Abstract:Natural gas hydrate is an ice-like substance which is sometimes called "combustible ice" since it can literally be lighted on fire and burned as fuel. Natural gas hydrate is characterized by widespread distribution, large reserves and little pollution. This paper introduced the distributions of hydrate, hydrate reserves and properties of hydrate. The main exploration methods, such as geophysical exploration and geochemical exploration have been presented. In addition, the main production techniques of natural gas hydrate including depressurization, thermal stimulation and chemical injection have been summed up. Finally, the challenges and outlooks of natural gas hydrate production have been proposed.
Horizontal wells with multi-stage fractures have been widely used to improve coalbed methane (CBM) production from coalbed methane reservoirs. The main focus of this work is to establish a new semi-analytical method in the Laplace domain and investigate the transient pressure behavior in coalbed methane reservoirs. With the new semi-analytical method, flow regimes of a multi-fractured horizontal well in coalbed methane reservoirs were identified. In addition, the sensitivities of fracture conductivity, diffusion model, storability ratio, inter-porosity flow coefficient, adsorption index, fracture spacing, fracture asymmetry, non-planar angle, and wellbore storage were studied. Results indicate that six characteristic flow regimes can be identified for multi-fractured horizontal wells in coalbed methane reservoirs, which are bilinear flow, first linear flow, desorption-diffusion flow, first pseudo-radial flow, second linear flow, and second pseudo-radial flow. Furthermore, the sensitivity analysis shows that the early flow is mainly determined by the fracture conductivity, the asymmetry factor, the non-planar angle, and the wellbore storage; while the desorption-diffusion flow regime is mainly influenced by the diffusion model, the storability ratio, the inter-porosity flow coefficient, the adsorption index, and the fracture spacing. Our work can provide a deep insight into the fluid flow mechanism of multi-fractured horizontal wells in coalbed methane reservoirs.
Abstract:The Pseudo Steady-State (PSS) constant b Dpss is defined as the difference between the dimensionless wellbore pressure and dimensionless average pressure of a reservoir with a PSS flow regime. As an important parameter, b Dpss has been widely used for decline curve analysis with Type Curves. For a well with a finite-conductivity fracture, b Dpss is independent of time and is a function of the penetration ratio of facture and fracture conductivity. In this study, we develop a new semi-analytical solution for b Dpss calculations using the PSS function of a circular reservoir. Based on the semi-analytical solution, a new conductivity-influence function (CIF) representing the additional pressure drop caused by the effect of fracture conductivity is presented. A normalized conductivity-influence function (NCIF) is also developed to calculate the CIF. Finally, a new approximate solution is proposed to obtain the b Dpss value. This approximate solution is a fast, accurate, and time-saving calculation.
Studies of the hydrate cores have shown that natural fractures can be frequently observed in hydrate reservoirs, resulting in a fracture-filled hydrate. Therefore, it is highly necessary for industries to predict the gas well productivity of fracture-filled hydrate reservoirs. In this work, an embedded discrete fracture model is applied to characterize the natural fractures of fracture-filled gas-hydrate reservoirs. The non-linear mass and energy conservation equations which are discretized with the finite-difference method are solved by the fully implicit approach, and the proposed model is justified by a commercial simulator. On the basis of the proposed model, we investigate the influences of natural fractures, fracture conductivity, and hydrate dissociation rate on the gas well productivity and the distributions of pressure, temperature, and hydrate saturation. The simulation results show that hydraulic and natural fractures exert significant impacts on the gas well productivity of the fracture-filled hydrate reservoirs, and the cumulative gas production is increased by 45.6% due to the existence of the connected natural fractures. The connected natural fractures can impose a more important influence on the gas well productivity than the unconnected natural fractures. The cumulative gas production is increased by 6.48% as Nnf is increased from 2 to 50, whereas the increase is 43.38% as Nf_con is increased from 0 to 4. In addition, A higher hydraulic fracture conductivity can be more favorable than a higher natural fracture conductivity for improving the gas well productivity, and a higher hydrate dissociation rate can lead to a lower temperature along fractures due to a more noticeable reduction of solid hydrate. This study provides a theoretical basis for developing fracture-filled hydrate reservoirs efficiently in the future.
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