The Effect of Cement Mechanical Properties and Reservoir Compaction on HPHT Well Integrity

In the life of a well, the cement sheath not only provides zonal isolation but also supports casing and increases casing-collapse resistance. Due to the high-pressure, high-temperature (HPHT) conditions, the cement sheath plays an important role in maintaining wellbore integrity. During the production process in HPHT wells, the pressure differential inside the casing and the surrounding formation is larger than the conventional wells. The stress induced by fluid withdrawal in highly compact reservoirs can cause the cement and the casing failure in these wells. These present a greater challenge to the wellbore integrity than the conventional wells.

To have reliable data, extensive experimental work on Class G cement was carried out to measure the principal parameters for mechanical structural calculations. The experiment was also set up to simulate conditions under which cement low-cycle fatigue failure could occur. Zero-based cyclic pressure was applied to the casing in the cement low-cycle fatigue test. Three types of cement (72-lbm/ft3, 101-lbm/ft3 and 118-lbm/ft3) were cured and tested at 300?F to study the cement mechanical properties under high-temperature conditions over the long term. The tests included a 1-year mechanical properties measurement such as compressive strength development; i.e., Young?s modulus and Poisson?s ratio. Finite element methods (FEM) were used to study the casing buckling deformation characteristics of reservoir compaction in some south Texas wells.

The 2D and 3D FEM models were built to study the effects of mechanical properties and reservoir compaction on HPHT well integrity. As the confining pressure increases, the cement shows more plasticity and can withstand more pressure cycles. The cement with a higher Poisson?s ratio and lower Young?s modulus showed better low-cycle fatigue behavior. Casing collapse resistance is very sensitive to void location, cement Poisson?s ratio, cement Young?s modulus, and pore pressure. Casing eccentricity and voids shape have minor effect on the casing-collapse resistance. Casing shear failure, tension failure, and buckling failure are the most likely failure modes in reservoir compaction. For different casing wall thickness, the critical buckling strain is almost identical.

This study presents a better understanding of casing failure and cement failure in HPHT wells. The results of the study will help improve cement and casing design to maintain wellbore integrity that can in turn be expected to extend throughout the life of the well.