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The Importance of Using Geologic Information to Complete Wells*

By

J. B. (Jack) Thomas1

Search and Discovery Article #40032 (2001)

*Adapted from oral presentation to Tulsa Geological Society, September 25, 2001

1Jackrock Consulting, Tulsa, Oklahoma ([email protected])

 

Summary

In the continuum of the energy business, the greatest volume of geological and geophysical information is created in the early stages of Opportunity Capture through Prospect Definition. Engineering information grows rapidly in abundance once a well is drilled. When well completion costs may exceed one quarter of the total expense for drilling a wildcat well, the application of key geologic information should be a significant factor in reducing the business risk. Geologic data are important for engineering decisions like… (1) “If I use limited entry perforating in a laminated pay sand, will I optimize production?”…or (2) ”What happens if I use mud acid on a volcanogenic sandstone?”

Four major areas of geological information are important in well completions: depositional environment, reservoir and seal composition, log interpretation, and pore geometry. For example, internal reservoir stratification affects perforating design; illite clay rich reservoirs resemble shales on logs; and small pore-throated carbonate wackestone reservoirs have higher log porosity net pay cutoffs than carbonate grainstone reservoirs. An informal polling of reservoir engineers involved in field discovery and development shows that such information is the most significant application of geologic knowledge for engineering decisions and failure to apply it can make a discovery well non-economic.

The impact of geology does not end when total depth is reached!  Completion is a business risk element that requires geologic and engineering expertise to optimize results.

 

uSummary

uFigure Captions

uIntroduction

uGeologic factors

tPore geometry

tDepositional environment

tComposition

wMineralogy

wClays

-Composition

-Habit vs. productivity

tLog interpretation

uFormation damage

uCompletion examples

uPutting it together

uCompletions vs.    composition

tImproving completions

tChecklist

tBottom line

uReferences

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

uSummary

uFigure Captions

uIntroduction

uGeologic factors

tPore geometry

tDepositional environment

tComposition

wMineralogy

wClays

-Composition

-Habit vs. productivity

tLog interpretation

uFormation damage

uCompletion examples

uPutting it together

uCompletions vs.    composition

tImproving completions

tChecklist

tBottom line

uReferences

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

uSummary

uFigure Captions

uIntroduction

uGeologic factors

tPore geometry

tDepositional environment

tComposition

wMineralogy

wClays

-Composition

-Habit vs. productivity

tLog interpretation

uFormation damage

uCompletion examples

uPutting it together  uCompletions-composition

tImproving completions

tChecklist

tBottom line

uReferences

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

uSummary

uFigure Captions

uIntroduction

uGeologic factors

tPore geometry

tDepositional environment

tComposition

wMineralogy

wClays

-Composition

-Habit vs. productivity

tLog interpretation

uFormation damage

uCompletion examples

uPutting it together  uCompletions-composition

tImproving completions

tChecklist

tBottom line

uReferences

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

uSummary

uFigure Captions

uIntroduction

uGeologic factors

tPore geometry

tDepositional environment

tComposition

wMineralogy

wClays

-Composition

-Habit vs. productivity

tLog interpretation

uFormation damage

uCompletion examples

uPutting it together  uCompletions-composition

tImproving completions

tChecklist

tBottom line

uReferences

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

uSummary

uFigure Captions

uIntroduction

uGeologic factors

tPore geometry

tDepositional environment

tComposition

wMineralogy

wClays

-Composition

-Habit vs. productivity

tLog interpretation

uFormation damage

uCompletion examples

uPutting it together  uCompletions-composition

tImproving completions

tChecklist

tBottom line

uReferences

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

uSummary

uFigure Captions

uIntroduction

uGeologic factors

tPore geometry

tDepositional environment

tComposition

wMineralogy

wClays

-Composition

-Habit vs. productivity

tLog interpretation

uFormation damage

uCompletion examples

uPutting it together  uCompletions-composition

tImproving completions

tChecklist

tBottom line

uReferences

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

uSummary

uFigure Captions

uIntroduction

uGeologic factors

tPore geometry

tDepositional environment

tComposition

wMineralogy

wClays

-Composition

-Habit vs. productivity

tLog interpretation

uFormation damage

uCompletion examples

uPutting it together  uCompletions-composition

tImproving completions

tChecklist

tBottom line

uReferences

 

Figure Captions

Figure 1. A continuous process: Exploration – exploitation - reservoir management.

