It is gradually accepted that pores can be created in burial settings via dissolution by organic acid,TSR derived or hydrothermal fluids.The role of deep-buried carbonate reservoirs is becoming more and more important,since the degree and difficulty in petroleum exploration of shallow strata is increasing.Understandings of the development scale and prediction of deep-buried carbonate reservoirs is economically crucial.In addition to the formation mechanism,scale and distribution of burial dissolution pores in burial settings are the focus in recent studies.This paper,based on case studies of deep-buried (>4 500m) carbonate reservoirs from the Tarim Basin and Sichuan Basin,and dissolution simulation experiments,proposes that open system is of crucial importance in the development of large scale burial dissolution pores,the distribution pattern of which is controlled by lithologies,pre-existing pores and pore throat structures.These findings provide a basis for the evaluation and prediction of deep-buried carbonate reservoirs.
In the Tarim Basin,dolostone strata widely distribute in the Lower Proterozoic with great exploration potential.Based on 2D and 3D seismic materials explanation and geologic materials analysis,the strata residual thickness of the Lower Cambrian,Middle Cambrian,and Upper Cambrian to Lower Ordovician were reconstructed,and the structural paleogeography and the lithofacies paleogeography from Cambrian to Early Ordovician are studied.Structural paleogeography features from Early Cambrian to Middle Cambrian in the Tarim Basin present western-high-eastern-low and southern-high-northern-low characteristics,and can be divided into eight departments,which are southern Tianshan ocean,Kuman Depression,western Kunlun ocean,southern Tarim Uplift,Middle slope,Tabei Uplift,Luoxi Uplift,and Manxi Depression.Because of the formation of back-arc basin at Northern Altyn platform,southern Tianshan ocean,Kuman Depression,Altyn back-arc basin and western Kunlun ocean were connected together from Late Cambrian to Early Ordovician,leading to the indeed formation of western Tarim isolated platform.Lithofacies paleo-geography features from Cambrian to Early Ordovician demonstrate a framework of platform in the west and basin in the east,and sedimentary patterns evolved from paleoterrene-semirestricted platform open platform-edge-slope-basin in Early Cambrian,to evaporated platform-edge-slope-basin in Middle Cambrian,to semi-restricted platform-open platform-edge-slope-basin in Late Cambrian and Early Ordovician.Sedimentary facies maps of Early Cambrian,Middle Cambrian,Late Cambrian and Early Ordovician were modified at last.
Based on mechanical parameters testing by uniaxial/triaxial compression experiment and compressinal & shear acoustic velocities testing,the relationship between the static and dynamic elastic parameters of the tight sandstone reservoirs of Daniudi Gasfield in the Ordos Basin was established,then a single-well calculation model of mechanical parameters was constructed referring to predecessors′ achievements,further the characteristics of mechanical parameters field were investigated with log data and three-dimensional seismic interval velocity data.The results show that the cohesion of the Upper Paleozoic tight sandstone reservoirs ranges from 2MPa to 10MPa,the angle of internal friction being between 30° and 42°,the tensile strength mostly being between 5MPa and 15MPa,and the compressive strength mostly being between 150MPa and 250MPa.The modulus of elasticity of C2t2-P1x1 sandstone reservoirs is mostly between 15GPa and 40GPa,and their Poisson′s ratio is between 0.22 and 0.27.The rock elastic parameters show obvious inhomogeneities horizontally and their values increase with depth vertically.The research results can provide basic mechanical parameters for stress field simulation,gas drilling & completion design and target-layers′ fracturing,additionally the quantitative relationship between the static and dynamic mechanical parameters and that between the shear and longitudinal wave velocity might supply a reference for similar study in surrounding areas.
