Enhancement of gas production from natural gas hydrate reservoir by reservoir stimulation with the stratification split grouting foam mortar method

Highlights

• The stratification split grouting foam mortar is a novel method of gas production from NGH.

• The method could improve the fracture width and maintain the fracture during gas production.

• The gas production and energy efficiency are effectively improved by SSGFM method.

• The FMLs will help to maintain reservoir stability while improving gas production.

• The optimizing parameters of FLMs in view of the hydrate reservoir at SH2 site were proposed.

Abstract

The stratification split grouting foam mortar method (SSGFM) was first proposed to stimulate the low-permeability gas hydrate reservoir through constructing the fast flow channel for enhancing gas production. Based on the low-permeability hydrate-bearing sediments (HBS) at SH2 site in the Shenhu area of South China Sea, the foam mortar layer (FML) reservoir model was constructed. The numerical simulation that coupled thermal-hydraulic-mechanical processes was employed to evaluate the efficiency and feasibility of this method. Results show that the FML could effectively promote the expansion of the low-pressure zone into the hydrate reservoir, and enhance gas hydrate dissociation rate, cumulative gas production and energy efficiency. The foam mortar layers (FMLs) have significant influence on the spatial distribution and the evolution characteristics of reservoir thermophysical and geomechanical parameters during gas production. The sensitivity analysis of FML shows the number of FMLs, thickness and permeability of FML exist the critical values for meeting the demand of promoting hydrate dissociation and gas production. Although the cumulative gas production and gas production rate will greatly increase with enlarging the radius, the gas-to-water ratio increase slightly. In view of the hydrate reservoir at SH2 site, the recommended number of FMLs is 4–6, and the distance between the FML and overburden/underburden should be more than 7.5 m. In addition, the optimal values of thickness, radius and permeability of FML are 5 cm, 40 m and 1 × 10⁻¹⁰ m², respectively. The geomechanical response indicates that the FMLs are beneficial for reservoir stability while improving gas production.

Introduction

Natural gas hydrate (NGH) is one kind of cage compound formed by natural gas (mainly CH₄) and water under low-temperature and high-pressure coexisting conditions (Sloan and Koh, 2008). NGH occurs mainly in deep oceanic sediments and permafrost regions (Max, 2003). Technically recoverable global volumes of NGH are estimated to be on the order of 3 × 10¹⁵ m³ (Boswell et al., 2011). However, roughly 90% are incased in clayey-silt sediments, which are difficult to exploit owing to low permeability and high content of clays (Boswell et al., 2011; Li et al., 2018a). In 2017, the field production test in South China Sea was the first trial on NGH bearing in clayey-silt sediments in the world, and the test lasted for 60 days with total gas production of 3.09 × 10⁵ m³ (Li et al., 2018a).

The production test area is located in the Baiyun sag in the middle of the continental slope of southeast Shenhu area, northern South China Sea between Xisha Trough and Dongsha Islands, as shown in Fig. 1 (Zhang et al., 2019b; Liu et al., 2017). In 2007, 2015 and 2016, three gas hydrate drilling expeditions (GMGS1,3 and 4, as shown in Fig. 1) were conducted in the Shenhu area, northern South China Sea by Guangzhou Marine Geological Survey (GMGS) (Zhang et al., 2007; Yang et al., 2015, 2017). The results show the hydrate-bearing sediments (HBS) in the Shenhu area of China, is mainly composed of fine-grained clayey-silt with low permeability (Li et al., 2018a; Liu et al., 2017; Zhang et al., 2007; Yang et al., 2015, 2017). The content of clay minerals is 12.8–30.33%, which are mainly montmorillonite and illite. The mean median grain size of HBS based on the core analysis of production well (SHSC-4) is about 12 μm (Li et al., 2018a). The permeability of HBS is in the range of 1.5 mD-70 mD (Li et al., 2018a). In addition, the high content of clay minerals results in high bound water concentration, which will reduce the gas and water relative permeability (Liu et al., 2016). The very low permeability of HBS is one main challenge to be overcome in order to improve the gas production from NGH reservoir. So, we should construct fast flow channel in clayey-silt NGH reservoir to improve gas and water flow speed during gas production from NGH. The reservoir stimulation is an important measure to construct fast flow channel and promote the gas recovery from low-permeability hydrate reservoir.

