Although the available body of information on production from hydrates is still limited, there is sufficient information to begin identifying particular features, prop­erties, conditions, and production methods that are linked to a higher gas production potential and increase the desirability of hydrate deposits, and to use this informa­tion to develop a set of guidelines for the selection of promising production targets.

5.1 Desirable Features and Conditions

These include the following [125]:

• Large formation k and j, which are almost invariably associated with sandy and gravely formations that are characterized by low P, S and S, leading to

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relatively high permeability to gas and aqueous flow.

• Medium SH (i. e.,30% < SH < 60%) corresponds to optimal QP in terms of magnitude and length of time to attain it. The effect of SH on production is not monotonic, but a complex function of SH and the timeframe of observation. A lower SH has the advantage of higher keff, leading to an earlier evolution of gas and a larger initial QP [131,132]. The disadvantages of a lower SH may include a larger water production and a lower total gas production because of early exhaustion of the resource. A high SH leads to slower evolution of gas and lower initial QP, but a higher maximum QP and total production.

• The most desirable targets can be easily identified from the inspection of the phase diagram (Fig. 14). The larger T provides a larger source of sensible heat to support the endothermic dissociation, and a larger initial P allows a larger pres­sure drop, leading to larger production rates. Thus, (a) hydrates that occur along the equilibrium line are very desirable, and (b) the desirability increases with an increasing equilibrium P (and, consequently, T). The production potential decreases as the stability of the hydrate deposit at its initial conditions increases (as quantified by the pressure differential DP = P-Pe at the prevailing reservoir T). In practical terms: we target the deepest, warmest reservoirs that are as close as possible to equilibrium conditions. In addition, the deeper reservoirs have larger overburdens and are therefore less prone to adverse geomechanical impacts.

• For reservoirs with the same hydraulic properties, SH, and P: the warmest possi­ble deposit is the most desirable. For reservoirs with the same hydraulic proper­ties, SH, and T: the reservoir with the lowest possible P is the most desirable.

• In terms of deposit classes: All other conditions and properties being equal, Class 1 deposits appear to be the most promising targets for gas production because of the thermodynamic proximity to the hydration equilibrium. Additionally, the existence of a free gas zone guarantees gas production even when the hydrate contribution is small.

• Class 2 and Class 3: Class 2 deposits can attain high production rates, but are also burdened by longer lead times of very little gas production; Class 3 deposits may yield gas earlier and can attain significant production rates, but there are indica­tions that these are lower than in Class 2. The relative merits of these two types will likely be determined by site-specific conditions.

• All classes: The difficulties of site access notwithstanding deeper and warmer oceanic accumulations appear to be more productive than permafrost ones they can have (a) a higher T (14°C is the maximum equilibrium temperature observed in permafrost-associated deposits) and a larger sensible heat available for disso­ciation and (b) a higher P, increasing the depressurization effectiveness, in addi­tion to (c) the beneficial dissociating effect of salt.

• All classes: The importance of impermeable or near-impermeable upper bound­aries cannot be overemphasized.

• In terms of production method: Depressurization appears to have a clear advantage in all three classes.