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

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

|-

|-

! Mineral or Material

! Mineral or Material || Fluorescence Color

! Fluorescence Color

|-

|-

| ''' Minerals'''

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

|-

|-

| 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

|-

|-

| Chalky limestones

| Chalky limestones || Purple

| Purple

|-

|-

| Foliated shales

| Foliated shales || Tan to grayish brown

| Tan to grayish brown

|-

|-

| Anhydrite

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

| Blue to mid-gray

|-

|-

| Pyrite

| Pyrite || Mustard yellow to greenish brown

| Mustard yellow to greenish brown

|-

|-

| ''' Artificial Materials '''

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

|-

|-

| Diesel fuel

| Diesel fuel || Dull brown

| Dull brown

|-

|-

| Pipe dope

| Pipe dope || Bright blue

| Bright blue

|-

|-

| Oil-based mud

| Oil-based mud^a || Varies

| Varies

|}

|}

===Cut fluorescence===

===Cut fluorescence===

[[Category:Wellsite methods]]

[[Category:Wellsite methods]]