Shale reservoirs are (often) unconventional plays
Posted by D Nathan Meehan June 10, 2010
Category: Reservoir, Shale   |  Tags:

Shale reservoirs are (often) unconventional plays

According to the US Geological survey(1), production from unconventional reservoirs exists in geographically extensive accumulations . Their deposits generally lack well-defined hydrocarbon/water contacts and include coalbed methane, some tight sandstone reservoirs, chalks, and self-sourced oil and gas in shale accumulations. The assessment methodology and production practices of unconventional reservoirs vary from those used for conventional resources. General categories of unconventional petroleum include:

  • Deep Gas
  • Shallow biogenic gas
  • Heavy Oil/Natural Bitumen
  • Shale Gas and Oil
  • Gas Hydrates
  • Coalbed Methane
  • While the level of maturity of these unconventional resources varies, oil and gas production from shale reservoirs is growing at a phenomenal rate. It is important to note that the type of shale production being discussed here is generally conventional oil or gas produced from relatively deeply buried (2) shales that are produced in a manner roughly similar to conventional wells. We refer to this as shale oil and shale gas. The term oil shale is widely applied to petroleum extracted from shallow rocks with very high kerogen content. Many of the rocks referred to as oil shales are not actually shales and theses rocks are often physically mined rather than having the hydrocarbons produced through wellbores. Oil shales contain typically solid or ultraviscous hydrocarbon material in their pores. While some oil shales can be burned directly, most require some sort of extractive process coupled with upgrading to yield oil like hydrocarbons. Oil shales are not discussed in this blog.

    A “resource play” is a relatively large hydrocarbon accumulation that occurs over a broad geological area. In a resource play, the geological likelihood of encountering the hydrocarbon bearing strata is nearly certain within the play area. A resource play may nonetheless have wide variability in well performance; however it is often the case that such variability cannot easily be predicted in advance or even correlated to conventional measurements (e.g. porosity, thickness). Resource plays have alternately been described as “statistical” plays in which an operator must drill a large number of wells and can expect fairly repeatable results if enough wells are drilled.

    This is really the key concept. Is there a clear cut methodology for predicting the quality of offsetting well locations, or must we approach this estimation process using a more statistical approach? It is essential to be able to estimate recoveries from as yet undrilled wells of course, but (with the level of knowledge and data available) it is often impossible to meaningfully differentiate the quality of any specific locations. Structural location rarely correlates strongly to performance. There may be no reliable spatial predictors of conventional reservoir variables even if the measurements and tests required to estimate these variables have been made. Conventional reservoir descriptions are also unlikely to give us a big leg up on estimating reservoir performance. In our next few blog entries we will look at:

  • Criteria for defining “resource” plays.
  • What does a shale need to be a commercial reservoir?
  • What is a critically stressed fracture and how does it affect my shale play?
  • So, what is different about hydraulic fracturing lately?
  • 1.

    2. Generally more than a thousand feet and usually at depths comparable to land drilling for oil and gas reservoirs. Oil shales requiring mining would necessarily be quite shallow.

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