One of the main conditions for ensuring high-quality well cementing is full replacement of fluid in the annular space, which can be achieved, among other things, by ensuring the concentric position of the casing string in the wellbore. The algorithms based on ISO 10427-2: 2004 are usually used to calculate the centralizer placement for ensuring a required eccentricity. The formulas presented in ISO 10427-2: 2004 are only applicable for calculating casing deflection between two identical centralizers. It is assumed that if the deflection of the string between the two centralizers exceeds the nominal value of the gap between the string and the wellbore wall, then the eccentricity over the entire interval between the centralizers is equal to one. At the same time, actual measurements in wells using downhole GR-density fault detector show that the eccentricity of the casing strings extremely rarely turns into one.
In the paper the new approach to casing eccentricity calculation is suggested. It considers the possible sagging of the casing between two adjacent collars. First, the deflection between two nearest centralizers is determined. The calculating procedure of casing deflection at each point will depend on the resultant gravity force, Archimedes' force, tensile force and bending force. If centralizers are installed on each casing, then the maximum deflection and therefore the minimum clearance will be noted in the middle of each casing. If the gap in the middle of the distance between the centralizers exceeds the half-difference between the outer diameters of the collar and the pipe body, then it is considered that the collar does not touch the borehole wall. Otherwise, an additional fulcrum appears in the middle between the two centralizers. If there are more than two casing between the centralizers, then in this case there are several likely outcomes: all collars touch the wellbore walls, all collars do not touch the wellbore walls, only some collars touch the wellbore walls. The calculation of the casing deflection between two collars or between the collar and the centralizer is carried out in the same way as the calculation of the casing deflection between two centralizers. It is shown that the procedure that has been described leads to more accurate eccentricity prediction in comparison with existing ones. The proposed approach was implemented in specialized software for centralizer placement calculation in Surgutneftegas PJSC.
References
1. ISO 10427-2:2004. Petroleum and natural gas industries. Equipment for well cementing. Part 2: Centralizer placement and stop-collar testing.
2. Juvkam-Wold, H.C., Baxter, R.L., Discussion of optimal spacing for casing centralizers, SPE-13043-PA Drilling Engineering, Dec. 1988, https://doi.org/10.2118/13043-PA.
3. Juvkam-Wold H.C., Wu J., Casing deflection and centralizer spacing calculation, SPE-21282-PA, 1992, https://doi.org/10.2118/21282-PA.
4. Kinzel H., Kiithan T., Planning the cementing job incorporates data management and technical expertise – A new software to calculate the optimum placement of mechanical cementing products, SPE-38130-MS, 1997, https://doi.org/10.2118/38130-MS
5. Blanco A., Ciccola V., Limongi E., Casing centralization in horizontal and high inclined wellbores, SPE-59138-MS, 2000, https://doi.org/10.2118/59138-MS
