Reliability-Based Casing Design Unlocks Reserves in a High-Pressure Gas Field
Authors: Wilson Adam 
Published: June 2018

This article, written by Special Publications Editor Adam Wilson, contains highlights of paper IADC/SPE 189669, “Unlocking Reserves in a BP-Operated High-Pressure Gas Field Through Reliability-Based Casing Design,” by Richard A. Miller, SPE, Rishi Ramtahal, SPE, and Oladele O. Owoeye, BP, prepared for the 2018 IADC/SPE Drilling Conference and Exhibition, Fort Worth, Texas, USA, 6–8 March. The paper has not been peer reviewed.
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.
The section of the 10-in. liner acts as a barrier to high-pressure water-bearing sands below and possibly above the production packer. If a well is put on compression, the wellbore pressure will be reduced, thereby increasing the differential pressure across that section of the 10-in. liner. This induced high pressure differential can result in a collapse failure of the liner, and, subsequently, high-pressure water may enter the well, compromising well integrity and the ability to produce from the well.
The field basis of design stated that the wells should be able to withstand a minimum bottomhole pressure of 2,500 psi. This limit reflected collapse ratings for the 10-in. liner. At this downhole pressure, the feasibility of a compression project is marginal, with the incremental recovery insufficient to support the capital investment. Reservoir-management studies indicate that, if the WDOL can be reduced to 1,000 psi, the compression phase of the project could be economical, with recovery potentially increasing by several hundred million BOE. The challenge was to find a way to lower the WDOL while maintaining well integrity and reliability, considering that many of the wells had already been drilled.