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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])
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.
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The energy business continuum may be stated as follows:
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 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:
One obtains the geologic information for completions from:
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. In
depositional mineralogy, provenance and transportation energy are the
keys. In authigenic mineralogy, provenance, original 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):
Clay minerals in carbonate rocks (Figure 4) occur as:
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 1. Discrete clays – kaolinite and chlorite 2.
3.
Surface area increases in habits from 1 to 3 (itemized above), and capillary pressure illustrates reservoir potential decreasing with increasing surface area (Figure 9). Clay
not only affects
Formation damage mechanisms - lowering permeability near wellbore - may be due to:
Stimulation fluids versus composition, as they interrelate to formation damage, include the following considerations:
General relationships of stimulation versus permeability are given in Figure 10 and listed below:
Barrier Bar: Cretaceous Grieve Sandstone, Wyoming, with: 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:
Recommended procedure is: Drill underbalanced; perforate underbalanced, high density perforation; use little or no water. Why? Because of mineral solubilities. 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.
Improving Completions: Using
information about reservoir
A recommended checklist is as follows:
The most successful completions result from applying a combination of GEOSCIENCE and ENGINEERING Knowledge!
Neasham,
J.W., 1977, The morphology of dispersed clay in sandstone reservoirs and its
effect on sandstone shaliness, 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. |
Figure
1. A continuous process: Exploration – exploitation - reservoir management.
Figure
2. The cost-impact of geologic information on well completion.
Figure
3. Idealized 
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. 