Production-induced instability of a gentle submarine slope: Potential impact of gas hydrate exploitation with the huff-puff method

Highlights

• A slope stability model is developed for slopes undergoing hydrate production.

• The impact of hydrate huff-puff production on submarine slope instability is analyzed.

• A concept of multi-objective optimization for hydrate production scheme is proposed.

Abstract

Natural gas in clathrate hydrates is regarded as a potential energy source that has received increased attention to optimize production strategies with controllable impacts on the environment. This paper investigates possible instability of a gently sloping reservoir of oceanic hydrates induced by gas production using the huff-puff method through a horizontal well. The geomechanical stability of the slope is analyzed within the framework of the limit equilibrium method by considering the dynamic change in the pore pressure and the strength parameters of the slope during gas production. The production process is simulated by a coupled analysis of heat and flow transport considering thermal effects of hydrate dissociation and formation, and the time-dependent pore pressure and strength parameters are attained from this analysis and passed to the slope stability analysis. Parametric studies are performed to screen the optimal production scenario under different site conditions. Being part of the huff-puff production process, thermal stimulation during the huff stage poses a risk of production-induced instability to the slope. Overpressure is the dominant cause for slope failure, and strength reduction due to hydrate dissociation plays a secondary role in the studied scenario. The production-induced slope failure likely takes place at a site with interbedded geological structures that promote overpressure expansion in a laterally extending band beneath the potential failure surface. Thus, the geological structures should be properly modelled in reservoir simulations, as they could impact the production effectiveness and geomechanical response of the reservoir remarkably. This study demonstrates a need for a multi-objective optimization procedure to seek the overall optimal production strategy, since the economically optimal option is not necessarily free of risk of production-induced geo-hazards.

Introduction

Natural gas hydrate, mainly methane hydrate (MH), is abundant in voids of sediments in permafrost regions and continental slopes, and it is a promising source of future energy drawing worldwide attention of research. The global reserve of gas hydrates locks the carbon content about two times as much as that of conventional fossil fuels (Boswell, 2009; Piñero et al., 2013). A number of possible production methods have been proposed to extract natural gas from hydrates by breaking the equilibrium temperature-pressure conditions of stable hydrates. Differently from conventional oil and gas production, producing gas from an oceanic hydrate reservoir could affect the mechanical stability of the reservoir where hydrates have grown in pores of unconsolidated deposits (Lee et al., 2011). During production, dissociated hydrates are no longer part of the load-bearing frame of the reservoir (Miyazaki et al., 2011), and meanwhile pore pressure could rapidly build up due to volume expansion arising from liberated gas and water (Xu and Germanovich, 2006). As a result, inappropriate production schemes could trigger submarine landslides, which may pose a catastrophic risk to offshore facilities and coastal cities, and even accelerate global warming with a large amount of released methane, a greenhouse gas 84 times stronger than carbon dioxide (Ruppel and Kessler, 2017). Several extraordinary submarine landslides in the history, such as Storegga Slide (Paull et al., 2007), Hinlopen-Yermak Megaslide (Geissler et al., 2016) and Amazon Fan failure (Maslin et al., 1998), were likely primed by hydrate dissociation. Hence, it is crucial to include the assessment of production-induced seafloor instability into the process of decision-making to rule out risky production sites and schemes.

Several methods for extracting gas from oceanic hydrates are theoretically feasible, such as depressurization, thermal stimulation, inhibitor injection and carbon dioxide replacement (Li et al., 2016). Among these methods, depressurization is superior due to technical simplicity and economic efficiency. However, depressurization alone hardly provides long-term stable production especially in fine-grained reservoirs of low permeability where production is highly hampered by sanding due to high pressure drop and wellbore plugging by secondary hydrates due to temperature drop (Moridis et al., 2011a; Sun et al., 2016; Yamamoto et al., 2014). Alternatively, a method combining depressurization and thermal stimulation appears plausible, for instance, the huff-puff method (Falser et al., 2012; Song et al., 2015; Wang et al., 2014), also known as the cyclic steam stimulation. This method is a multi-cycle process, and each cycle consists of three stages: (1) the huff stage for injecting hot water or steam into the reservoir via a well; (2) the soaking stage for poising the well to drive sufficient heat to the reservoir; and (3) the puff stage for extracting gas from the well by depressurization. It was numerically illustrated that the huff-puff method is promising in heterogeneous low-permeable reservoirs of gas hydrates (Hou et al., 2016). A few studies were conducted to improve the economic efficiency of the method by optimizing well configurations and production scenarios (e.g. Feng et al., 2014, Feng et al., 2015; Li et al., 2011; Li et al., 2012a; Liu et al., 2018a, Liu et al., 2018b; Su et al., 2012; Wang et al., 2014). As suggested in these studies, production with a horizontal well (Feng et al., 2015) and multiple vertical wells (Wang et al., 2014) apparently leads to higher efficiency than with a single vertical well. To seek optimal production schemes, efforts were made to explore the effects of production parameters such as the injection rate, the production rate, the injected brine temperature, and the permeability of reservoirs (e.g. Li et al., 2011; Liu et al., 2018a, Liu et al., 2018b). In addition to the economic issue, the concern of production-induced seafloor instability associated with the huff-puff method deserves particular attention. During the huff stage, a reservoir could be greatly over-pressured due to volume increase in pore fluids arising from phase change in addition to thermal expansion and mass injection. The mechanical stability of the reservoir could be reduced to a significant extent, given that the overpressure resulting from hydrate dissociation under undrained conditions could reach up to several to tens of megapascals (Xu and Germanovich, 2006).

