Causal analysis and countermeasures on B sustained casing pressure of shale-gas horizontal wells in the Sichuan Basin

In the Sichuan Basin, the percentage of shale gas wells with sustained casing pressure (SCP) is higher, bringing about the challenges to the efficient development of gas reservoirs and the wellbore integrity of gas wells. In this paper, the actual SCP situations in domestic shale gas development blocks were analyzed specifically. The gas source was determined by using the isotope analysis method, and SCP mechanisms of shale gas wells were studied by means of physical simulation experiment. Accordingly, the failure laws of cement sheath in different cement slurry systems in the process fracturing were fi gured out, and the key technologies to improve the sealing capacity of cement sheath and relieve the SCP phenomenon in staged fracturing wells were developed. When the cement slurry column can structurally satisfy the sealing requirement completely, a cement slurry system of low porosity and Young's modulus was developed by means of the interphase fi lling technique and the set cement porosity decreasing method. Its Young's modulus is only 4.2 GPa, and its porosity is 27.1% (the porosity of conventional cement set 35%). And the elastic deformation capacity of cement set is enhanced dramatically. The new cement slurry system has been practically applied in 5 wells. And its high-quality cementing rate is 100%with no SCP. This research provides the good reference for SCP control.