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.

Characterization of the pore system in an over-mature marine shale reservoir:A case study of a successful shale gas well in Southern Sichuan Basin,China

During the past two years the shale gas exploration in Southern Sichuan basin received some exciting achievements.Data of a new appraisal well showed that the gas producrtions of vertical well and horizontal well are^1.5×104 m3/day/well(with maximum^3.5×104 m3/day/well)and^12.5×104 m3/day/well(with maximum^40×104 m3/day/well),respectively,indicating a good gas potential in this area.Eight core samples from the reservoir were investigated by using a carbon sulfur analyzer,microphotometry,x-ray diffractometry,field-emission scanning electron microscopy(FE-SEM),mercury injection porosimetry(MIP),and low-pressure nitrogen adsorption to obtain a better understanding of the reservoir characteristics of the Upper OrdovicianeLower Silurian organic-rich shale.Results show that the total organic carbon(TOC)content ranges from 0.5%to 5.9%,whereas the equivalent vitrinite reflectance(VRr)is between 2.8%and 3.0%.Pores in the studied samples were observed in three modes of occurrence,namely,interparticle pores,intraparticle pores,and intraparticle organic pores.The total porosity(P)ranges from 1.6%to 5.3%,and MIP data sets suggest that pores with throats larger than 20 nm contribute little to the pore volume.Low-pressure N2 adsorption isotherms indicate that the total specific surface area(SBET)ranges from 9.6 m2/g to 18.9 m2/g,and the pore volume(V)ranges from 0.011 cm3/g to 0.020 cm3/g.The plot of dV/dW versus W shows that the fine mesopores(pore size(BJH)<4 nm)mainly contribute to the pore volume.The P,SBET,and V show a good positive correlation with TOC and a weak positive correlation with the total clay mineral content,thus indicating that the nanopores are mainly generated by the decomposition of organic matter.The reservoir characteristics of the Upper OrdovicianeLower Silurian organic-rich shale are comparable with commercial shale gas plays in North America.The sample gas contents with TOC>2%are more than 3.0 m3/ton.The observation can be a good reference for the future exploration and evaluation of reservoir in this area.