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Abstract

This paper demonstrates the benefits of using hollow glass microspheres as a low density additive in fluids formulated for use in workover of wells operating under depleted pressure conditions as found in JV “Vietsovpetro” White Tiger (Bach Ho) Oil Fields, Cuu Long Basin in Vietnam. The paper details the requirements on hollow glass microsphere selection process as a function of bottom hole hydrostatic pressure and fluid density. Assessments on cost of fluids of different densities are also made on the basis of microsphere selection and concentration for the workover program in the White Tiger Oil Field. The paper proposes options to bring fluid costs to more economical levels based on microsphere regeneration. Detailed analysis has been made on all operating JV “Vietsovpetro” wells calculating the required amount of microspheres for workover in accordance with the selected grades. The data reported in this paper can be used for mud design and to provide technical support for well workovers whenever low density fluids (less than 1 g/cm³) are considered. In addition to mud properties and formulating practices, the paper also provides useful tips in the preparation, handling, and waste management of low density fluids for workover programs.

Introduction

Challenged by “sub-hydrostatic pressures”, workover activities in the fractured basement rock of the Southern Vietnam shelf is highly susceptible to circulation losses and poor well performance due to reservoir damage. To resolve this serious problem, JV Vietsovpetro has developed a new fluid technology that is based on light weight “Hollow Glass Micropheres” in the preparation of low density drilling and workover fluids. Fluid density management has been proposed as the most practical method to protect the basement pay zone / collector properties during workover and drilling.

Mud losses during workover and drilling in basement rocks of Southern Vietnam shelf is dependent on fracture characteristics and, as such, occurs in the pay zones of the Central and Northern Blocks, typically between 3151 m to 4490 m depth.

Basement Collector Properties, Characterization of Pore Space Structure in the Basement Reservoir.

The basement collector properties of JV Vietsovpetro’s field are typically made up of vuggy or cavernous fractured rock that exhibits complex lithology and porous structures. The fractured character of the rock has been established through various core sampling and geophysical surveys.

The most numerous by number (up to 55 - 60 %) are made up of pores with prevailing diameter of 0.05 - 0.5 mm, playing the main role in making of the structure of the pore space of granitoids (and granitoid like rocks).

Prevailing (47 - 67 %) fractures open to 0.01 - 0.06 mm make up the void space of granitoids. There can be found open-joint (gaping) fractures opened to 1.5 - 2.0 mm and even to 10.0 mm. The size of caverns may reach 0.5 - 1.0 mm and sometimes their diameter can be up to 50.0 mm. Smaller sized fracture opening to 0.05 - 0.10 mm are also quite common.