Hydrate-bearing Layer Research Articles

Gas hydrate accumulation and saturations estimated from effective medium theory in the eastern Pearl River Mouth Basin, South China Sea

Pore- and fracture-filling gas hydrates were identified from the core samples at several sites during the second Guangzhou Marine Geological Survey (GMGS2) expedition. Well logs indicated that gas hydrate occurred in three distinct layers at site GMGS2-08. The gas hydrate saturations calculated from well-log data and the seismic responses for the three gas hydrate-bearing layers, especially within the middle carbonate layer, were poorly known. We estimated gas hydrate saturations using isotropic and anisotropic models based on the mineral composition of the sediments and the effective medium theory. In the upper and lower gas hydrate-bearing layers, saturations estimated from anisotropic models are close to those estimated from pressures cores and chlorinity data. The average saturation using an anisotropic model in the upper (fracture-dominated) hydrate layer is approximately 10% with a maximum value of 25%. In the lower (fracture-dominated) layer, the horizontal and vertical gas hydrate-filled fractures and visible gas hydrate were formed with a maximum saturation of approximately 85%. For the middle layer, well logs show high P-wave velocity, density, high resistivity as well as low gamma ray, porosity, and drilling rate, together indicating a carbonate layer containing gas hydrate. The hydrate saturations calculated from isotropic models assuming hydrate formed at grain contacts are less than 20%, which fit well with two values calculated from chlorinity data for this layer. The upper gas hydrate layer shows no clear seismic response and probably consisted of small fractures filled with gas hydrate. The middle carbonate and lower fracture-filled gas hydrate-bearing layers show pull-up reflections, with the carbonate layer exhibiting relatively higher amplitudes. Pore-filling gas hydrate was also identified just above the depth of the bottom-simulating-reflector (BSR) from the GMGS2-05 drill site. Below the BSR, the push-down reflections, polarity reversal, and enhanced reflections indicate the occurrence of free gas in the study area.

Characterization of hydrate-bearing sediments recovered from the Shenhu area of the South China Sea

The characteristics of the hydrate-bearing layer are important parameters for gas hydrate exploration and exploitation, which can be provided by laboratory analysis. We have carried out a systematic analysis of modern instruments, including the laser grain size analyzer, X-ray computed tomography, scanning electron microscopy, X-ray diffraction (XRD), Raman spectroscopy, gas chromatography, and isotope mass spectrometry, on the hydrate-bearing sediment samples recovered from the Shenhu area in the South China Sea in 2016. We focused on the structure, hydration number, and gas composition of the gas hydrate samples. We also evaluated the effects of sediments on the occurrence and distribution of gas hydrate. Our results showed that the sediments are much finer than those recovered from the Shenhu area in 2007. Some samples are found to contain calcareous microfossils and foraminifera, which are beneficial for dispersed gas hydrate formation. Whereas the only visible hydrate sample (FC4) has not found any bioclast in the sediments. Based on the Raman data and the XRD analysis, the gas hydrates demonstrate typical structure I (sI) hydrates, with cage occupancies of methane molecules higher than 99.3% in large cages and 81.5%–91.4% in small cages, respectively. The crystal lattice parameter is 11.89 Å with the hydration number being approximately 6.0. Methane is obviously the dominant component, ranging from 97.6% to 99.95% in these samples. Comprehensive study of carbon and hydrogen isotopes suggests that the hydrate-bound gases are from biogenic and thermogenic gases in the study area.

Mechanical Properties of Stratified Hydrate-bearing Sediments

Abstract Methane hydrate–bearing layers will be disturbed during the production of methane hydrates, which may induce the deformation or settlement of the layers, and the destruction of engineering structures, so it's important to study the mechanical properties of the hydrate-bearing sediments. In this paper, specimens were prepared in which the methane hydrates were in different location of the specimens by a special deposition method in which the methane hydrate was put in the mold separating with the kaolin, the volume of methane hydrate was 40% of the whole volume of the specimen. A series of triaxial shear tests were carried out under different confining pressures of 1.25 MPa, 2.5 MPa, 3.75 MPa and 5 MPa, conditions with temperature of -6 o C and strain rate of 1%/min. The results indicated that the maximum deviator stress of the sediments increased with the declining of the clay of hydrate and the failure strength achieved maximum when the hydrate clay was in the center of the sediments; the failure strength increased with the increasing of the confining pressure in the low confining pressure stage.

