Comprehensive Logging Evaluation For Shale Gas Reservoir In WY Area

Shale gas reservoir usually has following characters:compliex mineral components,rich in organic content,low porosity,complex pore structure,low permeability,heterogeneity and very strong anisotropy caused by stress,fracture and thin layered formation,it cannot produce gas naturally in general.In order to explore and develop shale gas,besides using exploration technologies such as seismic,it is required to systematically optimize the drilling design,completion strategy and stimulation,through horizontal drilling and large-scale multi-stage hydraulic fracturing to maximum the stimulation of reservoir volume,hence to achieve industrial gas production capability.There is rich shale gas resource in Silurian Longmaxi and Cambrian Qiongzhusi formations in Sichuan basin and exploration prospects are broad.WY shale gas met problems in exploration and development,including long drilling cycle,high cost well construction,and poor production performance after hydraulic fracturing.Fully utilizing a variety of logging evaluation techniques in shale gas exploration and development,it will provide key parameters and strong support for the horizontal well drilling,completion and multi-stage fracturing with maximum stimulation reservoir volume.The main conclusions and understanding are listed as follows:1)According to the characteristics of shale gas reservoir,using optimized specific logging programs,combined with core experiment data from WY exploration wells,established local empirical formula to calculate mineral components,total organic carbon,adsorption gas content and permeability,it initially forms the logging interpretation methods for shale reservoir evaluation that is suitable for WY block at present stage.2)From accurate logging evaluation,favorable organic shale locates in bottom section of WY Silurian Longmaxi,the gas resource is about 3998.23×108m3.The favorable organic shale showed thickness 44.66m,effective porosity 4.1%,TOC 3%,total gas content 3.47 m3/t in average.It is seriously heterogeneity in WY shale and Wei204 locates in sweep point.3)In WY Longmaxi shale gas,the rock mineral components are 43.94%silica minerals(quartz and feldspar),37.76%clay content,11.2%carbonate content,3.5%pyrite content in average.Clay type is mainly illite and chlorite.High silica minerals and low clay content are good for proppant hydraulic fracturing.4)Combined with Sonic Scanner and FMI borehole imaging,the present maximum horizontal stress direction is the East-West(90°)in Longmaxi,it is affected by structural deformation slightly,and it becomes 135° in well Wei201.Nature fractures are not well developed in vertical wells,however there are lots of small faults.The directions of high conductive fractures varied.Micro Seismic monitoring revealed that the drilling azimuth of horizontal well completely along the minimal principal stress direction is not the best solution,natural fractures and small faults in far field of wellbore would be re-opened during hydraulic fracturing,and complex fracture network could be created,however it's difficult to create the ideal fractures transverse to horizontal wellbores due to complexity of geology,structure,stress and natural fractures in WY block.5)Processing Sonic Scanner logging data with modified MANNIE model,it gets more accurate geomechanic parameters in TIV anisotropic shale of WY block.It showed that there are two good high stress barriers for hydraulic fracturing extension between top shale formation and bottom Baota carbonate formation according to the minimal horizontal stress gradient profile,and it is proven by Micro Seismic monitoring,the micro seismic events mainly occurred in the organic shale.6)Using a new plane of weak(PoW)anisotropic wellbore stability model,accurate safe mud density window is calculated for vary deviation wellbore from vertical to horizontal.The safe mud density has to be increased sharply to keep wellbore stable when wellbore deviation increases from 400°to 75°,The safe mud density window of horizontal section is very narrow,minimal safe mud density is 2.0g/cm3.The direction of maximum horizontal principal stress will dramatically affect stress distribution surround wellbore.Directional drilling along the minimum horizontal principal stress is the most risky azimuth for wellbore stability,this observation is different from previous knowledge based on isotropic formation.7)Gas flow profiling was successfully obtained in two horizontal wells with multi-stage hydraulic fracturing,it showed the gas contributions from each stage are not unified.70%of gas production comes from 30%of fracturing stages.Finally,it suggested that evaluating unconventional shale gas should be overall planning with key control,according to different phases(exploration,evaluation,development experimental and "factory" development)and respective critical objectives,optimizing logging program can be conducted in block,Pad or single well.Comprehensive logging evaluation should be performed in pilot hole same as that for exploration well,to control the horizontal wells placement in the Pad,then optimized logging programs can be performed in these horizontal wells.The implementation of "factory" logging,combined with "factory"process for drilling and fracturing,will ultimately gain the "factory" scale benefit.