 

Figure 2. The cost-impact of geologic information on well completion.

 

 

Figure 3. Idealized pore system models.

 

 

Figure 4. Clay minerals in coarse siliciclastic rocks (Pittman and Thomas, 1979) and clays in carbonate rocks.

 

 

Figure 5. Two layer clays--kaolinite group.

 

 

Figure 6. Three layer clay--illite.

 

 

Figure 7. Three layer clay--smectite.

 

 

Figure 8. Three layer clay--chlorite.

 

 

Figure 9. Reservoir capability vs. clay (authigenic) habit: diagram and capillary pressure curves (after Neasham, 1977).

 

 

Figure 10. Stimulation versus permeability reduction--General relationships.

 

 

Figure 11. Discrete clay in central bar reservoir, Grieve sandstone.

 

 

Figure 12. Grain-coating clay in bar margin reservoir, Grieve sandstone.

 

 

Figure 13. Smectite and glass in volcaniclastic Markley sand.

 

 

Figure 14. Completion problems and remedies for reservoirs with the different clay groups.

 

Figure 15. Pore geometries and completion problems and favored 
treatment(s).

 

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Introduction

The energy business continuum may be stated as follows:

  • Business decisions are driven by technology.

  • Geology and engineering data are needed in all phases!

  • Specific geologic and engineering data should be focused on the well. 

Impact of geologic information may be shown in the continuous process in exploration – exploitation – reservoir management (Figure 1), involving the following:

1.      Opportunity capture

2.      Basin analysis

3.      Play, area analysis

4.      Prospect analysis

5.      Well analysis

6.      Play/field extension

7.      Reservoir optimization, management

Data volume and problem complexity increase as the number given above increases.

The cost-impact of geologic information on well completion, as an example of the influence-cost curve, is shown in Figure 2. Impact of geologic information decreases with time, whereas the impact of cost increases with time. Key types of information include pore geometry, reservoir composition, and depositional environment.

Geologic information in completions impacts business risk by improving “the answer” on reservoir type and quality. For engineering decisions, this is THE MOST SIGNIFICANT application of geologic information. A geologist can create a geologic “checklist” of information for completion decisions! 

 

GEOLOGIC FACTORS AFFECTING COMPLETIONS

In regard to geologic information in well completions, one should consider four major areas of information: 

  • Depositional environment (e.g., How did reservoir rock form?) 

  • Composition (e.g., Is it “clean” sandstone; a lime mudstone?)

  • Log interpretation (e. g. Shaly sandstone?)

  • Pore geometry (e.g., Will it flow?) 

One obtains the geologic information for completions from:

  • Mud logs -- Patterns of Total Gas, each gas (C1-5)

  • Cuttings study -- Can you see pores?

  • Cores -- Porosity/permeability ties!

  • Logs -- Stratification, composition, porosity!

  • SEM -- Pore geometry!

  • Thin sections -- Cements, pore geometry.

  • X-ray diffraction – Composition.

Pore Geometry

Pore geometry includes two critical elements:

  • Surface area

  • Pore throats

Pore geometry relates pores to pore throats. Pore throats are critical to fluid flow. Depositional energy and diagenesis determine the resultant pore geometry. Reservoirs can have same pore geometry with different composition or depositional environment.

Pore throats are the key to flow (Figure 3). Idealized pore system models may be grouped into (1) pater noster (beads) and (2) tabular (blocks).

