The geological conditions in front of Kuqa piedmont in Tarim Oilfield are extremely complex. High pressure salt water and weak layer coexist in the salt layer, fractures are developed in the target layer, and oil/gas shows are active. Therefore, the contradiction between pressure stabilization and leakage prevention in cementing is prominent. The safety density window of cementing is only about 0.03~0.10g/cm3. Based on the theory of wall shear stress, a set of narrow safe density window cementing technology suitable for Kuqa piedmont front is formed by finely evaluating leakage pressure, optimizing construction displacement and slurry rheological properties, which can effectively improve the cementation quality of cement-stone interface. From 2020 to 2021, more than 40 wells of the narrow safety density window cementing technology will be applied on site, and the cementing qualification rate will be increased by 30%. The effective isolation rate of high-pressure brine and the pass rate of negative pressure channeling test of the target layer are both 100%.At the same time, the managed pressure cementing technology was successfully applied for 7 wells, which provided effective technical support for narrow-density window cementing.
The engineering geological conditions of the Tarim Kelasu tectonic belt are complex. The narrow safe density window severely restricts the implementation of techniques to improve displacement efficiency, such as increasing displacement, increasing slurry density difference, and fully circulating wells, making it difficult to guarantee cementing quality. High pressure brine (pressure coefficient 2.30-2.60 g/cm3) and weak layer (pressure bearing capacity between 2.05-2.20 g/cm3) coexist in the piedmont salt layer (Johnston, et al., 1992; Yan, et al., 2019). Fractures are developed in the target layer and the risk of gas channeling is high. In order to stabilize the gas-water layer, the drilling fluid density is forced to increase, resulting in a further narrowing of the safe density window. Some downhole casings are lost or lost, and only positive injection and reverse extrusion can be used. Although reducing the displacement can barely achieve one-time upstroke, the interface cementing quality of the cement sheath is poor, and it is easy to generate micro-annulus. At the same time, in order to safely run the casing under the salt layer to the bottom of the well, some wells use the casing without couplings. In the case that the centralizer cannot be matched, the average degree of the pipe string is only 20%. The eccentric annulus causes poor displacement geometry, and cementing displacement efficiency is more difficult to ensure displacement.