Field Development Geologic Evaluation Of Tight Gas Sandstones In Taiyuan Formation, Dingbei Area, Ordos Basin,China

Dingbei area,with an area of 888.43km²,is located in the Dingbian County of Shanxi Province and Yanchi County of Ningxia Province.It is tectonic location is at the connection zone of the Yishan Slope and the Tianhuan Depression in the Ordos Basin,China.The gas production zone is T₂ Member in the Carboniferous Taiyuan Formation of Upper Paleozoic.The sedimentary system is barrier bar-tidal flat-lagoon.Up to now,there are 34 exploration wells and a few horizontal wells.The main problems are the variable gas production and the limited understanding of origins of“sweet spot”and characteristic of“sweet spot”.In addition,the tight reservoirs show small resistivity difference between gas and water layer,which lead to the inaccuracy of water and gas layer prediction.All these factors affect the prolific gas area selection.A systematic study of the geological characters of the Dingbei district is the essence of better production of the gas reservoir.An integration geology,rock physics experiments,sonic velocity and electrical conductivity numerical simulation,well logs and analytical methods are used in this study.The origins of the high-quality reservoir are analysis based on the stratigraphy,structure,sedimentology and diagenetic research.After the characterization of reservoir lithology,porosity,permeability and pore structure,a pore structure model is constructed considering the structure of micron-nano pores using micro-CT,high pressure mercury intrusion.The reservoir evaluation standards are then established.Rock physics experiments and numerical simulation are conducted to understand the factors that affect the sonic and electrical properties of tight sandstone.Rock physics model and electrical conduction model are established.Well logging interpretation of gas reservoir properties and gas layer identification are conducted based the results of rock physics study.Finally,the factors affecting the high production zones are analyzed and gas production prolific areas are selected.The main conclusions are listed as following:?1?T₂ Member in the Dingbei district is structural high in the Northeast and low in the Southwest.A few strike-slip faults with fault displacement of 20³0m occur in the North part of the area.The sedimentary system is barrier bar-tide flat-lagoon.The barrier bar is composed of medium to coarse quartz sandstones and lithic quartz sandstones.Typical beddings are parallel bedding,low angle swash cross bedding and wedge-shaped cross bedding.Tide flat micro-facies is mainly composed of thin sandstones and dark grey mudstones,tidal bedding can be frequently found.Lagoon micro-facies is mainly composed of muddy limestone.Barrier bar is the micro-facies that composite the reservoir.?2?The sandstones are of low clay content,good sorting and well-rounded.The main cements are quartz overgrowth,kaolinite,illite and a few calcareous cements.Other cements are rarely found.Dissolution is found in clay,feldspar and early formed calcite.Four diagenetic facies are identified based on the sedimentary facies and diagenetic processes.The four diagenetic facies are quartz cemented diagenetic facies,kaolinite partially filled diagenetic facies,soluble fraction dissolution diagenetic facies and tightly compaction diagenetic facies.The former two diagenetic facies are mainly found at the barrier bar close to the shallow sea,however the latter two diagenetic facies are observed close to the tidal flat.?3?Porosity of the production zone is mainly between 2%and 10%,with an average of 5.75%.Permeability mainly ranges from 0.1mD to 1.6mD,with an average of 0.41mD.Porosity do not show much difference in the barrier bar,however larger permeability is found in the West part of barrier bar.?4?The pore types in the study area are residual intergranular pores,clay intercrystal pores and dissolutions pores.Micro-CT results shows that pores in the tight sandstones mainly composed of micron scale pores,which are isolated and may connected by the nano scale pores.High pressure mercury intrusion experiments shows that pore throat radius is mainly ranges from 0.1?m to 1?m.Pore throat radius sorting increases as the increase of porosity and permeability.Mercury withdrawal efficiency decreases as the increases of porosity and permeability.Topological structure of the tight sandstones are established based on the analysis of micro-CT,high pressure mercury intrusion.The pore structure model shows that micron scale pores?1³0?m?have good connectivity and have minor effects on fluid flow.Nano scale pores in the intergranular pores illustrate tree like structure and semi-connected with the intergranular pores.The higher content of nano pores,the lower movable fluid saturation.The parameter K?/V_sh can be used to predict movable fluid saturation.?5?The water film thickness is obtained based on the NMR and high pressure mercury intrusion.The obtained minimum pore throat radius is 35nm and the corresponding lower limit of permeability is about 0.016mD.Four types of reservoirs are obtained based on the lithology,porosity and permeability,pore structure and movable fluid saturation.Type I and type II are regarded as the high-quality reservoir.The high-quality reservoir is mainly controlled by deposition environment and also affected by dissolution.Lithology of the high quality reservoir is medium to coarse quartz sandstone with good sorting and well-rounded.It is located close to the shallow sea and the diagenetic facies are quartz cemented diagenetic facies and kaolinite partially filled diagenetic facies.Dissolution occurs in the lithic quartz sandstone with soluble fractions and close to the source rock at the T_2-2 small layer.Dissolution increases porosity,however has minor contribution on permeability,thus the soluble fraction dissolution diagenetic facies can also be the part of the high-quality reservoir.?6?Sonic velocity experiments show that good linear correlation can be obtained among sonic velocity,porosity and clay content when take into account the different diagenetic facies.P-wave velocity illustrate good linear relationship with S-wave velocity.Biot exponent is sensitive to the porosity.When considering the effects of clay distribution on the elastic properties of tight sandstones,structural clay in the grain-grain contact,structural clay as grains and dispersed clay show an decrease on strengthen the bulk modulus,however clay distribution do not affect shear modulus.P-wave velocity,bulk modulus and Poisson's ratio are all sensitive to the water or gas saturation,which can be used for gas layer identification using the dipole array acoustic logging system.A rock physics model is established considering the effect of clay content and distribution,pore structure.?7?Rock electrical properties experiments and numerical simulation results show the cementation exponent have good linear relationship with pore-throat ratio,thus the parameters such as porosity,mercury withdrawal efficiency and V_sh/?can be used to predict cementation exponent.Saturation exponent is affect by the permeability,content of dispersed clay and content of nano scale pores.Saturation exponent decreases as the increase of permeability.In addition,it also decreases as the increase of dispersed clay content and increases as the increase of nano scale pores.?8?Gas layer identification is done by methods such as the three porosity overlapping method,resistivity intrusion property analysis,the Pickett plot and elastic properties obtained from the dipole array acoustic logging system.The results show that the methods based on the resistivity logs are less capable to identify gas layer,gas-water mixing layer and water layer with gas.Much better gas identification results can be obtained using the dipole array acoustic logging system compared with the resistivity methods.The gas layer limit is determined using different cross plots and the gas layer is subdivided into four types.The first and second types are the main gas production layers.?9?The controlling factors for the high production gas layers are faults,good micro-facies and reservoir quality.High gas production layers are far from the faults,close to the shallow sea of the barrier bar and high porosity and high permeability sandstones.High gas production layers have porosities higher than 5.5%and permeability larger than0.3mD,some of the high gas production layer occur in zones with permeability about0.1mD.Two types of gas production zones are decided based on the above analysis,the results match well with the horizontal well production results.