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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 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:
One
obtains the geologic information for completions
Pore geometry includes two critical elements:
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). 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 pore geometry, and fluid throughput are the keys. One must know the effects on logs and the stimulation types. 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 Illite
is one of the three layer clays; an example of its occurrence in a reservoir
is in the Eocene Markley Smectite
is another three layer clay; an example of its occurrence is in the Yegua
A
third type of three layer clay is
chlorite; it also occurs in the Yegua 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
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 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:
Formation
damage mechanisms - lowering
Stimulation fluids versus composition, as they interrelate to formation damage, include the following considerations:
General
relationships of stimulation versus
Barrier
Bar: Cretaceous Grieve
In
central bar 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
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: Pore Geometry Information In Complex Reservoirs Using information about reservoir pore geometry (Figure 15 and listed below) in drilling and well completions reduces damage.
A recommended checklist is as follows:
The
most successful completions result
Neasham,
J.W., 1977, The morphology of dispersed clay in 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 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 
Figure
11. Discrete clay in central bar reservoir, Grieve 
Figure
12. Grain-coating clay in bar margin reservoir, Grieve 
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 