Introduction to Unconventional Gas

Recent years have brought a rapid expansion of activities around shale gas. The successes in shale gas exploitation have been mainly attributed to advances in engineering. In particular, hydraulic fracturing has contributed immensely towards this success. To date, geophysics has yet to assume a major role in the development of this important resource. There are three major reasons for the tepid response to geophysical applications for shale gas exploration and development. In this issue, we show a few applications for shale gas development. There are still areas needing urgent attention in shale gas exploration. Some of these are listed below:

• Resolution: Shale gas reservoirs can be very thin. Seismic mapping has traditionally been confounded by existence of thin beds. Resolving them can be a non-trivial task.
• Pressure and fluid sensitivity: Many shale gas reservoirs are found at large depths. These are competent rocks that demonstrate very little sensitivity to changes in fluids or in depths. Thus, there is little understanding about any Direct Fluid Indicators for gas saturations in shale gas reservoirs
• Heterogeneity: Shale gas lithologies are very heterogeneous. For example, the Monterey shale lithology can encompass quartz (opals and porcellanites), clay minerals, and limestone-dolomite, in addition to organic matter. Detecting the mineralogy and distinguishing between changes in maturation, mineralogy, and saturations can be a difficult task.
• Anisotropy: Although the shales are anisotropic, the changes in anisotropy with maturity are still poorly understood.
• Organic Content: Velocity of shale formations depends on the clay and the organic contents and maturities. However, elastic properties and its changes with maturity are not well understood.
• Organic Maturity: Empirical correlations have been derived between elastic moduli and maturity. The exact reason for these correlations is poorly determined and thus useable and useful theoretical models are lacking.
• Resistivity: Resistivity increases with maturity – it is used for mapping potential sweet spots. Again, the reason for this change in resistivity is disputed. For example, it could be due to changes in saturation or wettability, or due to dehydration of clay minerals.

Shale gas has become profitable (some say too profitable!) mainly due to advancements in horizontal drilling and fracing. The resources devoted to these two are much higher than for other branches of the oil field business. The resources used to gather information to make informative decisions for frac designs pale in comparison to those needed for fracing. Furthermore, these decisions are based on empiricisms that might be fraught with ever more errors. Even the terminology used is inexact. Shale reservoirs need not necessarily contain any clay minerals. They need not even have a common lithology: some shale reservoirs are carbonate muds while others are siliciclastic. Numerous alternate terms have been used for shale reservoirs: unconventionals, self-resourcing rocks; organic-rich rocks, mudstones, etc. But, the term shale appears to have stuck.

Other branches suffer from similar problems of inexact terminology. For example, for fracturing a rock, the brittleness ratio is often used. This Brittleness Ratio is derived from sonic data from well logs – mostly Young’s modulus and Poisson’s Ratio. However, stiffness, modulus and velocity are elastic properties, while brittleness (or its cousin, the newly minted term “Fracability”), hardness, and toughness are fracture or static deformation properties. Brittleness is the ratio between Hardness and Toughness, where Hardness is resistance to deformation and Toughness is resistance to fracture! In relating the elastic properties to static deformation properties, we need site-specific dynamic to static conversions. Young’s modulus and Poisson’s ratio are derived from Vp and Vs measured in well logs by assuming an isotropic and homogeneous system. If the reservoir rock is anisotropic, Poisson’s ratio calculations will be in error. If the reservoir rocks change from gas saturation to water saturation, the Young’s modulus – Poisson’s ratio plots will be reversed. At the very least, operators might want to account for changes in saturation and use anisotropic equations to calculate the stiffness coefficients before using them for Fracability.

With the articles in this focus section, we hope to provide you with highlights about possible ways to think about the problems. We show how seismic information might be understood to derive important properties of fluids. We also show how flow properties might change over the life span of a reservoir given pressure changes that it experiences.

In the first article entitled “Assessing Knudsen flow in gas-flow models of shale reservoirs”, by Kuila et al. uses a theoretical framework to calculate the gas flow in nanoporous systems. These calculations show that diffusion flow can be important in gas shales and indiscriminate drawdown might move the reservoir from Darcy flow to diffusion flow regimes.

In the second article in this section entitled “Characterization of sandstone reservoirs using Poisson impedance inversion”, Sharma and Chopra show how to detect and characterize the often thinly-bedded reservoir sequences. They use the difference between Vp and Vs to demarcate gas zones. They show in a λρ – μρ cross-plot, gas-saturated zones often plot in a distinct location away from water- or oil-saturated zones: gas-saturated zones have lower λρ values and higher μρ values than the background shale. Their lithology impedance (LI) can help differentiate clean zones from shaly zones while their fluid impedance (FI) can help predict fluid contents.

In the last paper entitled “Conventional approach for characterizing unconventional reservoirs”, Sharma and Chopra discuss various workflows to characterize shales. They present an integrated workflow to determine P- and S-impedances. They use a model based inversion to compute P- and S-impedance, and further using these impedance information, they derive other attributes, such as λρ, μρ, and Vp-Vs ratios. They demonstrate usefulness of their approach with an example from the Montney formation and are able to make maps of seismic attributes that might help delineate areal extent of fracable zones.

Exploration and exploitation of shale reservoirs is an exciting new field that has rejuvenated the industry. We hope that the readers find these articles interesting and inspiring!