Reservoir stimulation method (micro-fracturing around the borehole by hydraulic slotting) has been used to increase the reservoir exposing area exposed to the borehole in the first field production test in South China Sea (Li et al., 2018a). However, the stimulation range is relatively small, and the reservoir exposing area exposed to the borehole is only more than three times than before (Li et al., 2018a). Hydraulic fracturing can greatly increase reservoir exposing area, and several researches have discussed the feasibility and enhancement effect of this method on gas production from gas hydrate by numerical simulations (Chen et al., 2017; Yang et al., 2018; Iseux, 1992; Wang et al., 2018; Feng et al., 2019; Sun et al., 2019a; Shan et al., 2020). Chen et al. (2017) and Yang et al. (2018) made the sensitivity analysis of fracture quantity, fracture spacing, initial NGH saturation and intrinsic permeability of reservoir on gas production from NGH reservoir of the SH7 site in the Shenhu area. Iseux (1992) proposed to developed gas hydrate reservoir with the thermal fluid fracturing technique. Wang et al. (2018) proposed a coupled temperature field model for thermal fluid fracturing to accurately simulate the temperature fields considering the effects of the changes in the hydrate dissociation enthalpy and the formation permeability on the heat and mass transfer. Feng et al. (2019) proposed the combination of hydraulic fracturing and depressurization method to enhance the efficiency of gas production from methane hydrate reservoir. They set a radial high-permeability zone in the methane hydrate reservoir to mimic the impact of elliptic fractured zone induced by hydraulic fracturing and analyzed the effect of fractured zone absolute permeability and initial reservoir temperature on gas production from NGH reservoir at SH2 site in the Shenhu area (Feng et al., 2019). Sun et al. (2019a,b) assessed the gas production potential of a depressurization horizontal well that was assisted by the hydraulic fracturing (horizontal fracture, vertical fracture) according to field data at SH2 site. They found that the effect of horizontal fracture on gas production was better than that of vertical fracture. Shan et al. (2020) proposed the frac-packing technique to stimulate production for gas hydrate reservoir and derived an analytical model to predict the propagation of a horizontal fracture and to assess the well productivity in frac-packed gas well in a gas hydrate reservoir. Based on analysis of the above literatures, we know hydraulic fracturing will greatly enhance gas production from NGH reservoir. In order to verify the feasibility of this technology on gas hydrate reservoir stimulation, several researches did some laboratory tests (Ito et al., 2008, 2011; Konno et al., 2016; Too et al., 2018a, 2018b). Ito et al. (2008, 2011) employed a triaxial hydraulic fracturing system with a cubical specimen of 200 × 200 × 200 mm³ to study hydraulic fracturing behavior in silica sand which had kaolinite interlayer. They found that fracture formed preferentially at the junction of two different layers and increasing injection rate would make the fracture have more branches. Konno et al. (2016) used the hydrate-bearing sandy core in a triaxial apparatus to study its fracturability and permeability change after fracturing test. They found that the observed fracture behavior of hydrate-bearing sandy yielded the tensile failure mode, and the permeability was increased after fracturing and maintained even after re-confining and closing the fractures. Too et al. (2018a) used three approaches to determine the apparent fracture toughness of frozen sand using hydraulic fracturing in a penny-shaped crack. Too et al. (2018b) also tested the frackability of synthesized high saturation methane hydrate bearing sand specimens (approximately 50–75%) using hydraulic fracturing in a penny-shaped crack. In addition, the apparent fracture toughness and tensile strength and characteristic length were estimated (Too et al., 2018b). Except for hydraulic fracturing, another reservoir stimulation method (enlarging highly permeable well wall) was proposed and discussed by Zhang using numerical simulations (Zhang et al., 2019a).

Based on analysis of the above literatures, we can know hydraulic fracturing could construct fast flow channel in HBS and improve gas production from gas hydrate. However, the fracture width of hydraulic fracturing was usually small. In addition, the clayey-silt HBS may appear cavity structure or become starchy due to the dissociation of gas hydrate, which will lead to the failure of reservoir stimulation due to fracture closure and permeability reduction. In order to solve this problem, we proposed the reservoir stimulation with the stratification split grouting foam mortar method (SSGFM) for the first time to improve the fracture width and maintain the fracture during gas production. To evaluate the efficiency and feasibility of this method, we constructed a foam mortar layer (FML) reservoir model based on the low-permeability hydrate-bearing reservoir at SH2 site, and investigated the behaviors of hydrate dissociation and gas production and geomechanical responses by numerical simulation, as well as the factors affecting them. It was hoped that this research may provide a new idea for improving gas production from low permeability hydrate-bearing reservoir in future.