Microscopic pore structure characteristics of tight sandstone reservoir of the 8th member of Lower Shihezi Formation in the Su6 area of Sulige Gasfield was quantitatively analyzed by means of casting thin sections,scanning electron microscope,3D X-ray computed tomography,high pressure mercury injection,constant-rate mercury penetration.The purpose was to discuss the causes of differences of microscopic pore structures,and further to screen evaluation parameter which could reflect the microscopic pore structure characteristics of tight sandstone reservoir.The results are as follows.Reservoir space types include (grains and cements) dissolved pores,clay-mineral intercrystalline pores and primary remained intergranular pores.Reservoirs with different permeability have less differences in pore radius but obvious differences in throat radius.The reservoir is more tight with smaller range and greater proportion of fine pore-throat.The throat also accounted for higher proportion of effective reservoir space.The medium-to-coarse grained lithic sandstones and medium-to-coarse grained lithic quartz sandstones have higher textural maturity and primary porosity,which have experienced strong dissolution.The dissolved pores are large proportion of total pore content,and the pore-throat radius content increases significantly,which are more than 1μm in the two types of medium-to-coarse grained sandstones.The small grained (feldspar) lithic sandstones have lower textural maturity and primary porosity,which have experienced weak dissolution.The pore types in the small grained sandstones are mainly clay-mineral inter-crystalline pores,and the pore-throat radius in which is less than 0.1μm.Mainstream throat radius plays the main controlling effect on reservoir permeability,and can well reflect the microscopic pore structure,such as reservoir pore-throat distribution,effective reservoir space and heterogeneity.In conclusion,mainstream throat radius should be an important reservoir evaluation parameter for the tight sandstone reservoir.
Based on geological conditions of Ordovician Kelimoli Formation in western Ordos Basin,fracture-cavity reservoir and source rock conditions as well as gas accumulation characteristics are studied,migration and accumulation features of natural gas are analyzed,and accumulation models in different areas are built.The results show that Kelimoli Formation is mainly composed of brecciated limestones.The storage space is a fracture-cavity system with logging response of high GR,high AC and low RT and it shows good connectivity through well-connecting profiles.Its origin is dominantly controlled by lithology and slope topography.Mud limestones from Ordovician Wulalike Formation to Lashizhong Formation and Carboniferous-Permian coal-bearing strata are effective source rocks.Gas source correlation results show that natural gas in the Kelimoli Formation is mainly from coal-measure source rocks.Kelimoli Formation mainly comprises dissolved fracture-cavity reservoir and local development of dolomite lens reservoir,both of which belong to dry gas reservoir and are featured by bedding-parallel development and stratiform-like distribution.Combined with methane carbon isotopes and components of natural gas,three gas accumulation models in the Kelimoli Formation are built,migration downward along carrier bed in the deposition area of Lashizhong-Wulalike Formation,migration downward directly in the exposed area of Kelimoli Formation,and migration downward vertically along faults in faulted zones.
There are a certain number of free gases in the Carboniferous-Permian sandstone in the Qinshui Basin,which are always accompanied by coal-bed methane.It has been a complement to the resource of coal-bed methane.As the accumulation has the features of low porosity,low permeability and continuous distribution like the tight gas,it is always confused with the deep basin gas.It is difficult to know the main controlling factors on accumulation correctly.On the basis of a lot of actual data,with the method of statistical analysis,this study investigates the main controlling factors of reservoir and discusses the genetic mechanism of such kind of gas reservoir more deeply.The results show that this kind of gas accumulation was formed when the CBM was lost and temporarily stays in the sandstone formation,which are in dynamic equilibrium with the CBM.On the premise of the coal-seam with gas,the displacement pressures of mudstone in sandstone roof is the main controlling factor,then followed by the porosity of sandstone.So we can look for this type of gas in the area with high content of CBM,high displacement pressures of roof mudstone and high porosity of sandstone.
