Abstract

This article presents the development of a computational tool to guide horizontal gravel pack design for long horizontal offshore wells. Mechanistic model formulation, experimentation at a large scale flow loop and software development are detailed. The computer simulation results are then compared with field data collected in Campos Basin operations, offshore Brazil. A discussion on design alternatives for long horizontal well at low frac gradient formations is presented. This discussion includes a sensibility analysis on screen eccentricity, open and closed BOP configurations and alpha vs alpha plus beta wave displacement options.

Introduction

Gravel Packing is today the most frequently applied sand control technique in Campos Basin. Due to the critical conditions, such as the deep and ultra deepwater and low frac gradients, a lot of precision is required to assure gravel packing success. Most models available in the industry for horizontal gravel pack design are essentially empirical, resulting in imprecise predictions for extrapolated conditions.

The new scenario for offshore development in Brazil includes heavy oil fields in deepwaters where 2000m horizontal sections are required. Sand control options are a major issue and gravel packing is a strong candidate if pressure loss issues can be overcome.

These aspects were the main motivators for a research project including theoretical and experimental developments. A mechanistic model to describe the whole operation, including sand injection and alpha/beta waves propagation, fluid leakage, multi zonal isolation and beta wave pressure reduction optimization was developed. The main core of the model, aiming the definition of alpha wave height, is based on a two layer model approach. Initially developed for hydrotransport applications, this kind of model have been adapted by several authors for drilled cuttings transport analysis. It is a consensus among design and operation engineers that a physically based software is a necessary rigsite tool for determining operational parameters, specially when last minute data have to be considered.

Several authors present experimental results of horizontal gravel packing performed in test facilities (Forrest1, Penberthy2, Sanders3). In the present study, 15 runs on a full scale displacement loop where the effects of pipe eccentricity, particle diameter, particle shape, fluid flow rate and return flow rate could be quantified. The results allowed the adjustment of fundamental coefficients in the mechanistic model.

Theoretical Model

The proposed model consists on the following steps: pressure propagation during string injection, alpha wave height calculation, pressure propagation during alpha wave propagation and pressure propagation during beta wave propagation. A brief description of each step follows while more details are highlighted in Martins4et al.

Alpha Wave Height Prediction

In order to predict alpha wave deposition heights, a two layer model was adopted. The present model is an extension, for horizontal gravel packing applications, of the model proposed by Martins5 for drilled cuttings transport analysis.

The following formulation was developed to describe the eccentric horizontal annular flow a solid-Newtonian fluid mixture, aiming the prediction of an equilibrium alpha wave bed height. The solids are characterized by their average diameter and sphericity.

The model consists of a stratified two layer configuration which allows, with an unique formulation, the simplified representation of the system in different flow patterns (stationary and moving beds). Fig. 1 shows schematically the proposition.

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