Evaluation of the behaviour of the lateral boreholes in the Gorm chalk field

Abstract

Open hole wellbores present cost efficient completion solution; however, instability of such boreholes in weak formations such as chalk in the course of production raises a concern. This paper presents a method for predicting the stability of the radial jet drilling laterals in the Danish Gorm chalk field. The method is based on four main parts: (1) rock mechanics testing in the triaxial cell; (2) the single lateral hole testing consisting of loading and fluid flow test; (3) utilizing CT imaging for identifying the damaged zone and its extension; and (4) numerical simulations, utilizing a chalk model that takes into account the post-peak softening and the rate dependency of the pore collapse stress. Simulation of bottom hole and reservoir pressure decline over a year by 17 MPa and 9 MPa, respectively, showed a small development of the plastic strain at the borehole wall. Further simulation up to four years with constant stress (creep) resulted in a change in the cross-sectional area of the borehole, where shear cracks developed at the wall and some distance away from the hole stress concentrations associated with pore collapse was observed. By the end of the simulation, the borehole was likely to change its geometry by removal of the plastified area. The single lateral hole test with flow carried out with differential pore pressure, drawdown, of about 2.5 MPa within 5 h and 1 h, respectively, provided insignificant permeability change during the flow test.