FROZEN HEAT | Volume 2

(oil, gas, and/or water) from flowing up the well uncontrolled. By pumping a portion of the fluid out of the well casing, the pressure exerted on the bottom of the well (and thus on the reservoir in contact with the well bore through the perfora- tions) can be reduced in a controlled manner. In the case of a gas hydrate reservoir, once the pressure is reduced below the gas hydrate stability condition, dissociation of gas hydrates will occur in the vicinity of the perforations, releasing gas and wa- ter that will then flow to the well. The efficiency of this tech- nique is influenced by the abundance and inter-connectivity of pores containing liquid water, which enable the transmission of the pressure change into the formation. For some reservoir settings, particularly those near the base of the gas hydrate stability zone, a free-gas interval may directly underlie the gas hydrate deposit (Makogon 1981; Moridis et al. 2007; 2011). In these cases, the well could be perforated in the free-gas zone, enabling production of the free gas. As envisaged by Makogon (1981) and shown by Grover et al. (2008) for the Messoyakha gas field, the resulting pressure reduction within the free-gas interval can be transmitted to the overlying gas-hydrate-bearing sediments, inducing disso- ciation of their gas hydrate content. In theory, such settings should yield promising productivity, although no significant deposits of this type have been verified to date. One practical consideration of the depressurization tech- nique is that gas hydrate dissociation is an endothermic (heat absorbing) process that induces cooling of the local forma- tion. If the magnitude of the temperature reduction is suffi- ciently large, gas hydrate dissociation can be impeded. If the dissociation-inducing depressurization leads to pressures below that at the quadruple point of the hydrate (that is, the point where free gas, liquid water, ice, and hydrate coexist), the liquid pore water can actually freeze. Preliminary reser- voir simulation modelling suggests that this process depends on the initial reservoir conditions and the production rate (or the constant bottomhole pressure at which the well may be operated), with transfer of heat resulting from pore-water movement being particularly important. A similar consideration, commonly encountered with conven- tional gas wells, is the temperature regime of the free gas as it flows to the well and up the production tubing. In this case, the

(Dallimore and Collett 2005), a Japanese study in the Nankai Trough (Takahashi and Tsuji 2005), a 2007 drilling program in northern Alaska (Hunter et al. 2011), and a 2012 testing program conducted also in Alaska (Schoderbek, 2012). Makogon (1981) has suggested that gas production from the Messoyakha gas field in Siberia was enhanced by significant long-term dissociation of an overlying gas hydrate deposit in contact with the conventional free-gas reservoir below. While there is evidence to suggest that some of this gas was indeed produced from the hydrate deposit by depressurization, as ex- traction of free gas from the underlying conventional reservoir decreased local formation pressures (Grover et al. 2008), there is continuing debate about this interpretation (Collett and Gins- burg 1998). Unfortunately, the lack of field data to confirm the initial conditions at Messoyakha or to quantify the production response greatly limits any modern engineering evaluation. 3.4.1 Depressurizationof the reservoir Currently, the depressurization technique is considered the most cost-effective and practical way to dissociate gas hydrates (Moridis et al. , 2009). The primary method involves reducing reservoir pressure by mechanical means. This can be done by directly reducing the reservoir pressure or by reducing the pressure in the overlying or underlying sediments in con- tact with the gas hydrate reservoir and allowing this pressure change to transfer to the reservoir naturally.. Originally, it was assumed that the formation of gas hydrates consumed all free water in the sediment pores, creating a relatively contiguous solid hydrate phase that effectively prevented the transmission of a pressure change into the formation. However, field pro- grams (Kleinberg et al. 2005) and laboratory studies (Kvamme 2007; Jaiswal et al. 2009; Minagawa 2009) have found that even the richest gas hydrate accumulations retain small but measurable volumes of mobile liquid water, sufficient to sup- port the propagation of a pressure field into the formation. Using conventional oilfield technology, depressurization can be accomplished by perforating the production well casing at the target interval and reducing the weight of the fluid within the well. Normally, a well is filled with fluid from top to bot- tom. The weight of the fluid is balanced against the pressure of the reservoir in order to prevent the contents of the reservoir

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