The second natural gas hydrate production test in the South China Sea

Hydrate phase transition and seepage mechanism during natural gas hydrates production tests in the South China Sea: A review and prospect

The horizontal well technology was successfully applied in the Chinese second natural gas hydrate (NGH) field test in the Shenhu area of the South China Sea in 2020. However, the results show that the threshold for commercial exploitation has not been broken, judging from daily gas production and cumulative gas production. Consequently, the paper presents the effects of dual horizontal well systems for exploitation in this area. The NGH reservoir model in the Shenhu area was established with CMG software. The influence of various layout options and various spacing of dual horizontal well systems on the production capacity was investigated. Further, we simulated the production effect of dual horizontal well systems joint auxiliary measures, such as well wall heating, heat injection, etc. The results show that the production capacity of dual horizontal well systems increased by about 1.27~2.67 times compared with that of a single horizontal well. The daily gas production will drop significantly, no matter which method was used, when exploitation lasts for about 200 d. Meanwhile, well wall heating and heat injection have limited effects on promoting production capacity. In conclusion, attention was drawn to the fact that the synergistic effect could be fully exerted to accelerate NGH dissociation when dual horizontal well systems are applied. The NGH reservoirs in the Shenhu area may be more suitable for short-term exploitation. The research results of this paper can provide a reference for the exploitation of the Shenhu area.

Most natural gas hydrate (NGH) reservoirs have poor permeability characteristics, which is unfavorable to fluid flow and gas production. To enhance gas production, a new method of radial water jet slotting and grouting (RWJSG) was proposed to reconstruct the formation around horizontal wellbores of a horizontal well. To evaluate the effectiveness of the proposed method, a three-dimensional (3D) gas production model was constructed with reference to the NGH reservoir at the SH2 site in the South China Sea (SCS). The results show that RWJSG can effectively enhance the gas recovery of the horizontal well. Compared with an unreconstructed case, the hydrate dissociation efficiency, cumulative gas production, and average ratio of gas to water production in the RWJSG case are increased by 121.0, 116.3 and 46.1%, respectively, during the 720-day production period. RWJSG can enlarge reservoir exposed area, promote pressure drop transfer, and improve gas/water flow near the wellbore. In addition, the effect of increasing production mainly depends on the slot radius and number. Within a certain range, the decrease in slot spacing and increase in slot filler permeability have positive effects. For the reservoir at SH2 site, the scheme with a radius of 5 m, a spacing of 5 m, a number of 5 and a permeability of 500 mD could achieve remarkable stimulation effect. This work provides ideas and guidance for stimulation of horizontal wells in NGH reservoirs by RWJSG.

As an important unconventional gas resource, shale gas has become an important part of gas production in recent years with the advantage of horizontal well drilling and large-scale multi-stage hydraulic fracturing completion technologies. The shale gas reservoir numerical simulation advances were reviewed and a multi-stage fractured horizontal well numerical simulation was performed to qualitatively modeling the well productivity in over-pressured shale gas reservoir based on actual shale properties and well completion parameters. A single horizontal well model was established on the basis of dual-porosity model and logarithmically spaced grid refinement. A comprehensive comparison and analysis of the initial average gas production, daily gas production, cumulative gas production, adsorbed gas and free gas cumulative production were provided to investigate the influence of matrix permeability, SRV permeability, hydraulic fracture conductivity and half length, SRV size, bottomhole pressure on the well performance. The research shows that for the high matrix permeability (K m > 10 −7 mD) and low SRV permeability (K SRV < 0.01 mD), the SRV permeability has a significant impact on the initial average gas production. For the high matrix permeability (K m > 10 −7 mD) and medium SRV permeability (0.01 mD < K SRV < 0.5 mD), the initial average gas production is controlled by both the matrix and SRV permeability. For the high matrix permeability (K m > 10 −7 mD) and high SRV permeability (K SRV > 0.5 mD), the initial average gas production is mainly controlled by the matrix permeability. When the matrix permeability is lower than 10 −9 mD, the cumulative gas production is too low to be of economic interest. For the matrix permeability (10 −9 mD < K m < 10 −5 mD), the matrix permeability and SRV permeability are all important factors that influence the cumulative gas production. For the matrix permeability (K m > 10 −5 mD), the matrix permeability has much more impact on cumulative gas production than that of SRV permeability. The daily gas production and cumulative gas production are independent of hydraulic fracture conductivity and half length. The initial gas production of multi-stage fractured horizontal well is also independent of SRV sizes. The SRV size mainly controls the gas production decline characteristic. With the increase of the SRV size, the daily gas production declines slowly. The SRV size determines the cumulative gas production directly. With the increase of the SRV size, the cumulative gas production increases linearly. The bottomhole pressure has a significant impact on cumulative gas production. With the decrease of the bottomhole pressure, the cumulative gas production of 20 years increases linearly.

