An appreciable amount of formation water exists inevitably in the porous media of reservoirs, but the existence of water is often neglected in the usual calculation of phase equilibrium. The commonly used equation of ...An appreciable amount of formation water exists inevitably in the porous media of reservoirs, but the existence of water is often neglected in the usual calculation of phase equilibrium. The commonly used equation of state is simple, but may have large deviations in estimating some thermodynamic properties. The existing equation of state, which has been modified, is not very suitable for the system containing polar substances (e.g. hydrocarbons, water, and alcohols). In this paper, the CPA (Cubic-Plus-Association) equation of state which considers both physical and association interactions between molecules was used to calculate the thermodynamic properties of water under the saturated and unsaturated states. Moreover, the disadvantages of the commonly used equation of state in the calculation were analyzed. The following results were obtained. First, the commonly used equation of state can be used to effectively calculate the saturated vapor pressure of water, but it presents certain differences in calculating water density and enthalpy, and the enthalpy calculation is not affected by volume modification. Second, the average absolute/relative deviations between the thermodynamic properties of water calculated by the CPA equation of state and the experimental data are approximately 1%. For the system containing polar substances, the CPA equation of state is preferred in calculating the thermodynamic properties. The accurate estimation of the thermodynamic properties of water is fundamental in identifying the fluid state in the water-bearing gas reservoirs, and it is of great significance in the development and production of such gas reservoirs.展开更多
This paper proposes an approach to determing the optimal cluster spacing for volume fracturing in shale oil reservoirs based on three scales,i.e.microscopic capillary displacement,large-scale core imbibition,and macro...This paper proposes an approach to determing the optimal cluster spacing for volume fracturing in shale oil reservoirs based on three scales,i.e.microscopic capillary displacement,large-scale core imbibition,and macroscopic reservoir nuclear magnetic resonance(NMR)logging.Through flow experiments using capillary with different diameters and lengths,and large-scale core counter-current and dynamic imbibition tests,and combing with the NMR logging data of single wells,a graded optimization criterion for cluster spacing is established.The proposed approach was tested in the shale oil reservoir in the seventh member of the Triassic Yanchang Formation(Change 7 Member),the Ordos Basin.The following findings are obtained.First,in the Chang 7 reservoir,oil in pores smaller than 8μm requires a threshold pressure,and for 2-8μm pores,the movable drainage distance ranges from 0.7 m to 4.6 m under a pressure difference of 27 mPa.Second,the large-scale core imbibition tests show a counter-current imbibition distance of only 10 cm,but a dynamic imbibition distance up to 30 cm.Third,in-situ NMR logging results verified that the post-fracturing matrix drainage radius around fractures is 0-4 m,which is consistent with those of capillary flow experiments and large-scale core imbibition tests.The main pore-size range(2-8μm)of the Chang 7 reservoir corresponds to a permeability interval of(0.1-0.4)×10^(-3)μm^(2).Accordingly,a graded optimization criterion for cluster spacing is proposed as follows:for reservoirs with permeability less than 0.20×10^(-3)μm^(2),the cluster spacing should be reduced to smaller than 4.2 m;for reservoirs with permeability of(0.2-0.4)×10^(-3)μm^(2),the cluster spacing should be designed as 4.2-9.2 m.Field application on a pilot platform,where the cluster spacing was reduced to 4.0-6.0 m,yielded an increased initial oil production by approximately 36.6%over a 100-m horizontal reservoir section as compared with untested similar platforms.展开更多
文摘An appreciable amount of formation water exists inevitably in the porous media of reservoirs, but the existence of water is often neglected in the usual calculation of phase equilibrium. The commonly used equation of state is simple, but may have large deviations in estimating some thermodynamic properties. The existing equation of state, which has been modified, is not very suitable for the system containing polar substances (e.g. hydrocarbons, water, and alcohols). In this paper, the CPA (Cubic-Plus-Association) equation of state which considers both physical and association interactions between molecules was used to calculate the thermodynamic properties of water under the saturated and unsaturated states. Moreover, the disadvantages of the commonly used equation of state in the calculation were analyzed. The following results were obtained. First, the commonly used equation of state can be used to effectively calculate the saturated vapor pressure of water, but it presents certain differences in calculating water density and enthalpy, and the enthalpy calculation is not affected by volume modification. Second, the average absolute/relative deviations between the thermodynamic properties of water calculated by the CPA equation of state and the experimental data are approximately 1%. For the system containing polar substances, the CPA equation of state is preferred in calculating the thermodynamic properties. The accurate estimation of the thermodynamic properties of water is fundamental in identifying the fluid state in the water-bearing gas reservoirs, and it is of great significance in the development and production of such gas reservoirs.
基金Supported by the China National Oil and Gas Major Project(2025ZD1404800)PetroChina Science and Technology Major Project(2023ZZ15YJ03)CNPC Changqing Oilfield Company Major Special Project(2023DZZ04)。
文摘This paper proposes an approach to determing the optimal cluster spacing for volume fracturing in shale oil reservoirs based on three scales,i.e.microscopic capillary displacement,large-scale core imbibition,and macroscopic reservoir nuclear magnetic resonance(NMR)logging.Through flow experiments using capillary with different diameters and lengths,and large-scale core counter-current and dynamic imbibition tests,and combing with the NMR logging data of single wells,a graded optimization criterion for cluster spacing is established.The proposed approach was tested in the shale oil reservoir in the seventh member of the Triassic Yanchang Formation(Change 7 Member),the Ordos Basin.The following findings are obtained.First,in the Chang 7 reservoir,oil in pores smaller than 8μm requires a threshold pressure,and for 2-8μm pores,the movable drainage distance ranges from 0.7 m to 4.6 m under a pressure difference of 27 mPa.Second,the large-scale core imbibition tests show a counter-current imbibition distance of only 10 cm,but a dynamic imbibition distance up to 30 cm.Third,in-situ NMR logging results verified that the post-fracturing matrix drainage radius around fractures is 0-4 m,which is consistent with those of capillary flow experiments and large-scale core imbibition tests.The main pore-size range(2-8μm)of the Chang 7 reservoir corresponds to a permeability interval of(0.1-0.4)×10^(-3)μm^(2).Accordingly,a graded optimization criterion for cluster spacing is proposed as follows:for reservoirs with permeability less than 0.20×10^(-3)μm^(2),the cluster spacing should be reduced to smaller than 4.2 m;for reservoirs with permeability of(0.2-0.4)×10^(-3)μm^(2),the cluster spacing should be designed as 4.2-9.2 m.Field application on a pilot platform,where the cluster spacing was reduced to 4.0-6.0 m,yielded an increased initial oil production by approximately 36.6%over a 100-m horizontal reservoir section as compared with untested similar platforms.
