Research Of Drilling And Completion Fluid Technology For Low Porosity And Low Permeability Reservoir In Weizhou Oilfield

In this paper the genetic types, the reservoir geological characteristics, the factors of formation damage caused in well drilling, and corresponding techniques of formation protection and as well as drilling fluid systems used in low permeability gas field are denoted. Weizhou 11-7 Oilfield which is a low permeability gas field is used as object of research. By means of scanning electron microscope and X-ray diffraction, the characteristics of Weizhou 11-7 Oilfield are studied. The reservoir lithologic property of El3 is conglomeratic mid-coarse quartzose arkose and sandstone, the next is fine sandstone, which is low porosity and low permeability reservoir. El3 reservoir is mainly of illite, the next is the interstratification of illite and montmorillonite, chlorite and kaolin. With the increase of the depth, the contents of illite and chlorite increases, while the contents of kaolin and the interstratification of illite and montmorillonite decrease. The mechanism of potential formation damage in Weizhou 11-7 Oilfield is the damage of solid invasion, water and salt sensitivity, stress sensitivity and water lock.The result of sensitivity evaluation shows that the damage level of velocity sensitivity is none to weak, the damage is mainly induced by the low permeability and tight formation, where diagenesis is strong and there is less moving micro-particles in the reservoir. The damage of water sensitivity is totally medium, the core damage level of deionized water is 56.3% in average. Anti-swelling is necessary in field operation, the optimal and appropriate salinity in the reservoir is > 16000mg/L. The level of damage of alkaline sensitivity is zero to weak, critical pH is 9 in the operating fluid, there is no damage of acid sensitivity induced by hydrochloride, the damage of mud acid sensitivity is 0.7%～25.7%, level of damage of acid sensitivity is weak-medium to strong. If pressure difference in reservoirs of El3 is < 5MPa, the damage of stress sensitivity is 56.9% in average, the damage is mid-strong. The high level of formation damage induced by stress sensitivity is that the reservoir is the one with low permeability and small pore throats, these throats are tubular and bending sheet types, the contact area is bigger in the pore throats, under effect stress, they are easily to be closed to reduce the permeability in reservoirs, but for the high content of rigid particles in the reservoirs, such as quartz, the recovery level is more than 80% after it is damaged by stress sensitivity.On the basis of studying the reservoir characteristics and damage factors, the technique of drilling fluids used in conventional and horizontal wells are studied in Weizhou 11-7 Oilfield. For non-reservoir intervals in conventional directional wells, the selected formula of PEM drilling fluid is: 3% sea water slurry + 0.2% NaOH + 0.15% Na₂CO₃ + 0.5% PF-PLUS + 0.4% PF-PAC + 2.5% SPNH + 2% SMP-2 + 1% DYFT-2 + 2% PF-GJC + 0.15% XC + 1.5% PF-CMJ + 2% PF-GBL + 5% KCl.When drilling in reservoirs, the technique of shielded temporary blocking is used to protect the reservoirs. For horizontal wells, the chosen formula for PRD spud-in drilling fluid is: sea water + 0.1% NaOH + 0.15% Na₂CO₃ + 1.0% PF-VIS + 2.0% PF-FLO + 1.5% SMP-2 + 3.0% JLX + 20% KCl + formate.The chosen formula for perforating fluid is: sea water + 0.5% HTA + 2.5% clay inhibitor HCS + 2% corrosion inhibitor CA101-3 + 1% cleanup additive SAA; break fluid formula is: sea water + 2% breaker HBK + 2.5% clay inhibitor HCS + 1.5% corrosion inhibitor CA101-4 + 1% cleanup additive SAA. when density is less than 1.15 g/cm3, KCL was weight material; when density is more than 1.15 g/cm3, KCL and formate were weight materials. The well completion fluid, mud cake removing agent and the filtrate of spud-in fluid are well compatible formation water.