The Anyue Sinian-Cambrian large-scaled gas field was recently found in the central Sichuan Paleo-uplift,revealing a huge natural gas resource potential of Lower Cambrian shale in this area.By synthesizing the relevant data,this paper constructed the geology and geochemistry models of the Lower Cambrian shale in central Sichuan Basin.By the combined use of PetroMod basin modeling software and Easy% R O model,the paleo-geothermal gradient evolution at critical geologic period of typical borehole at the paleo-uplift was recovered,and the hydrocarbon generation and evolution of Lower Cambrian shale in this area was simulated.The results show that,Mesozoic is the critical period for the generation and evolution of the Lower Cambrian shale in central Sichuan Basin.The paleo-geothermal gradient in Mesozoic was relatively higher,and it decreased gradually to the present.The paleo-geothermal gradient model obtained is: 3.5℃/100m(≥96Ma) -3.2℃/100m(65Ma)-2.7℃/100m(0Ma).The Lower Cambrian shale in the central Sichuan Paleo-uplift has undergone an earlier oil generation and a later gas generation evolution history.The source kitchen is coincident with the paleo rift trough mentioned in this paper,with maximum oil and gas generation intensity of (8-10)×106t/km2 and (55-65)×108m3/km2,respectively.The oil generated at the oil generation window (Middle-Late Triassic) by Lower Cambrian shale in the paleo rift trough could migrate to the Gaoshiti and Weiyuan-Ziyang structure through the network migration system which is composed of the unconformities and faults,to form paleo-oil reservoirs.And the crude oil in the paleo-oil reservoirs was then cracked during the Jurassic to Early Cretaceous.The oil-cracked gas is the main source of the Upper Sinian Dengying Formation gas reservoir and Lower Cambrian Longwangmian Formation reservoir in the Gaoshiti and Weiyuan-Ziyang structure.
Middle and Lower Jurassic are main hydrocarbon generation layers in the eastern Qaidam Basin.It is used to being widely accepted that broad basin was developed during the Early-Middle Jurassic,and residual strata can be found in each sag at present.However,these ideas don't meet well with drilling practice.In face of such situation,the author made full use of field survey,drilling data,seismic data,and electric survey results,analyzed time correlation and lateral variation of outcrops,and studied original sedimentary system and distribution rules of residual strata.It is pointed out that during the Early-Middle Jurassic there developed four separated lake basins instead of broad basin namely Yuka,Hongshan,Delingha and Huobuxun,and Mahai paleohigh and Oulongbuluke low uplift separated the original lake basin into several small ones.Residual strata of Middle-Lower Jurassic were limited in sags such as Gaxi,Yuka,Hongshan,Xiaochaidan,Huobuxun,and Delingha,etc.Residual strata were developed in both sag area and piedmont thrust belt,but not correspond with present sags,such strata were not found in Ounan Sag and Dachaidan Sag,for instance.Distribution of residual strata was mainly controlled by the development of original sedimentation and reconstruction of uplift during Late Yanshan Movement.Laterally,residual strata distribution was strictly confined by NW striking mountain-controlling faults and sag-controlling faults such as South Qilian fault and Ainan fault,and NE striking or EW striking accommodation faults such as Maxian fault.Coupled influence of big faults and accommodation faults resulted in difference in development of residual strata between two walls of a fault and among sags.
The theory of “Phased Capture Principle” is a method to analyse oil and gas accumulation based on the segment process of hydrocarbon generation and accumulation.Inferring the accumulation process from the oil and gas geochemical characteristics in the current traps is the bright spot of this theory,which is fit to area with frequent tectonic activities.Take the North-West area of Qaidam Basin as an example,we confirm the formation time of gas reservoir according to the analysis of carbon isotopes.Different formation time of Nanyishan structure and Kaitelimike structure is the reason that why they accumulated condensate and gas respectively.Kaitelimike structure formed later than the E23 source rock reaching the hydrocarbon generation peak,just captured the later mature stage gas,while,Nanyishan structure captured the product of hydrocarbon generation peak,which always lead to the difference of hydrocarbon characteristic and scale of the two structures.