Heat and mass transfer analysis of depressurization-induced hydrate decomposition with different temperatures of over- and underburden

Natural Gas Hydrate (NGH) reserves are large and regarded as an important alternative future energy source. Safely and efficiently exploiting natural gas hydrates has recently become a hot issue around the world. The decomposition of natural gas hydrate requires a great deal of absorbed heat. The decomposition process is a complicated heat and mass transfer process with phase changes, such that heat and mass transfer will have a great impact on the decomposition results. Focusing on the heat transfer effect, a two dimensional mathematical model of natural gas hydrate decomposition was established. Using a numerical simulation method, the thermal conductivities of different porous mediums were selected and the bottom hole pressure (BHP) was changed under different overburden and underburden temperatures to simulate the depressurization exploitation of natural gas hydrates. The reservoir temperature distribution, hydrate decomposition front, gas production rate, and cumulative gas production under different cases were obtained. The results show that the incoming heat directly influenced the reservoir temperature distribution, hydrate decomposition rate, and gas production and the temperature at any point on the decomposition front is constant under certain BHP. When there was no external heat introduced into the hydrate-bearing layers, the impact of thermal conductivity on decomposition was greater, but decreased with increasing temperature of overburden and underburden. When no external heat was introduced into the hydrate-bearing layers, the BHP became the main controlling factor of the cumulative gas production, and the effect of the BHP on the cumulative gas production was gradually weakened with increasing temperatures of the overburden and underburden.

日本近海のメタンハイドレート胚胎層から単離した微生物を用いた土の強度増進効果に関する実験的検討

日本近海のメタンハイドレート胚胎層から単離した微生物を用いた土の強度増進効果に関する実験的検討

Analysis on geologic conditions affecting the performance of gas production from hydrate deposits

Potential of gas production from hydrate deposits depends on occurrence features of the hydrate deposits and production methods, and fundamentally it is determined by geologic conditions of the hydrate reservoir. The geologic conditions mainly include reservoir porosity (Φ), hydrate saturation (SH), formation permeability (K), thickness of hydrate-bearing layer (HBL) (H), initial pressure (P0) and temperature (T0) of the HBL. Profound differences of the conditions have been revealed by the global marine drilling programs. Orthogonal design, as a statistic method, is employed in this work to systematically investigate the relative importance of these diverse geologic conditions on hydrate decomposition as well as production performance of gas and water from the Class 2 offshore hydrate accumulations. The analysis results show that the order of the significance levels of the geologic conditions on cumulative gas production is K > Φ> SH > T0> P0> H; while the order of that on the water-to-gas ratio is P0 > K > Φ > SH > H > T0. The results suggest that it is favorable for production from the hydrate deposits characterized by the conditions of the high permeability, moderate porosity (40%), moderate-to-high hydrate saturation (38%–67%) and the low initial pressure (10.79–13.38 MPa). This research gives a forward sight for production prediction and provides a reference in selection of production target.

Evaluation of Gas Production from Marine Hydrate Deposits at the GMGS2-Site 8, Pearl River Mouth Basin, South China Sea

Natural gas hydrate accumulations were confirmed in the Dongsha Area of the South China Sea by the Guangzhou Marine Geological Survey 2 (GMGS2) scientific drilling expedition in 2013. The drilling sites of GMGS2-01, -04, -05, -07, -08, -09, -11, -12, and -16 verified the existence of a hydrate-bearing layer. In this work gas production behavior was evaluated at GMGS2-8 by numerical simulation. The hydrate reservoir in the GMGS2-8 was characterized by dual hydrate layers and a massive hydrate layer. A single vertical well was considered as the well configuration, and depressurization was employed as the dissociation method. Analyses of gas production sensitivity to the production pressure, the thermal conductivity, and the intrinsic permeability were investigated as well. Simulation results indicated that the total gas production from the reference case is approximately 7.3 × 107 ST m3 in 30 years. The average gas production rate in 30 years is 6.7 × 103 ST m3/day, which is much higher than the previous study in the Shenhu Area of the South China Sea performed by the GMGS-1. Moreover, the maximum gas production rate (9.5 × 103 ST m3/day) has the same order of magnitude of the first offshore methane hydrate production test in the Nankai Trough. When production pressure decreases from 4.5 to 3.4 MPa, the volume of gas production increases by 20.5%, and when production pressure decreases from 3.4 to 2.3 MPa, the volume of gas production increases by 13.6%. Production behaviors are not sensitive to the thermal conductivity. In the initial 10 years, the higher permeability leads to a larger rate of gas production, however, the final volume of gas production in the case with the lowest permeability is the highest.

