6859-PA.</ref>
== External links ==


<ref name="r6">Semmelbeck, M.E. and Holditch, S.A. 1988. The Effects of Mud-Filtrate Invasion on the Interpretation of Induction Logs. ''SPE Form Eval'' '''3''' (2): 386-392. SPE-14491-PA. http://dx.doi.org/10.2118/14491-PA.</ref>
Use this section to provide links to relevant material on websites other than PetroWiki and OnePetro
 
<ref name="r7">Ferguson, C.K. and Klotz, J.A. 1954. Filtration from Mud During Drilling. ''J Pet Technol'' '''6''' (2): 30–43. SPE-289-G. http://dx.doi.org/10.2118/289-G.</ref>
 
<ref name="r8">Holditch, S.A., Lee, W.J., Lancaster, D.E. et al. 1983. Effect of Mud Filtrate Invasion on Apparent Productivity in Drillstem Tests in Low-Permeability Gas Formations. ''J Pet Technol'' '''35''' (2): 299-305. SPE-9842-PA. http://dx.doi.org/10.2118/9842-PA. </ref>
 
<ref name="r9">Tobola, D.P. and Holditch, S.A. 1991. Determination, of Reservoir Permeability From Repeated Induction Logging. SPE Form Eval 6 (1): 20-26. SPE-19606-PA. http://dx.doi.org/10.2118/19606-PA.</ref>
 
<ref name="r10">Yao, C.Y. and Holditch, S.A. 1996. Reservoir Permeability Estimation From Time-lapse Log Data. ''SPE Form Eval'' '''11''' (1): 69–74. SPE-25513-PA. http://dx.doi.org/10.2118/25513-PA.</ref>
 
<ref name="r11">Staged Field Experiment No. 3: Application of Advanced Technologies in Tight Gas Sandstones—Travis Peak and Cotton Valley Formations, Waskom Field, Harrison County, Texas. Gas Research Inst. Report, GRI-91/0048, CER Corp. and S.A. Holditch & Assocs. Inc. (February). </ref>
</references>


==Noteworthy papers in OnePetro==
== See also ==
Use this section to list papers in OnePetro that a reader who wants to learn more should definitely read


==External links==
[[Tight_gas_reservoirs|Tight gas reservoirs]]
Use this section to provide links to relevant material on websites other than PetroWiki and OnePetro


==See also==
[[Log_analysis_in_shaly_formations|Log analysis in shaly formations]]
[[Tight gas reservoirs]]


[[Log analysis in shaly formations]]
[[Well_log_interpretation|Well log interpretation]]


[[Well log interpretation]]
[[Core_analyses_in_tight_gas_reservoirs|Core analyses in tight gas reservoirs]]


[[Core analyses in tight gas reservoirs]]
[[Permeability_estimation_in_tight_gas_reservoirs|Permeability estimation in tight gas reservoirs]]


[[Permeability estimation in tight gas reservoirs]]
[[Tight_gas_drilling_and_completion|Tight gas drilling and completion]]


[[Tight gas drilling and completion]]
[[Statistical_data_correlations_in_tight_gas_reservoirs|Statistical data correlations in tight gas reservoirs]]


[[Statistical data correlations in tight gas reservoirs]]
[[Modeling_tight_gas_reservoirs|Modeling tight gas reservoirs]]


[[Modeling tight gas reservoirs]]
[[Reserves_estimation_in_tight_gas_reservoirs|Reserves estimation in tight gas reservoirs]]


[[Reserves estimation in tight gas reservoirs]]
[[Hydraulic_fracturing_in_tight_gas_reservoirs|Hydraulic fracturing in tight gas reservoirs]]


[[Hydraulic fracturing in tight gas reservoirs]]
[[PEH:Tight_Gas_Reservoirs]]


[[PEH:Tight Gas Reservoirs]]
[[Category:5.8.1 Tight gas]][[Category:5.6.1 Open hole/cased hole log analysis]]

Revision as of 18:53, 10 June 2015

Logs provide the most economical and complete source of data for evaluating layered, complex, low porosity, tight gas reservoirs. The recommended logging suite for a tight gas reservoir consist of:

Preprocessing data

All openhole logging data should be preprocessed before the data are used in any detailed computations. The steps required to preprocess the logs are:

  • Digitize all log data
  • Depth shift the data as required
  • Perform all environmental corrections
  • Normalize data so that all logs from different wells are reading the same in zones, such as thick marine shales in which one expects the log readings to be consistent from well to well.[1][2]

Once the data have been preprocessed and stored in a digital database, a series of statistical analyses must be conducted to quantify certain evaluation parameters. These statistical analyses consist of a Picket plot to determine estimates of:

