Casing damage mechanism from short-term water injection development in the Yingdong Oilf

In the Yingdong oilfield, a large number of casing deformations occurred in loose sandstone and interbedded mudstone soon after oil wells were turned to water injection wells, which resulted in low efficiency of water injection for secondary recovery. In order to clarify the casing damage mechanism caused by short-term water injection, based on drilling data such as borehole trajectory, logging data, cuttings logging data and fault distribution map of the Yingdong oilfield, a geological model of the correlation between fault-lithology and perforation-casing damage was established using Petrel software, and the relationship between casing damage and water injection time, stratigraphic lithology, perforation location and fault location was analyzed. The main controlling factors of casing damage are water injection technology and stratigraphic lithology. The flow-solid coupling finite element model of short-term water injection in a sandstone reservoir is established, which could simulate and analyze the main factors controlling casing damage. The variation of pore pressure, ground stress and displacement of the loose sandstone reservoir after short-term water injection was studied, and three mechanisms of casing damage caused by short-term water injection after oil well transfer were revealed: (1) Water injection causes the reservoir to expand and tensile stress arises along the casing axis, the casing would be deformed shown as casing diameter shrinkage and deformation; (2) Hydration of mudstone layers forms shear stress and compressive stress radially along the wellbore. At the interface of the sand and mudstone, the casing is subjected to shearing and extension at the same time, resulting in dislocation damage, diameter shrinkage and severe deformation; (3) After the original sand production well is converted into a water injection well, the sand production layer will be compacted forming compressive stress along the casing axis, the casing would be liable to diameter expansion and deformation phenomena. In view of the above casing damage mechanism, the corresponding casing damage prevention countermeasures are put forward to ensure the long-term integrity of the wellbore such as (1) Avoiding cementing near the water injection layer, as the annular space would appear to buffer the mechanical action of the slip formation on casing; (2) High steel grade and thick casing should be used to give the casing sufficient strength and stiffness to resist formation stretching, compression and shear; (3) Sand control completion technology should be adopted in new well completions, and sand control technology should be supplemented in time to avoid formation weakening settlement after production wells are converted to water injection wells.