Reservoir Protection Strategy and a High Performance Drill-In Fluid for Deepwater Wells in the South China Sea

The protection of high porosity and high permeability reservoirs has not got enough attentions because the excellent hydrocarbon storage capacity would provide high oil and gas productivity even being damaged to some extent. Deepwater oil and gas reservoirs are mainly high porosity and permeability reservoirs, and the reservoir protection is important because severe reservoir damage will cause an enormous economic loss due to high investment and risk of deepwater operation. A new deepwater gas field in the western part of the South China Sea must be developed soon, and the reservoir protection strategy needs to be studied. The characteristics of the reservoir were studied by X-ray diffraction, scanning electron microscope, mercury intrusion porosimetry, hot rolling dispersion test and liner swelling test. Furthermore, the water and salinity sensitivities of the reservoir were studied. The results show that the reservoir has high porosity and high permeability with an average porosity larger than 30% and an average permeability higher than 500×10-3 μm. The large pore throats will be easily invaded and plugged by solids in drilling and completion fluid. Therefore, effective temporary-plugging of the pore throats with a wide size range is extremely important. The content of clay mineral is 9% to 19% with approximately 50% mixed-layer illite-smectite, presenting potential borehole instability and reservoir damage risk due to its strong hydration ability. In addition, the reservoir has strong salinity sensitivity and medium-weak water sensitivity. The reservoir protection strategy of drilling and completion fluid related to the properties of the reservoir was proposed. A multi-stage bridge temporary-plugging method was used to prevent the invasion of solids into the pore throats with a wide size range. A highly inhibitive polyamine was selected to inhibit the hydration of clays to maintain wellbore stability and reduce reservoir damage. Because of the requirement of gas hydrate inhibition and density adjustment, a high performance water-based drill-in fluid with formate brines was developed. It has excellent reservoir protection performance with the permeability recovery rate approximately 90%. Meanwhile, it performs well in inhibiting clay hydration, hydrate formation, and has good rheological as well as filtration properties over a wide temperature range from 4 °C to 75 °C. This work provides theoretical guidance of reservoir protection and a high performance drill-in fluid for the target deepwater gas filed. We hope it will also be useful for the protection of high porosity and high permeability reservoir in other deepwater oil and gas fields.