Study on Coalbed Methane Reservoir Sensitivity in Ningwu Basin and Drill-in Fluid Technology

Stress sensitivity, alkali sensitivity and water sensitivity conducted on rock samples taken from coalbed methane reservoir of the Ningwu Basin demonstrate that strong stress sensitivity, moderate alkali sensitivity and moderate-strong water sensitivity exist in the reservoir formations. It is suggested that the drilling fluid used in reservoir drilling in the Basin should have controlled mud weight and pH value, high temporary plugging capacity and inhibitive capacity, which help minimize damage to the permeability of reservoir rocks. A set of filming drill-in fluid, CQ-FDC, suitable for drilling the coalbed methane is as follows:(0.2%-0.3%)G304(organosilicon)+(2%-5%)FHY-2(compound salts)+(0.3%-0.5%) G310(viscosifier)+(1%-3%)G301(filter loss reducer)+(1%-2%) G325(temporary plugging agent)+ NaOH. This drill-in fluid showed in laboratory experiment percent reduction in linear swelling of cores by 71.56% after 12 h of test. Permeability recovery experiment showed that damage of the drill-in fluid to the permeability of coal samples 4^# and 9^# was less than 15%. CQ-FDC has been successfully applied in 6 coalbed methane wells in Ningwu Basin, average hole enlargement of these wells being 15.77%, and average rate of permeability impairment of the reservoir rocks was 16.73%. Average ROP of the six wells was increased in comparison with offset wells.