The Mesozoic source rocks haven't been systematically studied in the west Linqing Depression,which restricted the exploration of Mesozoic in this area.Source rocks were analyzed by drilling and seismic data combined with geochemical data,the study shows that Mesozoic source rocks mainly distribute in Qiuxian Sag and Nangong Sag.They include two types,one is coal seam of Fangzi Formation in Lower-Middle Jurassic,the other is dark mudstone and oil shale formed in semi-deep to deep lake of Qiucheng Formation in Lower Cretaceous.Coal seams of the Fangzi Formation are moderate-to-fair source rocks,dark mudstone and oil shale of the Qiucheng Formation are moderate-to-good oil prone source rocks.Mesozoic source rocks of Qiuxian Sag have experienced two thermal evolution stages,i.e.,the first hydrocarbon generation occurred in the Late Cretaceous,generated coalbed gas and a small amount of low-mature oil;the second oc[JP2]curred in the Paleogene,mainly generated pyrolytic gas.The source rocks of Nangong Sag in Paleogene,mainly generated mature oil.Lower Cretaceous has supper-thick dark mudstone with better organic matter,forming source-reservoir-cap assemblage of self-generation and self-accumulation,which is highly prospective resource.
The mechanism of using oil and gas remote sensing to detect the hydrocarbon microseepage has geochemical abnormality.Hyperspectral remote sensing has been introduced to petroleum prospecting and a lot of progresses have been made.Taking Ordos Basin as an example,based on the Hyperion hyperspectral data,the abnormal information of oil alteration,clay minerals,carbonate minerals and iron minerals in two study areas were extracted.Combined with surface spectrum and geochemical data,the reliability of the extracted remote sensing information was tested,and a correlation between satellite and ground information is established.According to the hyperspectral anomaly,the oil and gas prospecting areas are described,and the foundation of quantitative research and application of remote sensing is established.
For the present,key parameters in Variable-coefficient ΔLgR logging technique are estimated by experience,lacking of quantitative predicting method.To solve this problem,extrapolation effects of the key parameters (k and a) in this technique were studied,and practical methods for calculating key parameters using single well data were brought forward.Based on the TOC and well logging curves of Cretaceous stratus in the Songliao Basin,it is found that calculation of TOC value using other wells′ key parameters cannot always get satisfied results,especially in the complex strata in which TOC errors often became so great that it is necessary to calculate the key parameters according to single well.Practical methods for predicting key parameters were established,“k” was predicted form the slope of interval transit time versus resistivity,the predicting error is less than 19%;“a” was calculated from the averaged value of shale thickness,Ro and ΔLgR,with the predicting error is less than 20%.Methods in this paper are suitable for both stable and complex formations,but more effective and necessary for complex formations,and TOC prediction errors can be reduced over 10% compared with traditional method.Methods in this paper provided a foothold for the further application of Variable-coefficient ΔLgR technology in TOC evaluation of source rock,especially in complex formations.
Chang 7 tight oil is characterized by poor physical properties,low formation pressure coefficient,well-developed natural fractures and high friability.In order to realize the goal of effective development with low-pressure tight oil,research has been carried out on three aspects to improve the early stages of single well production,stabilize for long time and reduce the cost through a series of field tests.After the four-stage technical innovation,great breakthrough has been achieved on tight oil development test,key technology for low-pressure tight oil has been established such as reservoir reconstruction technology of horizontal well volume fracturing,horizontal well development technique policy optimization,technology of reducing cost and increasing economic benefits,and four aspects of main knowledge have been gained as following:(1)long horizontal length plus volume fracturing are the core of promoting well production;(2)currently it is preferable to utilize approximate natural energy at the initial stage and energy supplement at later stage for horizontal wells with long horizontal length,volume fracturing;(3)controlled oil production is beneficial to promote tight oil development effects;(4)cost-reducing and efficiency-enhancing are important ways of exploiting tight oil economically and effectively.