Experimental study on the mechanical properties of sediments containing CH4 and CO2 hydrate mixtures

CH4–CO2 replacement method for extracting methane from hydrate-bearing sediments has attracted more and more attention due to its twin advantages: CH4 recovery and CO2 geologic sequestration. In this study, a series of triaxial tests were conducted on sediments containing CH4 and CO2 hydrate mixtures to prove the mechanical stability of CH4–CO2 replacement in permafrost-associated methane hydrate-bearing sediments. And a comparative analysis was made between the mixed hydrate- and pure CH4/CO2 hydrate-bearing sediments containing ice. The stress–strain curves, failure strength, initial yield modulus, shear strength and the effects of CO2 hydrate volume ratio, confining pressure, temperature and strain rate on the mechanical properties of mixed hydrate-bearing sediments containing ice were investigated. The experimental results indicated that CH4–CO2 replacement was conducive to the stability of the methane hydrate-bearing layers during gas production.

Feasibility study of seismic monitoring for gas hydrate production in the Ulleung Basin in the East Sea

During gas hydrate production, the upper parts of the gas hydrate-bearing layer (GHBL) behave as a seal. If parts of the upper gas hydrates are dissociated, free methane gas present in the lower parts of GHBL may leak into the upper parts of the layer. Therefore, the dissociation of methane gas in the GHBL must be monitored to ensure strong stability during gas production. To investigate the feasibility of monitoring the dissociation of gas hydrates with a seismic survey, we established a rock physics model incorporating the geological characteristics of the region around the UBGH 2–6 well in the Ulleung Basin based on LWD data and core analysis data. We performed fluid substitution modeling to build a velocity model with the dissociated parts of the gas hydrates, and generated synthetic data. Diffractions from the edges of the dissociated parts allowed detection of gas hydrate dissociations, and the dissociated locations could be identified in the migrated image. In addition, sensitivity testing showed that the dissociated part was recognizable in the migrated image, even at a width of half the wavelength. Therefore, we conclude that the stability of gas hydrates during production can be monitored using a time-lapse seismic survey.

Evaluation of gas production from Qilian Mountain permafrost hydrate deposits in two-spot horizontal well system

In 2008–2009, gas hydrate deposits were confirmed to exist in the Qinghai–Tibet Plateau permafrost (QTPP) during the Scientific Drilling Project of Gas Hydrate in Qilian Mountain permafrost. Gas hydrate samples were successfully collected from three drilling sites: DK-1, DK-2, and DK-3. Based on the limited geological data from site measurements, gas was produced from the hydrate deposits of DK-2 zone in a two-spot well system through numerical simulation, and its commercial viability was evaluated. The two wells were placed on the same horizontal plane in the middle of the hydrate-bearing layer (HBL), and the depressurization and thermal stimulation methods were employed simultaneously in this system. Simulation results showed that desirable gas production performance could be obtained under suitable operation conditions when using this kind of well design. During the production process, large amount of free gas accumulated in the vicinity of the injection well until the flow channels between the two wells were gradually opened. It was found that the gas production performance was more favored with larger depressurization driving force, while the heat injection rate should be determined cautiously. Dependences of gas production performance on the various properties of hydrate deposits were also assessed.

Numerical Investigation of Hydrate Dissociation Performance in the South China Sea with Different Horizontal Well Configurations

Based on the available measurement data and literature on the hydrate deposits of the South China Sea, a numerical simulation with a new dual horizontal well system has been carried out. Warm brine stimulation combined with depressurization is employed as the production method. Two horizontal wells were situated in the same horizontal plane and they were placed in the middle of the Hydrate-Bearing Layer (HBL). The warm brine is injected from the left well (LW) into the reservoir, and the right well (RW) acted as the producer under constant pressure. The simulation results show that the effects of hydrate dissociation rate, gas to water ratio, and energy ratio are all better than the previous work in which the dual horizontal wells are placed in the same vertical plane. In addition, the sensitivity analysis indicates that a higher injection rate can enhance the hydrate dissociation rate and gas production rate, while a lower injection rate gives a more favorable gas to water ratio and energy ratio.

