These include the following [125]:

• Class 4 deposits: Earlier studies by Moridis and Sloan [140] have indicated the hopelessness of such deposits under any combination of conditions and produc­tion practices.

• Fine sediments (i. e., rich in silts and clays), deformed fractured systems, and hydrates in veins and nodules despite high SH, because of the geomechanical instabilities in such systems under production.

• In Class 2 deposits: Inappropriate well configurations [131].

• In Class 2 deposits: Constant-P production, because it can lead to early break­through and massive water production; however, such an approach can be used in a short-term flow test to determine the HBL properties [55].

• In Class 2 deposits: Deep WZ, and/or permeable overburden and underburden, can drastically reduce gas production [160]. Additionally, the use of multi-well (five — spot) systems involving simultaneous depressurization (at the production well) and thermal stimulation (through warm water injection) appears disappointing [128].

• In all Classes: Permeable upper boundaries drastically reduce production [157,160].

• In all Classes: Pure thermal dissociation methods and/or inhibitor methods have high cost and limited (and continuously eroding) effectiveness [132].

• In all Classes: SH that are so high that the remaining fluids are below their irre­ducible saturation levels. Such hydrates may not be prone to depressurization — induced dissociation.

• In all Classes: Fracturing appears to have limited effect on increasing productivity from hydrate deposits [53,94].