Changes

Wellsite show evaluation relies on the following:

Wellsite show evaluation relies on the following:

* Detection of formation gas or oil

* Detection of formation [[Natural gas|gas]] or [[Oil as an energy source|oil]]

* Detection of hydrocarbons in drill cuttings

* Detection of [[hydrocarbon]]s in drill cuttings

* Knowledge of drilling and wellsite activities

* Knowledge of drilling and wellsite activities

* Geological knowledge of the interpreter

* Geological knowledge of the interpreter

|+ {{table number|1}}Types of gas that result from the drilling process

|+ {{table number|1}}Types of gas that result from the drilling process

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|-

! Type of Gas

! Type of Gas || Description

! Description

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|-

| “Zero” gas

| “Zero” gas || Gas present in the mud circulating system when the bit is off-bottom and there is no vertical movement of the drill string. This reading results from the liberation of gases from the mud system or from the recycling of previously encountered gases in the wellbore. Although a “zero” gas value will constantly vary, it acts as a starting point for evaluating any subsequent formation gas shows.

| Gas present in the mud circulating system when the bit is off-bottom and there is no vertical movement of the drill string. This reading results from the liberation of gases from the mud system or from the recycling of previously encountered gases in the wellbore. Although a “zero” gas value will constantly vary, it acts as a starting point for evaluating any subsequent formation gas shows.

|-

|-

| Background gas

| Background gas || Gas that reflects the geological character of a consistent lithology. Background gas readings incorporate gas contributions due to the formation, but also those included as zero gas. Gas from the formation is due to the crushing of the rock as it is being drilled and typically has a low volume. These gas readings are plotted on the mudlog as background gas and represent the relative baseline against which all other gas shows are compared.

| Gas that reflects the geological character of a consistent lithology. Background gas readings incorporate gas contributions due to the formation, but also those included as zero gas. Gas from the formation is due to the crushing of the rock as it is being drilled and typically has a low volume. These gas readings are plotted on the mudlog as background gas and represent the relative baseline against which all other gas shows are compared.

|-

|-

| Liberated gas

| Liberated gas || Gas that is produced by the drilling process due to the crushing of the rock formation by the drill bit.

| Gas that is produced by the drilling process due to the crushing of the rock formation by the drill bit.

|-

|-

| Connection gas

| Connection gas || Formation gas that enters the wellbore while drilling and circulation are halted to make a connection. For this condition to occur, the contributing formation must be underbalanced by the mud system at some point within the borehole.

| Formation gas that enters the wellbore while drilling and circulation are halted to make a connection. For this condition to occur, the contributing formation must be underbalanced by the mud system at some point within the borehole.

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|-

| Produced gas

| Produced gas || Formation gas that enters the wellbore while drilling and circulating. This gas represents an underbalanced formation and, if left alone, will cause a blowout.

| Formation gas that enters the wellbore while drilling and circulating. This gas represents an underbalanced formation and, if left alone, will cause a blowout.

|-

|-

| Trip gas

| Trip gas || Formation gas that enters the wellbore when the drill string is being “[[trip]]ped” or pulled out of the wellbore. The contributing formation must be underbalanced at some point within the wellbore; such underbalance is due to the “swabbing” effect caused by pulling the drill string out of the hole.

| Formation gas that enters the wellbore when the drill string is being “tripped” or pulled out of the wellbore. The contributing formation must be underbalanced at some point within the wellbore; such underbalance is due to the “swabbing” effect caused by pulling the drill string out of the hole.

|-

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| Recycled gas

| Recycled gas || Gas that has been previously contributed to the borehole and not completely removed from the mud circulation system by surface equipment (such as a gas trap or degasser). Such gases that remain in the mud system are pumped back down the borehole to be subsequently re-recorded by the gas detection equipment. This recycled gas can usually be recognized because the “show” will be detected one full circulation cycle later than originally encountered and will appear more diffuse in character.

| Gas that has been previously contributed to the borehole and not completely removed from the mud circulation system by surface equipment (such as a gas trap or degasser). Such gases that remain in the mud system are pumped back down the borehole to be subsequently re-recorded by the gas detection equipment. This recycled gas can usually be recognized because the “show” will be detected one full circulation cycle later than originally encountered and will appear more diffuse in character.

|}

|}

Various gas ratios can be used depending on the data available. The most common gas ratios used are those with the most separation (C5/C1) and those with the heaviest composition (C4/C1 or C5/C1). Because gas ratio analysis is empirical in nature, it can sometimes prove inconclusive. However, the following “rules of thumb” can be useful<ref name=pt03r17>Exploration Logging, Inc., 1985, Mud Logging: Principles and Interpretations. Boston, MA, IHRDC, 92 p.</ref>:

Various gas ratios can be used depending on the data available. The most common gas ratios used are those with the most separation (C5/C1) and those with the heaviest composition (C4/C1 or C5/C1). Because gas ratio analysis is empirical in nature, it can sometimes prove inconclusive. However, the following “rules of thumb” can be useful<ref name=pt03r17>Exploration Logging, Inc., 1985, Mud Logging: Principles and Interpretations. Boston, MA, IHRDC, 92 p.</ref>:

* Zones with a high C1 value may represent dry gas, coal, biogenic gas, or a water wet zone.