  • Water resistivity (Rw), cementation factor (m), and saturation exponent (n)
  • Shale histograms to find the shale endpoints on all logs
  • Sand and/or limestone histograms to determine the clean zone endpoints on all the logs
  • Linear regressions between each porosity log and any core data to establish correlation constants
  • Linear regressions among the porosity logs to develop correlations that can be used to correct for bad hole effects on one or more of the logs

The series of articles by Hunt et al.[3] clearly describes the steps required to:

  • Preprocess the logs
  • Develop the correlation parameters
  • Analyze logs in shaly, low porosity formations

Computing porosity

To correctly compute porosity in tight, shaly (clay-rich) reservoirs, one of the first values to compute is the volume of clay in the rock. The clay volume is normally computed using either the self-potential (SP) or the GR log readings. The following equations are commonly used to compute the clay volume in a formation.

RTENOTITLE....................(1)

RTENOTITLE....................(2)

RTENOTITLE....................(3)

RTENOTITLE....................(4)

The SP provides reasonable estimates of VSH if the formation water and the mud filtrate do not have the same salinities. The GR log provides reasonable estimates of VSH as long as all the radioactive materials in the formation are part of the clays and not part of the sandstone, such as potassium feldspar.

Once the values of VSH are known as a function of depth, then the petrophysicist can compute values of clay-corrected porosity from the density, neutron, and sonic logs with Eqs. 5, 6, or 7.

If the petrophysicist only has a density, sonic, or neutron log, the clay-corrected estimates of porosity from Eqs. 5, 6, or 7 should be used to determine the porosity. However, if two or all three logs are available, crossplots should be used to determine the best estimate of porosity.[3]

RTENOTITLE....................(5)

RTENOTITLE....................(6)

RTENOTITLE....................(7)

Computing water saturation

There have been numerous water saturation equations published in the petroleum engineering and petrophysical literature. Worthington[4] published a complete review of all the commonly used water-saturation equations. For tight gas sandstones, the best method to compute the value of water saturation is normally the dual-water model.[5] Eq. 8 and Fig. 1 illustrate the dual-water model.

RTENOTITLE

and

RTENOTITLE....................(8)

It is possible to use a clay-corrected Archie equation, the Simandeaux equation, the Waxman-Smits equation, or any number of other equations as described by Worthington;[4] however, for many situations, the dual-water model provides accurate estimates of water saturation.

  • Fig. 1—Partitioning of shaly sand in a dual-water model.

    Fig. 1—Partitioning of shaly sand in a dual-water model.

In the Archie equation, all the electrical conductivity in the formation is assumed to be transmitted through the water in the pore space. The rock is assumed to be an insulator and does not conduct current. However, in clay-rich formations, the clays conduct an electric current. The Simandeaux and Waxman-Smits equations provide for a conductive rock but assume that the water associated with the pore space and the water associated with the clays have the same properties. In the dual-water model, there is free water and bound water. The free water is in the pores, and the bound water is associated with the clays. More accurate estimates of water saturation can be achieved by taking into account the current conducted by the clays using the dual-water model.

When the formation permeability in a gas reservoir is between 0.01 and 10 md, mud filtrate invasion from freshwater mud into a formation with saline interstitial water can substantially alter the resistivity profile near the wellbore during the time period before the openhole logs are normally run.[6] In such cases, dual-induction logs or array-induction logs should be run and used to make corrections to determine the true resistivity, Rt, of the formation. The log readings change with time because of mud filtrate invasion.

Most tight gas reservoirs are tight because they are highly cemented and have low porosity. The low porosity and cementation cause many tight gas reservoirs to become hard and abrasive, which may prevent the use of logging while drilling (LWD) equipment. In addition, the flow rates and ultimate recovery from individual wells are low, and the operator must control drilling, completion, and operating costs to improve the profitability of each well. For these reasons, LWD is not often used when drilling tight gas reservoirs. Most of the logging data come from openhole logs run after the well reaches total depth. See more discussion on logging practices in the section on petrophysics in Geothermal reservoir engineering

Mud filtrate invasion

In many tight gas formations, drilling mud mixed with fresh water is used to drill. Commonly, the formation water is more saline than the water in the drilling mud. When the drill bit penetrates a permeable formation, filtrate from the drilling mud invades the formation.[7] The factors that affect mud filtrate invasion are:

  • Mud cake properties
  • Reservoir pressure
  • Mud weight
  • Formation permeability
  • Formation porosity
  • Relative permeability
  • Capillary pressure

The factors that affect the resistivity profile around the well, in addition to the above factors, are:

  • The formation water salinity
  • The mud filtrate salinity
  • The initial water saturation in the formation

In low permeability gas reservoirs, mud filtrate invasion during drilling can affect the results from both drillstem tests and from openhole logs.[8][9] The mud filtrate invades the permeable zones, and the mud filtrate invasion profile changes with time. Therefore, the values recorded by logging tools are a function of when those logs are run. In addition, values such as mud weight, mud filtrate salinity, and mud circulation rate can change hourly or daily. As such, it is important to measure the mud properties daily and to keep accurate records during drilling operations.