Coupled thermal–hydrodynamic–mechanical numerical simulation of natural gas hydrate horizontal well depressurization production: Method and application in the South China Sea

Hydrate reservoirs in the South China Sea belong to muddy silt‐type hydrate reservoirs with low permeability. Horizontal well fracturing is one of the main stimulation methods for low‐permeability oil and gas reservoirs. For muddy silt‐type hydrate reservoirs, the stability and effectiveness of hydraulic fractures may be a severe problem due to poor reservoir cementation, reservoir deformation, and sand production. In this paper, the time variability of fracture conductivity is considered for the first time. According to the geological data at offshore gas hydrate production test site in the South China Sea, a multilayer hydrate reservoir model was established, and the influence of the time variable fracture conductivity on the production behavior in the process of horizontal well fracturing was analyzed. The simulation results show that, compared with the case of constant fracture conductivity, the gas production rate with the case of time variable fracture conductivity is greatly reduced, and the 5‐year cumulative gas production is reduced by 49.9%. Sensitivity analysis shows that the larger the initial fracture conductivity, the higher the peak gas production rate; the larger the attenuation magnitude of the fracture conductivity, the lower the gas production rate in the late production period; the larger the decline rate coefficient of the fracture conductivity, the higher the gas production rate in the early production period. The analysis of the orthogonal experimental design shows that the influence of the attenuation magnitude of fracture conductivity, the initial conductivity of fracture, and the decline rate coefficient of fracture conductivity on the cumulative gas production decreases in order. The actual production process should try to avoid excessive attenuation magnitude of fracture conductivity, while improving the initial fracture conductivity as much as possible to ensure high cumulative gas production.

The feasibility of hydrate exploitation technology has been verified by two rounds of trial productions in the South China Sea, but it is also faced with the problem of low gas production efficiency. Therefore, this paper proposed four different hydrate production cases with multihorizontal wells and established simulation models. Then, the influence patterns of reservoir stimulation and offset distances on hydrate dissociation, saturation change, and pressure distribution were studied. To understand hydrate dissociation behaviors and production performances, the coupling effects of multihorizontal wells were discussed. Through simulation, the following conclusions could be drawn: (1) hydrate reservoir stimulation could effectively increase gas production. In these four production models, the cumulative gas production in Case C (multihorizontal wells + reservoir stimulation) was 5.27 times that of Case A (only multihorizontal wells) in 330 days. However, in Case D with screen completions, the gas output was 15.58% less compared with Case C. (2) Different offset distances of horizontal wells had a relatively minor impact on the cumulative gas yield and daily gas capacity. In addition, the variation range of hydrate saturation changed with offset distances of horizontal wells, indicating that hydrate dissociation occurred mainly around horizontal wells and fractures. (3) Considering interwells coupling, the daily gas rate and cumulative gas production of inner wells were higher than outer wells, and the cumulative gas volume increased by 6.4% in 330 days. Temporally and spatially, the hydrate saturation variation between wells was significantly faster than the outward expansion from the horizontal wells. Moreover, nearer to the axis of the horizontal wells, the variation in the hydrate saturation was more pronounced. These results could provide theoretical data to optimize marine hydrate development with multihorizontal wells.

The prediction of gas productivity and reservoir stability of natural gas hydrate (NGH) reservoirs plays a vital role in the exploitation of NGH. In this study, we developed a THMC (thermal-hydrodynamic-mechanical-chemical) numerical model for the simulation of gas production behavior and the reservoir response. The model can describe the phase change, multiphase flow in porous media, heat transfer, and deformation behavior during the exploitation of NGH reservoirs. Two different production scenarios were employed for the simulation: depressurization and depressurization coupled with CO2 exchange. The simulation results suggested that the injection of CO2 promotes the dissociation of NGH between the injection well and the production well compared with depressurization only. The cumulative production of gas and water increased by 27.88% and 2.90%, respectively, based on 2000 days of production simulation. In addition, the subsidence of the NGH reservoir was lower in the CO2 exchange case compared with the single depressurization case for the same amount of cumulative gas production. The simulation results suggested that CO2 exchange in NGH reservoirs alleviates the issue of reservoir subsidence during production and maintains good reservoir stability. The results of this study can be used to provide guidance on field production from marine NGH reservoirs.

Evaluation of natural gas hydrate resources in the South China Sea by combining volumetric and trend-analysis methods