Effects of Impermeable Boundaries on Gas Production from Hydrate Accumulations in the Shenhu Area of the South China Sea

Based on currently available data from site measurements and the preliminary estimates of the gas production potential from the hydrate accumulations at the SH7 site in the Shenhu Area using the depressurization method with a single horizontal well placed in the middle of the Hydrate-Bearing Layer (HBL), the dependence of production performance on the permeabilities of the overburden (OB) and underburden (UB) layers was investigated in this modeling study. The simulation results indicated that the temperature and the pressure of the HBL were affected by the permeabilities of OB and UB and the effect of depressurization with impermeable OB and UB was significantly stronger than that with permeable boundaries. Considering the percentage of hydrate dissociation, the gas production rate and the gas-to-water ratio, the hydrate deposit with impermeable OB and UB was expected to be the potential gas production target.

Effect of thermal stimulation on gas production from hydrate deposits in Shenhu area of the South China Sea

The Shenhu area on the northern continental slope of the South China Sea (SCS) is one of the promising fields for gas hydrate exploitation. The hydrate-bearing layer at drilling site SH2 is overlain and underlain by permeable zones of mobile water. In this study a vertical well was configured with a perforated Interval I for producing gas and a coiled Interval II for heating sediment. The hydrate is dissociated by a small depressurization at Interval I and a thermal stimulation at Interval II. The numerical simulations indicate that the thermal stimulation has a significant effect on gas release from the hydrates in the production duration and improves the gas production in the late period. The gas released by thermal stimulation cannot be produced as quickly as the production gets operated because of the hard pathway for fluids to flow in the sediments. The gas production is enhanced due to the heating for 7242 m3 in the whole production. Increasing heating temperature at Interval II can improve gas production and restrain water output, and advance the arrival time of the gas flow from the zone at Interval II. The absolute criterion and relative criterion suggest that the thermal stimulation in the production schemes is pronounced for releasing gas from the hydrate deposit, but the production efficiency of gas is limited by the sediment of low permeability. The study provides an insight into the production potential of the hydrate accumulations by thermal stimulation with depressurization in two wells, and a basis for analyzing economic feasibility of gas production from the area.

Long-Term Gas-Hydrate-Production Test, North Slope, Alaska

This article, written by Senior Technology Editor Dennis Denney, contains highlights of paper OTC 22152, ’Evaluation of the Hydrate Deposit at the PBU-L106 Site, North Slope, Alaska, for a Long-Term Test of Gas Production,’ by George J. Moridis, SPE, and Matthew T. Reagan, SPE, Lawrence Berkeley National Laboratory; Heidi Anderson-Kuzma, SPE, East Donner Research; Yang Zhao, University of California at Berkeley; Katie Boyle, Lawrence Berkeley National Laboratory; and James W. Rector, University of California at Berkeley, prepared for the 2011 Arctic Technology Conference, Houston, 7-9 February. The paper has not been peer reviewed. To investigate the technical feasibility of gas production from hydrate deposits, a long-term field test (lasting 18 to 24 months) is under consideration in a project led by the US Department of Energy. A candidate deposit involving the C-Unit in the vicinity of the PBU-L106 site on the North Slope, Alaska, was evaluated. The results indicate that production from horizontal wells may be orders of magnitude larger than that from vertical wells. Introduction Gas hydrates (GHs) are solid crystalline compounds of water and gaseous sub-stances that are described by the general chemical formula GsNH H2O. In the GH clathrates, the molecules of gas, G (referred to as guests), occupy voids within the lattices of ice-like crystal structures. If gas and H2O availability is not a limitation, hydrate deposits can occur in two distinctly different geographic settings in which the necessary conditions of low temperature, T, and high pressure, P, exist for their formation and stability: in the Arctic (typically in association with permafrost) and in deep ocean sediments. CH4 is the dominant GH-forming hydrocarbon gas in natural hydrates that occur in geologic media. Under standard T and P (STP) conditions, each cubic meter of simple CH4 hydrate releases, upon dissociation, approximately 164 m3 of methane and 0.8 m3 of H2O. Classification. Natural GH accumulations are divided into three main classes on the basis of simple geologic features and initial reservoir conditions. Class-1 settings have two layers: a hydrate- bearing layer (HBL) and an underlying two-phase-fluid zone containing mobile gas and liquid water. A distinct feature of Class-1 settings is that the base of the GH-stability zone coincides with the bottom of the HBL. Class 1 is the most desirable target because it is the easiest to destabilize to release gas. In Class-2 settings, an HBL overlies a zone of mobile water. Class-3 accumulations have a single HBL with no underlying zone of mobile fluids.

