Abstract

Initial Open Hole Gravel Pack (OHGP) completions that have been installed in Greater Plutonio to date have all achieved complete annular packs and zero mechanical skin factors, resulting in well productivity indices that are significantly greater than expected.

The success of the Greater Plutonio OHGP completions has been attributed primarily to the rigorous design and field application of the fluid systems used at all stages of the well from drilling the reservoir through to the gravel pack itself and subsequent completion. An integrated approach was adopted for the design of the fluid systems involving extensive formation damage and fluid compatibility testing. To translate the robust design into a fluid system which can be applied effectively in the field, a thorough, fit for purpose QA/QC system for all drilling and completion fluids was developed, requiring extensive fluids testing and reporting at the well site.

The paper describes in detail the reservoir completion philosophy, drilling and completion fluids' systems and overall operational practices used in the Greater Plutonio OHGP completions. It also discusses the fluids design phase of the project and the QA/QC processes implemented in the field. Finally the paper presents the well productivity data from the wells completed to date.

Introduction

The Greater Plutonio Development is a 5 field deepwater project, located in Block 18, offshore Angola (Figure 1). All 43 planned development wells are subsea in water depths ranging from 1200 to 1500m. Of the 43 wells, 20 are producers and 23 are injectors. The development drilling programme began in 2005 with the aim of drilling and completing 15 wells prior to first oil in 2007. All the 5 fields produce from poorly consolidated Oligocene, turbidite reservoirs and consequently sand control is required in all development wells.

Each of the 5 fields is composed of multiple stacked reservoirs separated by shales. The formation sands are high permeability (Kaverage between 800 to 1500md) with low viscosity oil (0.5 to 0.8 cP). GOR ranges from 700 to 1100 scf/stb.

Formation Sand Particle Size Distribution

Laser Particle Size (LPSA) and sieve analysis was performed on the whole core recovered from the exploration and appraisal wells. The results of the analysis conducted on these wells are summarized in Table 1. These results are typical of all producing formations in the Greater Plutonio Development. The LPSA data for the main reservoir, the Plutonio O73, are shown in Figure 2. The majority of the sands are well sorted with low fines content. There are however some poorly sorted sands with higher fines content which result in the Uniformity Coefficient (D40/D90) ranging from 2 to 16 and the fines content from 1 to 15%. Overall the formation sands are poor to well sorted with low to high fines content. This particle size distribution data (PSD) combined with the limited amount of whole core data available for 5 fields, required the selection of a sand control system capable of providing well bore stabilization in all production wells.

Shale Characteristics

The deepwater Angola fields are located in shallow, immature sediments which typically have reactive shales that are incompatible with water based fluids. The experiences from the analogue fields in the basin clearly demonstrate the reactivity of the shales. The initial Open Hole Gravel Pack (OHGP) wells in an offset Angola deepwater block, were drilled with oil based mud and the hole was displaced to brine prior to running screens 1. Significant problems were initially encountered running screens due to shale instability and as a result the current system in the offset block deploys the screens in oil based mud.

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