Mechanical Performance of High Density Set Cement for HTHP Applications

The south rim of the Junggar Basin is a key block for reserve increase of the Xinjiang oilfield. Wells drilled in this area are 6,000-8,000 m in depth, the bottom hole temperature and pressure of which are 160 ℃ and 170 MPa respectively, typical “three high” wells. Studies were conducted on the basic properties of the high temperature high pressure and ultra-high density cement slurries used in this area as well as the mechanical performance of the ultra-high density set cement under high temperature high pressure. In general laboratory studies, the strength development of a cement slurry is always quite different than the strength development of the same cement slurry in field operation. To understand the causes for this phenomenon, set cement test samples made from the typical cement slurry compositions used in this area were tested at 120 ℃ and 160 ℃, respectively under simulated actual downhole pressures and the pressure specified in the standard GB/T 19139—2012, which is 20.7 MPa. The tests include compressive strength test, flexural strength test and uniaxial compression test at room temperatures and triaxial compression test at actual downhole temperatures. Through these tests, the effects of pressure on the compressive strength and flexural strength of ultra-high density set cement, as well as the mechanical characteristics (deformation, failure mode etc.) of the set cement samples were investigated. The test results showed that pressure is one of the key factors affecting the development of the early strength of ultra-high density set cement. As the curing pressure increases, the compressive strengths of the top and bottom of the set cement were increased by 61.53% and 120%, respectively, the flexural strength of the top and bottom of the set cement were increased by 65.2% and 62.8%, respectively. Set cement tested in uniaxial compressive test at room temperatures showed obvious brittleness. After curing under 20.7 MPa, the ends of the set cement samples were obviously damaged. As the curing pressure was increased, the peak stress and elastic modulus were both increased, and the ability of the set cement samples to resist deformation failure was improved. High temperature triaxial compression test results showed that all mechanical parameters of ultra-high density set cement were greatly increased compared with the mechanical parameters obtained in uniaxial compression test, and the deformation of the set cement was mainly in a form of axial compression deformation, no obvious macro cracks were ever developed in the set cement, indicating that the set cement performed very well in resisting deformation failure, and was more like a linear-elastic – ideal plastic material. It is suggested that in laboratory examination and simulation experiment, the effects of actual downhole conditions on the mechanical performance of set cement be taken into account.