Abstract

Industry successful cases and lessons learned have shown the challenges of setting cement plugs in open hole. Using industry best practices, it is possible to successfully set cement plugs in deepwater HPHT wells drilled with non-aqueous fluids. However, it is not uncommon to see cases where a single cement plug job has to be repeated more than once in order to achieve a competent barrier. Among different reasons that would cause a cement plug to fail, are: contamination with non-aqueous drilling fluid, insufficient cement and spacer volumes, density hierarchy, friction pressure hierarchy and cement plug slumping. These challenges become more critical as offshore drilling moves towards new frontiers: deepwater, salt formations, deeper measured depth reaching higher bottom hole temperatures and pressures. In Brazil, drilling operators do not face one single challenge in their operations; they are facing a combination of them in most of the new wells.

A successful case will be discussed, which describes the placement of a cement plug isolating an over-pressured gas reservoir and allowing the plug and abandonment operation to continue on a HPHT pre-salt offshore well in Santos Basin, in Brazil. The bottom hole pressure was over 137.8 MPa [20,000 psi] (ultra-high pressure well) and bottom hole temperature 171 degC [340 degF], required 2300 kg/m3 [19.2 lbm/gal] mud weight to maintain the well overbalanced after a gas influx happened when the mud had only a density of 2230 kg/m3 [18.6 lbm/gal]. In addition, the heavy-weight and non-aqueous fluid (NAF) added an extra challenge to this operation.

On this case, high density, high performance system (HDHPS) with engineered particle size distribution (PSD) was the selected cement slurry in order to overcome ultra-high formation pressure. Special chemistry was combined to the HDHPS to place the cement plug across the gas reservoir and salt formation, maintaining its stability and assuring isolation of the gas in the reservoir. The cement plug placement was designed with dedicated cement plug placement software, which brought a superior value in analyzing the risks involved, determining the placement technique and confirming best practices, thus aiding to define the pre-job conditions and consequently assuring the success of the P&A.

These practices can be successfully extended to other operators plugging and abandoning their wells in deepwater, HPHT wells and even in conventional environments.

Introduction

Placing cement plugs for sidetrack, remedial work, temporary or permanent abandonment is the highest cement activity offshore. It presents many challenges, including well cleaning, mud removal, minimizing contamination, cement slurry slumping, wait-on-cement sufficient time and more (Bogaerts et al. 2012). By adding to this condition a high pressure reservoir with high density drilling fluid to control the well and maintain it overbalanced, a set of new challenges are introduced to the cement job design and execution.

In the following paragraphs the reader will understand why HPHT offshore wells present new challenges for placing successful cement plugs, as well see a thorough discussion on the design, execution and evaluation (DEE) on setting the first cement plug, responsible for isolating the gas reservoir, in the plug and abandonment (P&A) operations of the well 1-OGX-63-SPS drilled in Santos Basin, offshore Brazil.

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