Evaluation of sweet spots and horizontal-well-design technology for shale gas in the basin-margin transition zone of southeastern Chongqing,SW China

The shale gas accumulation conditions in the basin-margin transition zone of southeastern Chongqing in SW China are complex.In order to improve single-well productivity in this area,the geologic characteristics,major factors controlling the occurrence of sweet spots,and drilling/fracturing optimization were investigated in this study.The sweet spot evaluation system and criteria were established,and the horizontal-well-design technology was developed.The following three conclusions were drawn.First,the accumulation and high-productivity-oriented approaches for sweet spot evaluation are proposed and the criteria are established based on screened key indicators.Second,the horizontal well was designed based on:(1)the“six-map”method,to identify both the geology and engineering sweet spots for well locations;and(2)seismic attributes,to predict the development of fractures and cavities,and thus,avoid mud loss and improve the drilling efficiency.The target window,well-azimuth optimization,and the curvature were forecasted to improve the fracturing performances.Third,the Pingqiao anticline,Dongsheng anticline,Jinfo slope,and Wulong syncline were selected as Type I sweet spots.Currently,shale gas has been successfully discovered in the basin-margin transition zone and is being commercially developed.

Limits and grading evaluation criteria of tight oil reservoirs in typical continental basins of China

Based on the microscopic pore-throat characterization of typical continental tight reservoirs in China,such as sandstone of Cretaceous Qingshankou and Quantou formations in Songliao Basin,NE China sandy conglomerate of Baikouquan Formation in Mahu area and hybrid rock of Lucaogou Formation in Jimusaer sag of Junggar Basin,NE China the theoretical lower limit,oil accumulation lower limit,effective flow lower limit and the upper limit of tight oil reservoirs were defined by water film thickness method,oil bearing occurrence method,oil testing productivity method and mechanical balance method,respectively.Cluster analysis method was used to compare the differences in pore-throat structure of different tight reservoirs,determine the grading criterion of tight reservoirs,and analyze its correlation with the limit of reservoir formation.The results show that the boundary between tight reservoir and conventional reservoir corresponds to the upper limit of physical properties,the boundary of classⅡand classⅢtight reservoirs corresponds to the lower limit of effective flow,the boundary of classⅢand classⅣtight reservoirs corresponds to the lower limit of reservoir forming,and the theoretical lower limit of tight reservoir corresponds to the boundary between tight reservoir and non-reservoir.Finally,the application results of the grading evaluation criterion show that the tight oil productivity is highly controlled by the type of tight reservoir,and classⅠand classⅡtight reservoirs are the favorable sections for high production of tight oil.

Geological characteristics and key exploration technologies of continental shale oil sweet spots: A case study of Member 2 of Kongdian Formation in the Cangdong sag in the Huanghua depression, Bohai Bay Basin

Under the general trend of stepping-up oil and gas exploration and development in China,the unconventional oil and gas resources such as shale oil and gas have become an important alternative.Abundant continental shale oil resources are developed in Member 2 of Kongdian Formation(Ek2)of the Cangdong sag in the Huanghua depression of the Bohai Bay Basin which has complex structure,strong heterogeneity,and large buried depth,thus,the geological characteristic of shale oil accumulation and key exploration technologies are investigated on base of the system coring of 635.8 m,centimeter-level fine description and analysis of over ten thousand samples,therefore,the“four accurate”technologies have been developed,including the accurate selection of sweet spot areas,accurate drilling of sweet spot layers,accurate fracturing of sweet spot layers and accurate measure of fracture-rich sections.The results show that the continental shale strata in Member 2 of Kongdian Formation can be divided into three types:felsic shale,limy dolomitic shale and mixed shale.The Member 2 of Kongdian Formation is characterized by diverse mineral components,abundant laminae,good source rocks and medium thermal evolution degree,tight reservoir with rich micropore and microfractures,high oil saturation and brittle mineral content,indicating good prospect of shale oil exploration.Through sedimentological study,TOC and Ro evaluation,brittle mineral calculation,and seismic fusion inversion,a total of 126 km^(2) of Class I sweet spots in Member 2 of Kongdian Formation was delineated comprehensively,which guides horizontal well deployment(Well GD1701H and Well GD1702H).The drilling rate of sweet spot of these two horizontal wells is up to 96%by tracing thin layer through fine calibration,locating compartment through fine inversion,ensuring window entry through precise positioning,and tracking sweet spot through adjustment.The “one-excellent and five-highs”criterion is used to select perforation points to realize differential designs of fracturing intervals and clusters.Micro-seismic and potentiometry monitoring show that the artificial fractures formed by volumetric fracturing are 300e400 m long and 120 m high,and control a volume of about 0.07 km^(3).The maximum daily oil production of two horizontal wells is 48 t and 66 t respectively to realize the economic exploitation of shale oil reserves controlled by fractures.The shale oil exploration in Member 2 of Kongdian Formation shows that the continental shale oil has great potential of exploration and development,and the above technologies are critical and effective for shale oil efficient recovery.

Micropore structure characteristics and quantitative characterization methods of lacustrine shale-A case study from the member 2 of Kongdian Formation, Cangdong sag, Bohai Bay Basin

To better understand the micropore characteristics of lacustrine shale and develop quantitative methodsfor characterizing lacustrine shale, the reservoir space types, structures and spatial distribution patternsof effective pores in the shale of Member 2 of Kongdian Formation in Cangdong sag are studied usingcores, thin sections, scanning electron microscopy (SEM), confocal laser scanning microscopy (CLSM),nuclear magnetic resonance (NMR), whole-rock X-ray diffraction (XRD) data, etc. Various shale porositycalculation methods are evaluated. The study results show that the reservoir spaces of the shale mainlyinclude three types, i.e., matrix pores, organic pores and fractures. Flaky pore throats formed bydissolution-induced pores and mould pores can improve the pore connectivity. NMR effective porosity ofthe shale varies from 0.59% to 4.42% with an average of 2.38%, accounting for 49.54% of the total NMRporosity and 34.53% of the gas porosity. The shale is divided into the unimodal-type felsic shale, bimodaltype felsic shale, carbonate shale and mixed shale. The different lithologies exhibit linear correlationsamong NMR, gas and effective porosities. The shale has effective porosity of 0.56%-4.53% with an averageof 2.12%. Furthermore, the shale reservoir is divided into four classes: Class I reservoir with effectiveporosities of more than 2.5%;Class II1 reservoir with effective porosity of 2.0%-2.5%;Class II2 reservoirwith effective porosity of 1.5%-2%;and Class III reservoir with effective porosity of less than 1.5%.