Evaluation on gas production potential from laminar hydrate deposits in Shenhu Area of South China Sea through depressurization using vertical wells

Gas hydrates are solid crystalline compounds in which gas molecules are lodged in lattices of ice crystals. Shenhu area is considered as one of the most promising fields on north continental slope of the South China Sea (SCS). Drilling and sampling at the site has indicated occurrences of methane hydrate in clay silty sediments. The thin Hydrate-Bearing Layer (HBL) is overlain and underlain by zones of mobile water, and the layer does not appear to be bounded by low-permeability strata. In this study we assess by means of numerical simulation the production potential of the laminar hydrate deposit at drilling site SH3 in the Shenhu area. We simulate the hydrate dissociation and the gas production induced by depressurization at a vertical well. To minimize gas losses through the overburden and excessive water production through proximity to the permeable, water-saturated zones, a perforated interval is limited to the middle section of the vertical well within the hydrate layer. The simulations show that productions from depressurization-induced dissociation through a vertical well at constant well pressure do not appear to be a promising approach in the deposits of low hydraulic diffusion. The production of hydrate-originating gas decreases at the beginning of the production and then keep stable in the reference case and the average production is 211m3/d. The deposit permeability is considered as the most insensitive parameter to enhance the gas production. And the production potential is much lower than that of deposits having an impermeable upper boundary as expectation.

A huff-and-puff production of gas hydrate deposits in Shenhu area of South China Sea through a vertical well

The Shenhu area on the northern continental slope of the South China Sea is one of the most promising fields for gas hydrate exploitation. Drilling and sampling has indicated high saturations of methane hydrate in clay silty sediments at drilling site SH2. The hydrate-bearing layer is overlain and underlain by permeable zones of mobile water, and the system does not appear to be bounded by low-permeability strata. In this study a huff-and-puff method is used to producing gas from the hydrate accumulation. We simulate numerically the hydrate dissociation and gas production by alternately injecting hot water and producing fluids at a vertical well. The simulations show the gas production rate in huff-and-puff operations is very small (50–140m3/d), and unacceptable for commercial production. The calculation also indicated secondary hydrates forms at the very early period of injecting operations, and then gas is released due to the thermal stimulation of hot water, but the amount of released gas in the injection periods of hot water is much smaller than that converts into secondary hydrates. In the production operations, much of gas is released from the hydrates due to a small depressurization at the well, but the released gas can not produce effectively due to the small pressure gradient, and thus remains in the reservoir and converts into secondary hydrates in the next operation of injecting hot water. The study provides an insight into the production potential of the Shenhu hydrate accumulations through the huff-and-puff method, and a basis for the analysis of the economic feasibility of gas production from that area.

Numerical simulation of gas production potential from permafrost hydrate deposits by huff and puff method in a single horizontal well in Qilian Mountain, Qinghai province

Based on the geological data of the Qinghai-Tibet plateau permafrost, such as the permafrost ground temperature, the thermal gradient within and below the frozen layer, we numerically investigate the gas production potential from hydrates at the DK-3 drilling site of the Qilian Mountain permafrost, which is located in the north of the Qinghai-Tibet plateau. We employ the huff and puff method using a single horizontal well in the middle of the Hydrate-Bearing Layer (HBL). The simulation results indicate that desirable gas-to-water ratio and energy efficiency can be obtained under suitable injection and production conditions in the huff and puff process. However, the absolute gas production rate remains low during the whole production process. The sensitivity analysis indicates that the gas production performance is strongly dependent on the intrinsic permeability of the hydrate deposits, the sediment porosity, the injection and production rates, the temperature of the injected water, the irreducible water saturation and P01. The relative permeability exponents appear to have limited effect on the gas production behavior using the huff and puff method. The sensitivity analysis also indicates that the production potential of the natural gas hydrate deposit will be better than that of pure methane hydrate in this simulation.

