The concerted international effort during the last decade has led to significant prog­ress and important advances [125, 127]. While most of the most important issues affecting gas production from geological GH systems have been addressed or are being addressed, some important issues need further attention and examination. Below we discuss the remaining challenges.

5.2 Challenges in the Analysis and Interpretation of Geophysical Surveys

This issue has been the subject of recent extensive review articles. The interested reader is directed to the papers of Bellefleur et al. [5] , Dvorkin and Uden [42] , Hardage et al. [62], Lee et al. [103], Inks et al. [71], and Boswell [9] for detailed discussions. In general, the current consensus is that GH exploration is fairly well advanced when assessing deep deposits at high SH, but challenges remain with respect to shallow hydrates and GH of complex geometries. The well-log analysis following the 2002 Mallik Test [35] still represents the state of the art in this subject. In log analysis, the approach appears to be well established for the sand reservoirs, but for grain-displacing, clay-hosted hydrates, further work is needed [102].

The current bottleneck in the state-of-practice of geophysical analysis is centered on the relationship between measured physical parameters and SH. In particular [103, 173]:

• Electrical conductivity and Archie’s Law: most applications use empirical and fitting parameters, leading to good fits to the data but uncertain predictions.

• P-wave velocity data—Biot-Gassman. The constrained modulus is strongly affected by the stiffness of the pore fluid, leading to potential error magnification in the assessment of SH from Vp.

• S-wave velocity dependence on SH and pore habit: proper constitutive models are needed for effective stress-dependent sediment shear stiffness and to account for the impact of hydrate on skeletal stiffness.

Laboratory-scale, borehole-scale and field-scale geophysical measurements are all conducted at different frequencies and wavelengths, hence they sense different temporal and spatial conditions, and caution is advised in the interpretation of mea­surements. An even bigger challenge is the need to expand to integrated geophysical analyses that include multicomponent seismic surveys [63, 174] . in combination with promising EM techniques [43, 163, 201, 211, 175].