Abstract
Many mature gas reservoirs in China have very low formation pressure, like Yakela, Dalaoba, Kekeya etc. The formation pressure coefficient ranges from 0.6 to 0.9. Conventional well killing fluids easily leak into the formation and damage the well productivity. There are alternative well killing fluids in the industry to kill low-pressure formations, including foamy fluid, oil-based emulsion fluid and well killing fluid with density reducing agent. However, the densities of these alternative well killing fluids are mostly higher than 0.8 g/cm3, and the cost is high if large volume of density reducing agent is used to decrease the density to lower than 0.8 g/cm3.
In this paper, a formula of nitrogen foamy well killing fluid is developed and successfully used in sand removal operations in low-pressure gas reservoirs. A series of tests are conducted to select the optimal foaming agent, foam stabilizer and other additives. Sodium dodecyl sulfate is selected as the foaming agent. However, the properties of Sodium dodecyl sulfate are not stable in hard water, so we use dodecyl dimethyl betaine with sodium dodecyl sulfate together to increase the foaming ability in tough water environment like in the desert area. Xanthan gum is selected as a foam stabilizer because it can thicken the fluid phase and reduce the drainage speed. Gelatin is also added into the formula because it can form stable coacervate with xanthan gum. The concentration of each additive is also optimized through lots of tests.
Then the properties of the foamy well killing fluid are tested. Its density is between 0.50-0.80 g/cm3 and is adjustable. Under the temperature of 150°C, its plastic viscosity is 51mPa.s; its yield point is 51.5 Pa; the half-life period reaches 3055min. These basic properties meet the requirements of being used as well killing fluid. Salinity tolerance and oil resistance tests are also conducted to see the toughest environment that the fluid can be used in. The results show that the formula can be used in the oilfield where the water salinity is less than 100000 mg/L and the oil content is less than 15%. A model of calculating the equivalent density of the foam is developed. Scenarios of field application and onsite maintenance are also established. If a completion operation or a workover operation lasts for more than 24 hours, the foamy fluid needs to be maintained every day to guarantee its performance in the wellbore.
The newly formulated well killing fluid has been used in three wells in Yakela condensate gas filed in Tarim Basin, western China, where the formation pressure coefficient is 0.67 and the formation is strongly water sensitive. A foamy well killing fluid density of 0.72g/cm3 was used (surface density 0.53 g/cm3) for the sand removal operation in well Ya2-2-4. No fluid loss has been observed during the whole operational process. Neither gas seepage nor oil overflow has been observed during well killing. The well has recovered from production after workover and the production rate reaches 90×103m3 natural gas with 12t condensate oil every day. The new formula of foamy well killing fluid not only shows good laboratory properties under 150°C, but also proves to be a good solution to the downhole operations in low-pressure and depleted reservoirs.