Depositional Environment

Clastic reservoirs (siliciclastics and detrital carbonates) are the result of depositional energy. Non-clastic reservoirs are the result of post-formation processes, such as fracturing and/or dissolution. Log signature indications are important.

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Composition

            Mineralogy

In depositional mineralogy, provenance and transportation energy are the keys. In authigenic mineralogy, provenance, original pore geometry, and fluid throughput are the keys. One must know the effects on logs and the stimulation types.

            Clay in Sedimentary Rocks

Occurrence of clays in reservoirs is as detrital and authigenic minerals. Clay minerals in coarse siliciclastic rocks occur as (Figure 4; Pittman and Thomas, 1979): 

  • Loosely packed authigenic clay in pores

  • Detrital clay-rich lamination

  • Detrital clay aggregate grains

  • Tightly packed detrital clay matrix filling pores

Clay minerals in carbonate rocks (Figure 4) occur as: 

  • Clay present along unconformity-erosion surface and in sinkhole

  • Clay in impure carbonate strata

            Composition of clays

Major clay groups are:

Two layer clays are represented by the kaolinite group. An example of kaolinite occurrence in a reservoir is in the Cretaceous Grieve Sandstone, Wyoming (Figure 5), that was deposited as a barrier bar.

Illite is one of the three layer clays; an example of its occurrence in a reservoir is in the Eocene Markley Sandstone, California (Figure 6); it is a volcaniclastic, deepwater deposit.

Smectite is another three layer clay; an example of its occurrence is in the Yegua Sandstone, in the Gulf Coast of Texas (Figure 7), where it was deposited in the outer shelf marine environment.

A third type of three layer clay  is chlorite; it also occurs in the Yegua Sandstone of Texas (Figure 8). Bladed chlorite + mixed layer clay are present in this unit at downdip positions, where it is a deepwater sandstone.

            Clay Habit versus Productivity

The relationship between clay habit and productivity has been documented. Most productive clay-bearing reservoirs contain discrete particles; in least productive reservoirs, clays are pore-filling/bridging. “Damage” or permeability reduction is worst with pore-filling-habit clays. The habits of types of clays occurring authigenically, shown diagrammatically in Figure 9, are as follows:

1. Discrete clays – kaolinite and chlorite

2. Pore-lining – illite, chlorite, smectite, and mixed layers

3. Pore-bridging – illite and chlorite

Surface area increases in habits from 1 to 3 (itemized above), and capillary pressure illustrates reservoir potential decreasing with increasing surface area (Figure 9). 

Log Interpretation

Clay not only affects pore geometry, as noted above, but also logs. Log responses, in addition to expressing stratification, are affected by, or reflect clays and/or porosity. A summary of these relationships is as follows:

  • Gamma ray tools -- illites, mixed layer clay

  • Neutron, Density, Sonic tools -- porosity

  • Resistivity/Conductivity tools -- chlorite

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FORMATION “DAMAGE” MECHANISMS

Formation damage mechanisms - lowering permeability near wellbore - may be due to:

  • Mobile solids (fines migration)

  • Swelling of certain clay minerals

  • Dissolution and re-precipitation

  • Relative permeability blocks

Stimulation fluids versus composition, as they interrelate to formation damage, include the following considerations:

  • Fresh water; smectite swelling.

  • Hydrochloric acid; best for removing carbonate cement, dissolves chlorite.

  • Hydrofluoric acid; dissolves all minerals.

  • Acetic, citric acids; partial solubility of clays and carbonates.

  • High pH cross-linked gels; partial linked gels and partial dissolution of clays and feldspars

General relationships of stimulation versus permeability are given in Figure 10 and listed below:

 

Technique Used

Precipitate   

Mobile Fines

Clay Swelling

Rel. Perm. "Blocks"

Acids-(HCl, HF, +)

Yes*

Yes*

Possible (mxlr)

Yes

Solvent (methanol, xylene) 

Yes*

Yes*

No

Yes?

