George J. Moridis

Abstract: The Effect of Reservoir Heterogeneity on Gas Production from Hydrate Accumulations in the Permafrost Matthew T. Reagan 1 , SPE, Michael B. Kowalsky 1 , George J. Moridis 1 , SPE, and Suntichai Silpngarmlert 2 , SPE Lawrence Berkeley National Laboratory, 1 Cyclotron Rd., Berkeley, CA 94720 ConocoPhillips, 600 North Dairy Ashford, Houston, TX 77252 Abstract The quantity of hydrocarbon gases trapped in natural hydrate accumulations is enormous, leading to significant interest in the evaluation of their potential as an energy source. Large volumes of gas can be readily produced at high rates for long times from methane hydrate accumulations in the permafrost by means of depressurization-induced dissociation combined with conventional technologies and horizontal or vertical well configurations. Initial studies on the possibility of natural gas production from permafrost hydrates assumed homogeneity in intrinsic reservoir properties and in the initial condition of the hydrate-bearing layers (either due to the coarseness of the model or due to simplifications in the definition of the system). These results showed great promise for gas recovery from Class 1, 2, and 3 systems in the permafrost. This work examines the consequences of inevitable heterogeneity in intrinsic properties, such as in the porosity of the hydrate-bearing formation, or heterogeneity in the initial state of hydrate saturation. Heterogeneous configurations are generated through multiple methods: 1) through defining heterogeneous layers via existing well-log data, 2) through randomized initialization of reservoir properties and initial conditions, and 3) through the use of geostatistical methods to create heterogeneous fields that extrapolate from the limited data available from cores and well-log data. These extrapolations use available information and established geophysical methods to capture a range of deposit properties and hydrate configurations. The results show that some forms of heterogeneity, such as horizontal stratification, can assist in production of hydrate-derived gas. However, more heterogeneous structures can lead to complex physical behavior within the deposit and near the wellbore that may obstruct the flow of fluids to the well, necessitating revised production strategies. The need for fine discretization is crucial in all cases to capture dynamic behavior during production. Introduction Objective. This investigation is part of an effort led by the U.S. Department of Energy to identify appropriate targets for a long-term field test of production from permafrost-associated hydrate deposits (Boswell et al, 2008). The main objective of this study is to determine through sensitivity analysis the possible effects of deposit heterogeneity on productivity, and assess the production strategies required to achieve economically viable production. Background. Gas hydrates are solid crystalline compounds in which gas molecules (referred to as guests) occupy the lattices of host ice crystal structures. Their formation and dissociation is described by the general equation: G + N H H 2 O = G•N H H 2 O, where N H is the hydration number, and G is a hydrate-forming gas. Natural hydrates in geological systems contain G = CH 4 as their main gas ingredient, and occur in two distinctly different geologic settings where the necessary conditions of low T and high P exist for their formation and stability: in the permafrost and in deep ocean sediments. The quantity of CH 4 contained hydrates is the subject of continuing debate, and estimates vary widely between 10 15 and 10 18 ST m 3 (Sloan and Koh, 2008; Milkov, 2004; Klauda and Sandler, 2005). The general consensus is that this quantity is huge, easily exceeding the total energy content of the known conventional fossil fuel resources. Even if only a fraction of the most conservative estimate of the resource is recoverable, the quantities involved are large enough motivate further evaluation of hydrates as an energy source (Makogon, 1987; Dallimore et al., 1999; 2005). As result, many studies have evaluated the technical and economic feasibility of gas production from natural hydrate accumulations (Moridis, 2003; Sun and Mohanty, 2005; Moridis and Sloan, 2007; Moridis and Reagan, 2007a;b;c; Kurihara et al., 2009; Moridis et al., 2009). Gas can be produced from hydrates by inducing dissociation using any of the three main dissociation methods (Makogon, 1997): (1) depressurization below the hydration pressure P e at the temperature T, (2) thermal stimulation, based on raising T above the hydration temperature T e at the prevailing pressure P, and (3) the use of inhibitors (such as salts and alcohols) that shift the P e -T e equilibrium. However, multiple studies (Moridis and Reagan, 2007a;b; Reagan et al., 2008) have demonstrated the depressurization is typically the most effective, efficient, and economically viable method to dissociate hydrates in situ and enable production of hydrate-derived methane. Thermal stimulation is reserved for localized control of secondary hydrate or ice formation, or for the initial dissociation of hydrate around the well production interval (Moridis and Reagan, 2007b), as

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