RIASSUNTO
Abstract
Oil production from some of wells in the White Tiger field producing from a fissured Basement reservoir; have been impaired by excessive water production. Excess water not only reduced the artificial lift efficiency, but also imposed various damages to the oil zones.
Since 2002, a joint industrial project was set up to study the feasibility of performing water shutoff treatments in the open-hole completion oil wells. The study involved evaluation of a high temperature polymer base water shut-off fluid for deep penetration of the fissure formation and a micro-fine cement system for sealing off the water entries. Based on this study, a cost-effective chemical treatment method was progressively developed. In 2005, the treatments were performed through-tubing with and without isolation packers.
Two Candidate wells were having 6.5"" open-hole size, at approximately 4,200 meter TD, and 150 deg C reservoir temperature. The water cut were 95% in one well and 30% in the other well. It was found that these two wells certainly had big difference in fluid injectivity, and original designed treatment was modified on site. This paper summarizes key lessons learnt including tool and packer conveyance, mixing and pumping of water shut-off fluids under offshore rig and wellsite conditions. It also shares a method of post treatment production evaluation and suggests operational change to improve the production.
Introduction
White Tiger Field in offshore Vietnam is producing from a highly fissured granite Basement formation. Basement consists of igneous crystalline rocks characterized by petrography heterogeneity because they were formed in different tectonic activities in their geological evolution. Since being formed to recent, the basement rocks of the Cuulong basin have been strongly affected by different alteration processes. These processes changed not only the composition, petrophysical characteristics, but also were principal causes creating good reservoir properties of some granitoid basement bodies. Some main alteration processes are volume shrinkage due to the crystallization of magma lavas, alteration due to the tectonic activities, alteration due to the hydrothermal activities, alteration due to the weathering activities. The inside volume of magma bodies is often shrank when the magma lavas crystallized and solidified. This volume shrinkage caused by sudden change of temperature as well as by viscosity increase during the times that these magma lavas crystallized and resulting in the formation of individual micro fractures and misco-vugs in granitoid rocks.
These micro-pore types can be only beneficial for reservoir if they had linked together by fractures and microfractures, which were formed due to tectonic activities at later times. The tectonic activities are principally factors making strong and widespread alteration of basement rocks. The basement rocks have been fractured, broken and catalazited at various degrees, developing different fracturing systems with different directions. The fracturing and breaking did not change the rock composition, but they strongly altered the structure, texture and particularly the petrophysical characteristics of the basement rocks.
The petrophysics characteristics of altered basement rocks in the White Tiger (Bach Ho) field change very strongly both with depth and area. Permeability ranges from less than 1mD to hundreds mD. Two principal porosity types that are fractured/micro-fractured and cavernous/micro-cavernous pores can always be observed in the altered granitoid rocks (figure 1).