Sweet spot identification

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Sweet spot identification is a method that finds the ideal drilling location for unconventional wells by determining reservoir characteristics such as reservoir quality and completion quality using surface seismic surveys and borehole geophysics.


What is a Sweet Spot?

Perforating Gun used for hydraulic fracturing. Credit: USGS[1]

Sweet spots are intervals within organic shales, which possess the highest relative hydrocarbon yield for drilling purposes. Sweet spots are necessary in unconventional reservoirs where permeability is low and viscosity is high. Potential sweet spots are found by observing shale characteristics such as reservoir quality and completion quality, which in turn estimate relative reservoir producibility[2]. Following is a brief definition of unconventional reservoirs, both reservoir and completion qualities, and an explanation on how to find sweet spots.


Unconventional Reservoirs

Unconventional reservoirs are locations where resources are not easily acquired due to high viscosity and ultra low permeability. Advanced technologies are necessary to economically extract resources from these unconventional reservoirs, such as hydraulic fracturing and horizontal wells [3].


Reservoir Quality (RQ)

Reservoir quality determines organic shales that have a high probability of producing hydrocarbons [2]. These qualities include mineralogy, porosity, water saturation, organic content, formation volume, and thermal maturity [3]. Kerogen, which is an organic component of unconventional rocks[4], plays a heavy role on reservoir quality. Kerogen concentration, distribution, density, and maturity all affect the RQ. [2]. A combination of multiple geophysical methods can be used to interpret these characteristics such as surface seismic surveys and borehole measurements.


Completion Quality (CQ)

Example of Horizontal well and Vertical Well. Credit: DCNR[5]

Completion quality predicts the producibility of a reservoir with hydraulic stimulation by using geomechanical parameters, horizontal stress, and fracture distribution [3]. Similar to reservoir quality, completion quality largely relies on mineralogy, but is also influence by elastic properties such as Young's modules, Poisson's ration, bulk modulus, and rock hardness [2]. More information on these parameters can be found Here. Kerogen lineations and concentrations also affect CQ. In order to have a reliable estimate of CQ, one must consider as many wells and surface seismic surveys as possible. Even then, QC can be difficult to decipher.


Locating a Sweet Spot

Locating sweet spots within a region or basin in previously unexplored locations heavily relies on preexisting seismic data. Once locations of interest are identified, basin structure analysis can be used to determine if a sweet spot is worth exploring. Even though seismic data is typically readily available and relatively easy to process, conclusions are difficult to make without a pilot well. Pilot wells allow geoscientists to accurately measure kerogen levels, porosity, permeability, and gas concentration. With a better understanding of the local structure and lithology, sweet spots can be identified[2].

Using Seismic Data for sweet spot identification

Schematic showing conventional and unconventional well locations.%=percentage; R0=vitrinite reflectance Credit: USGS[6]

Even though sweet spot identification should be confirmed by a pilot well, scientist can hypothesize sweet spot locations by analyzing seismic data. Since structural variations are typically easier to identify using seismic data, Seismic Attributes, specifically those highlighting frequency variations, can be used to predict micro fractures within a shale play. Other attributes can be explored in an attempt to decipher RQ or CQ, but a pilot well must be drilled to confirm any seismic observations.

References

  1. United States Geological Survey, 2016, Retrieved November 1, 2017, from https://www.usgs.gov/media/images/perforating-gun-hydraulic-fracturing
  2. 2.0 2.1 2.2 2.3 2.4 Glaser, K. S., C. K. Miller, G. M. Johnson, B. Toelle, R. L. Kleinberg, P. Miller, W. D. Pennington, 2014, Seeking the Sweet Spot:Reservoir and Completion Quality in Organic Shales: Oilfield Review, 25, 16–29.
  3. 3.0 3.1 3.2 Avanzini, A., P. Balossino, M. Brignoli, E. Spelta, and C. Tarchiani, 2016, Lithologic and geomechanical facies classification for sweet spot identification in gas shale reservoir: Interpretation, 4, SL21-SL31.
  4. Zhang, B., H. Daigle, 2016, Direct Determination of Surface Relativity in Isolated Kerogen by Pulsed-Field Gradient NMR: Unconventional Resources Technology Conference, San Antonio, Texas, 1-3 August 2016: pp. 2149-2158.
  5. Pennsylvania Department for Conservation and Natural Resources, n.d.,The Marcellus Shale Play in Pennsylvania Part 4:Drilling and Completion, Retrieved November 1, 2017, from http://www.docs.dcnr.pa.gov/cs/groups/public/documents/document/dcnr_007595.pdf
  6. Assessment of Continuous Oil Resources in the Wolfcamp Shale of the Midland Basin, Permian Basin Province, Texas, 2016, United States Geological Survey, Open File-Report 2017–1013, Retrieved November 19, 2017, from https://pubs.usgs.gov/of/2017/1013/ofr20171013.pdf

See Also