The fact that mud filtrate invasion in low porosity rocks does affect openhole logs can be used to the advantage of the log analyst. Semmelbeck et al.[6] explained how mud filtrate invasion in low permeability formations affect the deep induction (Rild) and the medium induction log (Rilm) differently as a function of time. Thus, if one has multiple logging runs, one can evaluate how the ratio of Rild /Rilm varies and can correlate that ratio with formation permeability. Fig. 2 shows simulated data that describes how the ratio of Rild /Rilm for one set of reservoir and drilling mud parameters varies over time as a function of reservoir permeability. Notice that the resistivity ratio changes with time as the mud filtrate continues to invade the formation. It is clear that the mud filtrate invasion affects the different resistivity logs more in high permeability formations than in low permeability formations. As such, evidence of mud filtrate invasion from log analyses can be used to estimate values of formation permeability.[6][9][10]

  • Fig. 2—Relation between history-matched absolue permeability and initial observed Rild/Rilmratio.

    Fig. 2—Relation between history-matched absolue permeability and initial observed Rild/Rilmratio.

The SFE No. 3, a GRI research well in East Texas, was logged four times while drilling to measure the effects of mud filtrate invasion on the readings from openhole logs.[11] Fig. 3 presents some of the data for a portion of the hole in SFE No. 3. Because the resistivity measurements are changing with every logging run, it is clear that mud filtrate invasion affects the openhole resistivity logging readings in the permeable zones. However, in the shales, where minimal invasion occurred, the effects of invasion are minimal as the resistivity readings do not vary between logging runs. As a rule of thumb, if the analyst sees evidence of mud filtrate invasion on the resistivity logs and/or mud cake across a gas-bearing zone, then that zone should have enough permeability to produce gas at measurable flow rates.

  • Fig. 3—Wireline logs from SFE No.3.

    Fig. 3—Wireline logs from SFE No.3.

Mud filtrate invasion also affects the sonic velocities, the bulk densities, and the hydrogen content of the portion of the rock near the wellbore that is invaded. As such, mud filtrate invasion also affects the sonic, density, and neutron log readings. As mud filtrate invasion proceeds, the properties change with time, and the readings from the sonic, density and neutron logs will also change with time.[11]

Nomenclature

A = surface area
C = conductivity, mho/m
I = index
ρ = density, g/mL
t = time, hours or days
Δt = travel time, μsec/ft
V = volume, fraction
φ = porosity, fraction

Subscripts

b = bulk
CL = clean
f = fluid or fracture
ma = matrix
N = neutron log
NC = neutron corrected for shale
RA = radioactive
SC = sonic corrected
SH = shale
t = true (for conductivity); total (for compressibility)
w = wellbore (for radius); water (for saturation)
wb = bound water (for conductivity and water saturation)
wf = well flowing; free water (for conductivity)
wt = total water

Superscripts

m = cementation factor
n = saturation exponent

References

  1. ↑ _
  2. ↑ _
  3. ↑ 3.0 3.1 _
  4. ↑ 4.0 4.1 _
  5. ↑ _
  6. ↑ 6.0 6.1 6.2 _
  7. ↑ _
  8. ↑ _
  9. ↑ 9.0 9.1 _
  10. ↑ _
  11. ↑ 11.0 11.1 _

Noteworthy papers in OnePetro

Use this section to list papers in OnePetro that a reader who wants to learn more should definitely read

External links

Use this section to provide links to relevant material on websites other than PetroWiki and OnePetro

See also

Tight gas reservoirs

Log analysis in shaly formations

Well log interpretation

Core analyses in tight gas reservoirs

Permeability estimation in tight gas reservoirs

Tight gas drilling and completion

Statistical data correlations in tight gas reservoirs

Modeling tight gas reservoirs

Reserves estimation in tight gas reservoirs

Hydraulic fracturing in tight gas reservoirs

PEH:Tight_Gas_Reservoirs

Retrieved from "https://petrowiki.spe.org/w/index.php?title=Log_analyses_in_tight_gas_reservoirs&oldid=47338"