Hydraulic frac.

Yes*

Yes

Yes

Yes

*Partial Solubility

 

 

 

 

 

COMPLETION EXAMPLES

Barrier Bar: Cretaceous Grieve Sandstone, Wyoming, with: 

  • Discrete particle kaolinite in central bar and 

  • Grain coating & pore filling in bar margins.

In central bar sandstone, with discrete clay (Figure 11), recommended completion is: Perforate and flow or HCl clean-up optimizes oil productivity.

In bar margin, with grain-coating clay (Figure 12), recommended completion is: Perforate underbalanced (foam ?), foamed HCl (7.5%); small foam frac results in gas or gas condensate wells.

Volcaniclastic Sandstone, represented by the Markley Sandstone, California, with:

  • 13-54% clays by X-ray

  • Grain coating, pore filling smectite+ GLASS (Figure 13)

Recommended procedure is:

Drill underbalanced; perforate underbalanced, high density perforation; use little or no water. Why? Because of mineral solubilities.

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HOW DO I PUT THIS TOGETHER?

Completions Versus Composition

To help improve completions for reservoirs rich in one or more of the minerals listed in Figure 14 (and below), special consideration should be given to drilling procedures and to treatments/stimulants in order to minimize damage or in considering remedial treatments.

 

Mineral

Potential Effect 

Maximizes Damage

Minimizes Damage

Remedy

Smectite

Swelling

Fresh Wtr., HF

Air, KCl, OBM drilling

HCl; Re-perf. & Frac.

Mixed Layer Clay

Swelling; Mobile Fines

Fresh Wtr., HF

Air, KCl, OBM drilling

HCl; Re-perf. & Frac.

Kaolinite

Mobile Fines

Hi WL muds, HF

Air, Foam drilling

Clay stab.; Low Flow Rates

Chlorite

Iron Gel Precipitate

Hi pH muds;  O2 Rich

Air, Foam; HCl sequestered

HCl (seq.), low strength

Carbonate Minerals

Complex CaF percipitates

HF

Salt muds; OBM

HCl

Quartz, Felds. Glass (silt)

Mobile Fines; Gel Precips.

Hi WL muds, Hi pH muds

Air, Foam; Bland Fluids

Clay stab.; Foam fracs.

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Improving Completions: Pore Geometry Information In Complex Reservoirs

 Using information about reservoir pore geometry (Figure 15 and listed below) in drilling and well completions reduces damage.

 

Pore Geometry, Rock Type

Potential Damage

Minimizes Damage, Drilling

Favored
Treatment(s)

Fine Pore-Throated Silicilastic 

Oil/Water Fluid Block

Foam, Air; Low FL Muds

Solvent; Foam frac; Foamed Acid

Fine Pore-Throated Carbonate 

Oil/Water Fluid Block

Foam, Air; Low FL Muds

Solvent; Foamed Acid Frac;

Fractured Reservoirs

Highly Varied; Fluid Blocks, etc.

Low FL Muds; Foam, Air 

Highly Varied; Foam Fracs.; Solvent 

 

“Checklist” for Completions

A recommended checklist is as follows:

  • What are the source(s), quality of information; cuttings, core, seismic?

  • What is the apparent pore geometry?

  • What is the formation composition?

  • What is the depositional environment?

  • What is the log signature; does it match pre-drill model?

What is the Bottom Line?

The most successful completions result from applying a combination of GEOSCIENCE and ENGINEERING Knowledge!

 

References

Neasham, J.W., 1977, The morphology of dispersed clay in sandstone reservoirs and its effect on sandstone shaliness, pore space and fluid flow properties: Soc. Petrol. Engineers paper #6858, 8p. 

Pittman, E.D., and J.B. Thomas, 1979, Some applications of scanning electron microscopy to the study of reservoir rock: Jour. Petrol. Technology, paper #7550, p. 1375-1380.

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