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.
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!
In regard to geologic information in well completions, one should consider four major areas of information:
One obtains the geologic information for completions from:
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 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.
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).
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 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.
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.
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 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, 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.