* Zones with a high C1 value may represent [[dry gas]], [[coal]], biogenic gas, or a water wet zone.

* Wet gas zones commonly have a C1 /C3 ratio that is higher than the C1 /C4 ratio.

* Wet gas zones commonly have a C1 /C3 ratio that is higher than the C1 /C4 ratio.

* Nonproductive zones tend to have a ratio trend where subsequent values are lower than preceding values.

* Nonproductive zones tend to have a ratio trend where subsequent values are lower than preceding values.

Near wellbore flushing occurs when the pressure or weight of the mud column exceeds the fluid entry pressure of the formation (for information on calculating mud weight, see chapter on [[Wellsite math]] in Part 3). This flushing by mud filtrate occurs above and ahead of the bit and is a function of time. If the mud system is overbalanced, gas shows can be reduced or totally suppressed. Even in a carefully balanced mud system where the fluid loss is minimized and the radius of wellbore flushing is small, problems can still occur in evaluating gas show quality if the rock's petrophysical properties are not considered. In zones of low effective porosity, even relatively small volumes of filtrate loss may result in deep invasion profiles. This causes a zone with a good gas show when drilled to recover only mud filtrate or to appear water saturated when later tested or electric logged. In zones of high effective porosity and permeability, the rocks will initially be flushed, then return to their native state soon after drilling, with little or no gas liberated. This causes a zone with minimal gas show when drilled to appear productive on electric logs or when later tested. Low permeability overpressured zones will not flush and will give high gas readings.

Near wellbore flushing occurs when the pressure or weight of the mud column exceeds the fluid entry pressure of the formation (for information on calculating mud weight, see chapter on [[Wellsite math]] in Part 3). This flushing by mud filtrate occurs above and ahead of the bit and is a function of time. If the mud system is overbalanced, gas shows can be reduced or totally suppressed. Even in a carefully balanced mud system where the fluid loss is minimized and the radius of wellbore flushing is small, problems can still occur in evaluating gas show quality if the rock's petrophysical properties are not considered. In zones of low effective porosity, even relatively small volumes of filtrate loss may result in deep invasion profiles. This causes a zone with a good gas show when drilled to recover only mud filtrate or to appear water saturated when later tested or electric logged. In zones of high effective porosity and permeability, the rocks will initially be flushed, then return to their native state soon after drilling, with little or no gas liberated. This causes a zone with minimal gas show when drilled to appear productive on electric logs or when later tested. Low permeability overpressured zones will not flush and will give high gas readings.

Information that can aid in the interpretation of flushed anomalies includes the following<ref name=pt03r17 />:

Information that can aid in the interpretation of flushed anomalies includes the following:<ref name=pt03r17 />

* Pump pressure

* Pump pressure

* Jet nozzle size(s) of the bit

* Jet nozzle size(s) of the bit

* Mud rheology (plastic viscosity and yield point)

* Mud rheology (plastic [[viscosity]] and yield point)

* Mud weight and effective circulation density

* Mud weight and effective circulation density

* Formation balance gradient (mud weight required to equalize formation pressure)

* Formation balance gradient (mud weight required to equalize formation pressure)

==Cuttings evaluation==

==Cuttings evaluation==

In many wells, drill cuttings collected may represent the only subsurface data available for geological interpretation. After a detailed lithology description, cuttings are analyzed for hydrocarbon indications (see [[Mudlogging: drill cuttings analysis]]). Traces of gas and oil in the cuttings represent formation hydrocarbons that have not been flushed by the [[drilling fluid]]. Gas in cuttings is analyzed by grinding a measured amount (approximately 100 mg) of unwashed cuttings in a blender, with any liberated gases analyzed by the standard gas detection system. This analysis is often divided into two components: total gas, comprising all combustible gasses; and petroleum vapors, comprising C2 through C5. This type of analysis can indicate the amount and composition of gases in the formation, even if the larger rock pores are flushed.