From the aspect of meso-structure heterogeneity of gas shale,the seepage- stress-damage coupling code was used to investigate the stress shadow effect during hydraulic fracturing.The experimental results show that: (1)The stress shadow effect is affected by horizontal differential stress,injection spacing,heterogeneity and fracturing sequence.It displays repel and attract mechanical behaviors,which influences the stress-umbrella area,fracture length and width.(2)With the decrease of rock heterogeneity,the stress shadow effect has weaker influence on the stress-umbrella area,area becomes smaller gradually.(3)When the horizontal differential stress is smaller,influence of injection spacing on stress shadow competes over the far-field stress;with higher horizontal differential stress,influence of far-field stress on stress shadow competes injection spacing.(4)The stress shadow effect displays dynamic variation with the evolution of hydraulic fractures.There is strong correlation between stress umbrella area and fracture length,and they obeyed power function.Through this research,it can provide certain theatrical basis for horizontal well spacing and multiple perforation spacing optimization.
For gas reservoirs with strong bottom or edge aquifer support,the most important thing is to avoid aquifer breakthrough in gas well.Because water production in gas wells not only results in processing problems in surface facilities,but also explicitly reduces well productivity and reservoir recovery.And there are lots of researches on prediction of water breakthrough time,but they are not practicable because of reservoir heterogeneity.This paper provides a new method with 3 diagnostic curves to identify aquifer influx status for single gas well,which is based on well production and pressure data.The whole production period of gas well can be classified into 3 periods based on the diagnostic curves:no aquifer influx period,early aquifer influx period,middle-late aquifer influx period.This new method has been used for actual gas well analysis,which can accurately identify gas well aquifer influx status and the water breakthrough sequence of all wells in the same gas field.And the evaluation results are very useful for well production rate optimization and gas field effective development.
Most production and injection wells in low permeability oil & gas reservoir have poor effect of measure,in which is due to the fact that these reservoirs have not established effective driving system,resulting in a low degree of rest oil and gas production.Due to the insufficient forecasting accuracy on ideal water flooding swept volume after hydraulic fracturing in the process of flooding development,on the basis of fine geological modeling of reservoir,combined with rock mechanics experiment of reservoir rock sample,we choose an injection well with low permeability in the Songliao Basin to carry out a quantitative forecasting of fracture in the range of 0°-360° around the wellbore which may produce fracture,finally put forward a new method to calculate the ideal water flooding extent after hydraulic fracturing and the concept called “water flooding D coefficient curve”,forecasting the ideal water flooding scale concept under contemporary technology.This will provide an important scientific basis for reservoir stimulation in the mid and late period of water flooding development and improve the residual oil and gas recovery degree.
Fractures intersected with borehole are extremely sensitive to borehole pressure fluctuations,which causes the fissures evolved loss channels.This causes the solid particles in drilling fluid failing to plug the deformable fracture stably,and results in lost circulation frequently.The plug zone formed by lost circulation materials (LCMs) is also simulated and investigated,which changes the deformable behavior of fractures,hoop stress on wellbore and stress intensity factor in fracture tip.The simulation results reveal that the plug zone has the function of preventing wellbore pressure propagation from fluid pressure in fracture to increase,reducing the deformable degree of fracture,weakening the hoop stress on wellbore,and decreasing the stress intensity factor in fracture tip.The deformable behavior of fracture in fractured-vuggy carbonate formations is investigated clearly,and the deformable degree of fracture is predicted,which can provide theoretical basis for mud loss controlling and improve the knowledge of the fracture deformation.
