A Novel Non-Acid Stimulation in Deep HPHT Naturally Fractured Gas Sandstone

Abstract KeShen gasfield is a low matrix permeability intensely naturally fractured tight sandstone reservoir in Kuqa gasfield of Tarim basin, West China, with the characteristics of high-pressure 120MPa-140MPa, ultra-depth 6700m-8000m, high temperature 160°C-190°C, and long lateral 200m-300m. A non-acid stimulation was trialed in five wells and the post-production was surprised. During drilling, a large number of barite is used to increase mud weight, and some calcium carbonate is used to avoid reservoir damage, But the damage is usually serious because of hundreds of tons of mud leakoff and the drilling fluid cakes. A non-acid cheating agent was trialed for removing the damage and increasing the production, based on the cheating mechanism, the divalent metal ions will be chelated by cheating agent, and barium or calcium ions will be preferentially chelated. The non-acid cheating agent is an alkaline chelate with density 1.03g/cm3, the dissolving capacity of chelating barium ions is increasing by the chelating agent concentration and temperature. The concentration is usually from 25% to 30%, and the capacity of chelating barite ions is about 12.5-12.8g/L at the temperature of 170 °C. The reason for using chelating agent to remove the damage is that fresh acid and reacted acid can make strong corrosion to tubing string in the process of acidizing in high temperature reservoir. On the condition of 140 °C, the laboratory experiments indicate that the corrosion rate of fresh acid and the residual acid is 17.4g/ (m2·h) and 3g/ (m2·h) respectively, however, there is essentially no string corrosion observed when using the chealting agent for removing the damage. The non-acid cheating agent was trialed for five Wells. The general pumping rate is from 3.0 m3/min to 4.4m3/min, and the total volume of cheating agent is about 400m3. The gas post-production is about from 60×104 to 70×104m3/d, similar to that of adjacent wells to which acidizing or hydraulic fracturing are applied. The flowback fluid of the non-acid stimulation wells is tested, the results indicate that the maximum content of barium sulfate is 3200 mg/l, and the maximum content of calcium ion is 4382mg/l. The non-acid stimulation is an effective method for high temperature intensely naturally fractured gas sandstone, no tubing corrosion.