In many wells, drill cuttings collected may represent the only subsurface data available for geological interpretation. After a detailed lithology description, cuttings are analyzed for hydrocarbon indications (see [[Mudlogging: drill cuttings analysis]]). Traces of gas and oil in the cuttings represent formation hydrocarbons that have not been flushed by the [[drilling fluid]]. Gas in cuttings is analyzed by grinding a measured amount (approximately 100 mg) of unwashed cuttings in a blender, with any liberated gases analyzed by the standard gas detection system. This analysis is often divided into two components: total gas, comprising all combustible gasses; and [[petroleum]] vapors, comprising C2 through C5. This type of analysis can indicate the amount and composition of gases in the formation, even if the larger rock pores are flushed.

Evaluation of oil in cuttings is performed on unwashed and washed bulk cuttings and on individual grains. Evaluation includes visual inspection and analysis using a microscope and ultraviolet (UV) box. Oil shows are described by their physical properties of visual stain, fluorescence, cut, and odor. Care must be taken always to evaluate hydrocarbon shows in cuttings with respect to their petrophysical properties (see review by <ref name=pt03r45>Swanson, R. G., 1981, Sample examination manual: Tulsa, OK, [http://store.aapg.org/detail.aspx?id=603 AAPG Methods in Exploration Series No. 1], 35 p.</ref>.

Evaluation of oil in cuttings is performed on unwashed and washed bulk cuttings and on individual grains. Evaluation includes visual inspection and analysis using a microscope and ultraviolet (UV) box. Oil shows are described by their physical properties of visual stain, fluorescence, cut, and odor. Care must be taken always to evaluate hydrocarbon shows in cuttings with respect to their petrophysical properties (see review by <ref name=pt03r45>Swanson, R. G., 1981, Sample examination manual: Tulsa, OK, [http://store.aapg.org/detail.aspx?id=603 AAPG Methods in Exploration Series No. 1], 35 p.</ref>.

===Visual stain===

===Visual stain===

Staining of the drill cuttings by oil is an indication that hydrocarbons have been in the formation at some point in time. The lack of sample staining, however, does not prove that a reservoir lacks producible hydrocarbons. The amount and distribution of staining is a function of the reservoir porosity and permeability. Stain color can be related to oil gravity, with darker staining indicating heavier hydrocarbons. If a stained sample does not fluoresce or cut, then this indicator is classified as thermally “dead oil” and is not considered a show. Staining is described in terms of its color, distribution, percentage of sample stained, and fluorescence (if any).

Staining of the drill cuttings by oil is an indication that hydrocarbons have been in the formation at some point in time. The lack of sample staining, however, does not prove that a reservoir lacks producible hydrocarbons. The amount and distribution of staining is a function of the reservoir porosity and permeability. Stain color can be related to oil [[gravity]], with darker staining indicating heavier hydrocarbons. If a stained sample does not fluoresce or cut, then this indicator is classified as thermally “dead oil” and is not considered a show. Staining is described in terms of its color, distribution, percentage of sample stained, and fluorescence (if any).

===Fluorescence===

===Fluorescence===

|+ {{table number|2}}Fluorescence of common minerals and artificial materials

|+ {{table number|2}}Fluorescence of common minerals and artificial materials

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! Mineral or Material

! Mineral or Material || Fluorescence Color

! Fluorescence Color

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| ''' Minerals'''

| colspan = 2 align = center | ''' Minerals'''<ref name=pt03r17 />

|-

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| Dolomite, magnesian limestones

| [[Dolomite]], magnesian limestones || Yellow, yellowish brown to dark brown

| Yellow, yellowish brown to dark brown

|-

|-

| Aragonite and calcareous mudstones

| Aragonite and calcareous mudstones || Yellow-white to pale brown

|

 

 

| Yellow-white to pale brown

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| Chalky limestones

| Chalky limestones || Purple

| Purple

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|-

| Foliated shales

| Foliated shales || Tan to grayish brown

| Tan to grayish brown

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|-

| Anhydrite

| [[Anhydrite]] || Blue to mid-gray

| Blue to mid-gray

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| Pyrite

| Pyrite || Mustard yellow to greenish brown

| Mustard yellow to greenish brown

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| ''' Artificial Materials '''

| colspan = 2 align = center | ''' Artificial Materials '''

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| Diesel fuel

| Diesel fuel || Dull brown

| Dull brown

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| Pipe dope

| Pipe dope || Bright blue

| Bright blue

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|-

| Oil-based mud

| Oil-based mud^a || Varies

| Varies

|}

|}

===Cut fluorescence===

===Cut fluorescence===

[[Category:Wellsite methods]]

[[Category:Wellsite methods]]