MD179調査より得られた日本海東縁域海底表層堆積土の物理・力学的性質

Gas hydrates are attracting attention as a next-generation energy source. On the other hands, Methane gas contained in the natural gas hydrates has approximately 20 times the greenhouse effect of CO2. There are concerns that dissociation of methane gas from the gas hydrates distributed in submarine surface layers (shallow type gas hydrates) by rising ocean temperatures or by vaporization at recovery of the hydrates for energy may contribute to global warming, which in turn may raise the sea level and causing climatic instability. In addition, many gas-hydrate-bearing areas are distributed near the boundaries of tectonic plates, as shown in the figure. Gas hydrates in the surface layer of seafloor may dissociate when seismic activities cause seafloor landslides that in turn cause gas hydrate-bearing layers to fail. There have been concerns over the environmental effects of shallow type gas hydrates, but surveys for shallow type hydrates have been few.The purpose of this study is to understand the soil properties of the sea-bottom sediment obtained from the Eastern Margin of Japan Sea. The samples were collected from 5 regions (Umitaka Spur, Joetsu Ridge, Toyama Trough, West of Tsugaru and west of Okushiri Ridge), and the physical (soil density and water content) and mechanical (cone penetration ratio and vane shear strength) tests were performed by on-board used for the sea-bottom sediments.From the test results, it is found that the physical properties of the sediment samples obtained in these areas have no marked difference regardless of the locations or depth of sampling. However, the soil strength of sea-bottom sediments obtained gas hydrates was lower than those of other sediments. It would seem that this is because the effect of methane hydrates dissociation and vaporization of dissolved gas in the pore water at sampling.

Drilling Through Gas-Hydrate Sediments: Managing Wellbore-Stability Risks

Summary As hydrocarbon exploration and development moves into deeper water and onshore Arctic environments, it becomes increasingly important to quantify the drilling hazards posed by gas hydrates. To address these concerns, a 1D semianalytical model for heat and fluid transport in the reservoir was coupled with a numerical model for temperature distribution along the wellbore. This combination allowed the estimation of the dimensions of the hydrate-bearing layer where the initial pressure and temperature can dynamically change while drilling. These dimensions were then used to build a numerical reservoir model for the simulation of the dissociation of gas hydrate in the layer. The bottomhole pressure (BHP) and formation properties used in this workflow were based on a real-field case. The results provide an understanding of the effects of drilling through hydrate-bearing sediments (HBS) and of the impact of drilling-fluid temperature and BHP on changes in temperature and pore pressure within the surrounding sediments. It was found that the amount of gas hydrate that can dissociate will depend significantly on both initial formation characteristics and bottomhole conditions) namely, mud temperature and pressure). The procedure outlined in the paper can provide quantitative results of the impact of hydrate dissociation on wellbore stability, which can help in better design of drilling muds for ultradeepwater operations.

The use of huff and puff method in a single horizontal well in gas production from marine gas hydrate deposits in the Shenhu Area of South China Sea

The Shenhu Area is located in the Pearl River Mouth Basin, the northern continental slope of the South China Sea. It is expected that the Shenhu Area will become a strategic area of gas hydrate exploitation in China. Based on currently available data from site measurements, including water depth, thickness of the Hydrate-Bearing Layer (HBL), sediment porosity, salinity and pressures and temperatures at key locations, it is possible to develop preliminarily estimates of the gas production potential by numerical modeling. We used measurements of ambient temperature in the sediments to determine the local geothermal gradient. Estimates of the hydrate saturation and the intrinsic permeabilities of the system formations were obtained from direct measurements. The hydrate accumulations in the Shenhu Area are similar to Class 3 deposits (involving only an HBL), and the overburden and underburden layers are assumed to be permeable. These unconfined deposits may represent a large challenge for gas production. In this modeling study, we estimated gas production from hydrates at the SH7 drilling site of the Shenhu Area by means of the stream huff and puff method using a single horizontal well in the middle of the HBL. The simulation results indicate that the hydrate dissociated zone expands around the well, and the hydrate formation occurs during the injection stage of the huff and puff process. The higher temperature of the injected brine appears to have a limited effect on gas production using the huff and puff method. Reasonable injection and production rates should be adopted to avoid the over pressurization and depressurization during each huff and puff cycle. Production is invariably lower than that attainable in a confined system, and thermal stimulation is shown to have an effect over a limited range around the well. The sensitivity analysis demonstrates the dependence of gas production on the level of the increment of the injection and production rates of the huff and puff process, the temperature of the injected brine and the existence of brine injection during the injection stage.