Analysis and countermeasures on sustained casing pressure in b-annulus of shale-gas horizontal wells in Sichuan basin of china

Abstract

In Sichuan Basin of China, the percentage of shale gas wells with sustained casing pressure (SCP) is high, bringing about 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 of fracturing were figured out, and the key technologies of improve the sealing capacity of cement sheath and relieve the SCP phenomenon in staged fracturing wells were developed. On the premise that the cement slurry column can structurally satisfy the sealing requirement completely, a cement slurry system of low porosity and low elastic modulus was developed by means of the interphase filling technique and the set cement porosity decreasing method. The elastic modulus was only 4.2 GPa, the porosity was as low as 27.1% (the porosity of conventional cement stone is 35.0%), and the elastic deformation capacity of cement stone was en­hanced dramatically. The new cement slurry system has been practically applied in 5 wells, and the high-quality cementing rate was 100% with no SCP. This research provides useful reference for SCP control.