Experimental Analysis of the Effect of Stress on Shale Reservoir Brittleness and Its Application

As well depth increases,formation rocks brittleness may be reduced and consequently manifest as ductile behavior.Conventional mineral composition can be used to assess the brittleness of shales.Because it does not take the impact of confining pressures (burial depths) into account,one arrives at less than satisfactory assessment conclusions.Under such circumstances,mechanical tests were conducted with continental deposit shale samples under different confining pressures and a strain method was deployed in a tested result analysis to highlight brittleness features of the shale.Research results showed that both elastic and plastic strains may increase with confining pressures while the brittleness index may decrease.Within the concerned Block,the brittleness index of shale at the depths of 1 250.00 m,2 500.00 m,3 500.00 m and between the interval of 3 500.00~5 000.00 m and 5 000.00~6 000.00 m fell approximately 5.97%,8.55%,10.74%,14.00% and 18.00%,respectively.With brittle minerals content in these shale reservoir formations over 65%,60%,55% and below 50%,the formation depth limitation for economic development of those reservoirs are determined to be 6 000.00 m,5 000.00 m,2 500.00 m and no commercial value,respectively.So the shale brittleness index determined from conventional mineral composition can be calibrated through quantification of impacts of stresses (burial depths) and it can provide help in the identification of intervals for staged hydraulic fracturing.