Water lock effect is an important factor affecting the production of gas well mining.In the tight sandstone gas reservoirs with low and ultra-low permeability,due to small pore and throat,effect of water lock is more obvious than that in other gas reservoirs.This paper has discussed the mechanism of the water lock effect in tight sandstone gas reservoir during the process of production by analyzing the stress characteristics of liquid phase in reservoir sandstone before and after development.Ascertained the relationships between pressure and water saturation,pressure and gas phase relative permeability by the analytical data of high pressure mercury and gas water relative permeability experiments.The results showed that,the pressure has good exponential relationship with water saturation,while the pressure and the relative permeability of the gas phase showed a significant logarithmic relationship.With the increase of production pressure difference,it will lead to the increase of water saturation.The increased water saturation will result in decrease of the gas phase percolation capacity.The result aggravated the water locking damage degree ultimately.Reasonable production pressure has great significance for reducing the water lock effect of gas well and delaying the water breakthrough time.
The complexity of hydraulic fracture network in shale gas reservoir depends on the behaviors of hydraulic fractures as they intersect natural fractures.The larger of the kink angle,the more likely to form the complex fracture network.The model of kink angle with consideration of remote stress,stress intensity near the tip of fracture and stress interference of multi-fractures is established based on linear elastic fracture mechanics.Compared with the existing single fracture models,the kink angle is larger,meanwhile,length and space of fractures,distribution position of natural fracture as well as net pressure are principal factors affecting kink angle.The model not only can be used in the analysis of kink angle in “well plant” fracturing and multi-cluster fracturing in horizon well,but also can evaluate the feasibility of network fracturing and guide the development programme of network fracturing.
The shale gas content plays an important role in quantitatively evaluating gas bearing potential.The desorption method and log interpretation method were used in this study to respectively determine shale gas content of Shanxi Formation in central and southern Ordos Basin.Results show that shale gas content is 1.189cm3/g by desorption method and 0.958cm3/g by log interpretation method.Results of the two methods are close.In addition,the accuracy and applicability of the desorption method and log interpretation method were compared and discussed.Gas content of shale can be directly measured by laboratory test in desorption method,but the result is easily influenced by coring methods and exposing time of cores.Consequently,effective coring is the key for exactly measuring gas content of shale.Gas content of shale for the whole well can be calculated by the log interpretation method,but that was based on abundant laboratory data and selection of appropriate parameters.Both methods have their own advantages and can complement each other.Combination of the two methods can provide an important basis for calculating shale gas content and searching for favorable exploration areas.
According to the fractal characteristics of porosity of porous media and fracture extending in the hydraulic strengthening process,the fractal theoretical relationship between porosity and permeability in the coal seam is established.Further,the fractal calculation models of the fracture toughness and the filtration coefficient of fracturing fluid under the hydraulic fracturing are also built.Based on all of these studies above,the fracture extending model is set up under the assumed condition of pseudo-three-dimensional fracture extending model.The geometric parameters of fracture for the eight CBM wells in Zhengzhuang block of Qinshui coal basin,Shanxi province,are calculated by means of the fractal models.The results show that the fractures′ lengths of hydraulic fracture vary from 106.8m to 273.1m,and the widths range from 3.6m to 12.7m in the study block.Through comparing the calculation results with different models,it shows the results of the fractal model fit well with the field measured value,which reflects the applicability of the model at some extent.
To analyze the change of pore characteristics of coal around the borehole after hydraulic slotting and gas pre-drainage systematically,considering the sensitivity,precision and the test principles of Nitrogen Gas Adsorption(N2GA) and Mercury Intrusion Porosimetry(MIP),a method,combining the two means organically,was put forward to characterize the pore structure.The results show that the combination condition is that the volume of the testing mediums filling the critical pore should be equal and the bonding point transits smoothly.With the decrease of the distance between sampling point and the slotted borehole,the proportion of the minipore(10-100nm) decreases remarkably,the macropore(>1 000nm) increases gradually,the mesopore(100-1 000nm) and micropore(<10nm) show little changes.The initial gas emission characteristic of the coal sample varies with the distance.The combined action of hydraulic slotting and gas pre-drainage can weaken the adsorptivity and strengthen the seepage capacity of the coal mass remarkably.The research results are of great significance to the gas pre-drainage after hydraulic slotting.
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