Research on the influence of geological factors on casing stress in casing-in-casing cementing in the horizontal section of shale gas wells

Abstract

Abstract Deep shale gas development has great potential, but the frequent occurrence of shale gas well casing change problems triggered by geological factors seriously restricts deep shale gas development. In order to investigate the influence of geological factors on the casing stress of the cemented casing in the sleeve, a model of formation-cement sheath-double casing assemblage was established, and the influence of three-way geostress and fault slip on the casing stress of the casing-in-casing cementing was investigated by using finite element analysis. The results show the following: the smaller the difference between the vertical geostress and the maximum horizontal geostress is, the lower the equivalent force on the casing is, and when the difference gradually decreases from 20 to 7 MPa, the maximum equivalent force of the inner casing under fracturing condition decreases by 9.4%; the increase of the minimum horizontal geostress leads to the increase of the equivalent force of the inner and outer casing. When the minimum horizontal stress gradually increases from 80 to 90 MPa, the maximum equivalent force of the inner casing under fracturing condition increases by 5.9%. The larger the fault slip and the fault angle, the larger the equivalent force generated on the casing. The shear resistance of the double-layer casing is significantly greater than that of the single-layer casing, with an average increase in fault slip distance that can be withstood of about 45.25% and 40.2% in the no internal pressure and fracturing conditions. The larger the casing steel grade and the thicker the wall thickness, the higher the shear resistance. It is recommended to reduce placing of wells in areas where the difference between the vertical and maximum horizontal ground stresses is large, and at locations where the fault slip angle is large, and to use both higher steel grades and larger wall thicknesses of casing. This research result demonstrates the feasibility of “milling + casing-in-casing” technology in severe casing change wells and also provides useful guidance for the application of this technology in the field.

Keywords

• Shale gas wells
• Casing change
• Casing-in-case cementation
• Ground stress
• Fault slip