A gas production system from methane hydrate layer by hot water injection using a pair of dual-horizontal wells has been proposed. Experiments with physical and numerical reservoir models have been carried out in order to simulate gas production characteristics with the system. In the experiments, the reservoir models consisting with ice of NaHCO3 aqueous solution formed in glass-bees porous medium were used to express the dissociation heat of methane hydrate by melting one of ice. Gas production at dissociation front of methane hydrate was simulated by gas generation with a chemical reaction of NaHCO3 included in the ice and HCl mixed in hot water injected at ice melting front. In the system, a dissociated region including the dual horizontal wells filled with hot water, named as hot water chamber, was generated to produce gas continuously. The gas production rate has the maximum peak just after breakthrough of injected water between dual horizontal wells, then it declined and gas was produced by almost constant rate. We have successfully developed the numerical model, and matched the history of physical gas production. Moreover, numerical simulations of gas production by the hot water injection into a Nankai Trough sediment layer model using a pair of dual horizontal wells 500m in length were carried out for a methane hydrate reservoir of 20 m in layer thickness, 46% of average methane hydrate saturation, 100 and 25 md in horizontal and vertical absolute permeabilities, respectively. The cumulative gas production is simulated as 5×106 std-m3 for initial two years. Furthermore, a new gas production scheme, which uses four pairs of dual horizontal wells in radian arrangement in a methane hydrate sediment layer with area of 1km×1km located at Nankai Trough, has been presented and evaluated with the numerical simulation as the cumulative gas production for 15 years is 1.3×108 std-m3. Introduction Reservoir characterization of methane hydrate bearing turbidite channel in the eastern Nankai Trough, Japan, has been proceeded to develop gas production method. The sediments are of the sand and mud alternation layers, which show heterogeneity characteristics especially on permeability(Suzuki et al., 2009). On the other hand, recent studies confirm that conserved methane hydrate (MH) deposits in sedimentation layers at a depth of more than several hundred meters from the bottom of the sea floor can be utilized as novel natural gas resources. In situ hydrate decomposition into water and gas is required to produce methane gas economically from these layers, since methane hydrate is a type of non-mobile solid energy resource. To trigger methane hydrate decomposition (see Masuda et al., 2002), decompression or temperature increase out of the equilibrium zone is necessary, while dissociation heat should be supplied for continuous gas production. Accordingly, new gas production systems that continuously supply the heat into the methane hydrate layers have been reported. The conventional methods of gas production to date include depressurization, inhibitor injection, and thermal recovery. Gas production by the hot water injection system is advantageous compared to depressurization (Kmath et al., 1991) and inhibitor injection in that the dissociation speed of methane hydrate and gas production rate may be relatively low due to a lower heat supply rate. On the other hand, the hot water injection method using a vertical wells requires drilling with high density into the targeted area (Masuda et al., 2002), since methane hydrates are formed in sand layers with very low permeability.

The China Geological Survey successfully completed the second trial of natural gas hydrate (NGH) production in 2020, and the daily gas production and total gas production were greater than those in the first trial, but the evaluation of NGH resources in the South China Sea (SCS) is still in the evaluation stage, specifically for the gas content. In 2021, some scholars used different methods to more accurately evaluate NGHs in the SCS to aid in NGH exploration and evaluation in the SCS; some of them analysed the main factors controlling NGH resources, added several factors to the volume method for optimization, studied the sensitivity of the evaluation of resources to each of the main controlling factors (NGH stability zone area, NGH area coefficient, NGH stability zone thickness, NGH thickness coefficient, NGH reservoir porosity, NGH reservoir saturation, NGH volumetric ratio, NGH resource ratio coefficient and NGH recovery coefficient) and found that the effective thickness coefficient, effective thickness and effective area coefficient are the most sensitive; these values are 23.15%, 15.66%, 13.98%, and 13.95%, respectively, while the other values are 0%, 15.66%, 8.42%, 8.41%, 2.50%, and 13.92%; taking into account the influence of the main controlling factors, the range of the water resource potential evaluation results in the SCS is 2.4 × 10 7 m 3 –5.9 × 10 14 m 3 , the modal value is 7.25 × 10 11 m 3 , and the confidence level of the modal ±25% interval is 77.8%; these values provide a higher evaluation accuracy and higher resource level, which is consistent with the current results of NGH research in the SCS. Although the development of NGHs in the SCS is progressing rapidly, basic exploration is relatively scarce, and the number of samples of effective thickness obtained through dozens of exploratory wells is too few. It is recommended that China carry out more exploration and research projects in the Shenhu area and the entire SCS to obtain more accurate thickness data.

The dissociation of hydrate plays an important role in restructuring the pore space and controlling the permeability, which determines the gas recovery from offshore hydrate reservoir. Based on the second production test in the Shenhu area, China South Sea (2020), this study investigates the production behaviors and permeability characteristics of the hydrate reservoir by using the depressurization method with a single horizontal well. A permeability adjustment model is proposed to revise the absolute permeability, and then the changes in permeability characteristics with hydrate dissociation are analyzed on a field scale for the first time. The dissociation of the hydrate can significantly improve the intrinsic permeability, thus promoting fluid flow and gas production efficiency. The simulation results show that it achieves a cumulative gas production of 89.41 × 104 m3 for 30 days, which is very close to the second production test of 86.14 × 104 m3. Also, the produced gas-to-water ratio is in the range of 15–55, and the ratio of dissociation-originating gas to the cumulative gas production reaches 0.50 for the 30 day simulation. The invasion of the underlying water with relatively high temperatures can promote the rate of hydrate dissociation in the bottom of the hydrate-bearing sediment due to the poor thermodynamic stability. The sensitivity analyses also indicate that the parameters of absolute permeability, irreducible gas saturation, and irreducible water saturation have significant influences on gas production. These insights verify the feasibility of the depressurization with the horizontal well and can be beneficial for future hydrate exploration.

Jet breaking tools for natural gas hydrate exploitation and their support technologies

Influence of horizontal well section length on the depressurization development effect of natural gas hydrate reservoirs

Analysis on a five-spot well for enhancing energy recovery from silty natural gas hydrate deposits in the South China Sea