The possibility of induced seafloor instability during gas production from oceanic hydrates has been explored in a few studies. It is reported that production with depressurization will not significantly compromise the stability of a gentle slope (Moridis et al., 2018; Zander et al., 2018) although subsidence will be expected (Lin et al., 2019). In contrast, thermal production poses a risk to sloping reservoirs as a result of overpressure (Moridis and Kowalsky, 2006). Rutqvist et al. (2010) numerically studied the stability of a sloping reservoir of oceanic hydrates subjected to thermal stimulation, and reported that the fluid pressure increases so much adjacent to the well that the sediment is completely unloaded with associated loss of mechanical strength and inelastic yielding. Song et al. (2019) concluded from their numerical simulations that a short-time thermal injection during depressurization can significantly increase yield at early stages and however rapidly reduce the slope stability. These studies suggest the destabilizing effect of thermal stimulation on submarine slopes. However, no study has been conducted to address the concern of slope stability for the huff-puff method, in which thermal injection is the essential part of the production scenario.

The challenge for studying production-induced slope instability lies in the complexity of hydrate production processes in addition to the difficulty in reservoir characterization. Hydrate production involves interplaying processes including heat transfer, multiphase flow, geomechanical deformation, and phase change due to hydrate dissociation/formation in the reservoir. The so-called Thermo-Hydro-Chemo-Mechanical (THMC) coupled analysis, which considers these processes in a fully coupled approach, is computationally demanding and hampered by immature geomechanical models of hydrate-bearing sediments. Alternatively, the mechanical part is decoupled for improving computational efficiency and meanwhile maintaining sufficient accuracy. In particular, the limit equilibrium method was modified for the slope stability analysis associated with hydrate dissociation (Kwon and Cho, 2012; Liu et al., 2020; Nixon and Grozic, 2007; Sultan et al., 2004). In this method, it is particularly important to precisely estimate the development of pore pressure in the slope with an appropriate approach. To commence this goal, characterizing the reservoir to a sufficient level is also necessary. The transport of pore fluids and, in turn, the development of pore pressure are controlled by the heterogeneity in the permeability of slope deposits, which results from geological structures such as faults, mud diapirs, and gas chimneys. Being widespread in fine-grained reservoirs of oceanic hydrates (Mangipudi et al., 2014; Miller et al., 2012; Sun et al., 2012; Yoo et al., 2013), these structures could be detrimental to slope stability. For instance, Elger et al. (2018) illustrated a slope failure related to a pipe structure at the base of gas hydrate stability zones, through which the overpressure transfers into an extensive shallow permeable bed and thereby destabilizes the slope. However, existing studies of production-induced seafloor instability are mostly based on idealized strata composed of homogenous layers without in-depth consideration of particular geological structures.

This paper explores the possibility of slope instability induced by gas production from a reservoir of oceanic hydrates by using the huff-puff method with a single horizontal well. To achieve rational results with affordable complexity, this study stays within the framework of the limit equilibrium method of the slope stability analysis, while the dynamic changes of the pore pressure and the soil strength during gas production are computed by using a numerical simulator developed for analyzing heat and flow transport in hydrate-bearing porous mediums. This Methodology has been applied to an infinite slope undergoing hydrate dissociation due to climate warming (Liu et al., 2020), and extended here for a two-dimensional (2D) slope. A parametric study is performed to examine the system behavior in response to different production scenarios from the perspective of economic efficiency and geomechanical stability. To investigate the worst case arising from adverse geological structures, the analysis is extended to a hypothetical slope with an interbedded lenticular layer of high permeability. The novelty of this study is to numerically identify the possibility of production-induced instability of sloping hydrate reservoirs undergoing the huff-puff stimulation, and highlight the need of a multi-objective optimization procedure for the overall optimal production scheme.