Increasing recovery was considered by lowering the reservoir-abandonment pressure below the initial design value. Design assumptions and equipment ratings were reviewed systematically to determine which aspects factored into the decision to change reservoir management. Collapse loading of the 10-in. production liner was identified as a key variable. This paper presents work performed to characterize the probability of collapse as a function of reservoir-abandonment pressure using reliability-based design (RBD).

Introduction

For the high-pressure gas field under consideration, future-field-development studies were conducted alongside execution of the development. One such study investigated the potential application of gas compression to the field. Gas compression reduces the pressures downstream of the wellhead to allow depleted reservoirs to flow at lower pressures. The potential benefit is increased reserves recovery with effectively no increase in well cost.

The reservoir design and operating limit (RDOL) defines the lower allowable pressure from a reservoir perspective. Engineering studies have shown that this limit is set by the well design and operating limit (WDOL). If compression options are pursued, proposed depletion pressures should be checked for consistency with the limits prescribed by the RDOL and WDOL. With compression, when the wellhead pressure is reduced, the wellbore pressures may approach the lower end of the WDOL. This increases the pressure differential across the 10-in. liner section, which can result in a collapse failure, as illustrated in Fig. 1 above.