A Successful Story of Integration Geological Characterization, Reservoir Simulation, Assisted History Matching and EOR in a Giant Fractured Granite Basement: A Road Map to Maximize Recovery in Unconventional Reservoirs
Authors: Dang Cuong Thanh Quy , Chen Zhangxing , Nguyen Ngoc Thi Bich , Bae Wisup , Phung Thuoc H. , Dong Chao Charlie 
Published: May 2011

Abstract
Near shore oil reservoirs have become significantly depleted, forcing oil companies to explore the unconventional reservoirs with huge investments and the latest technology. Among them, naturally fractured reservoirs (NFR) are found throughout the world and contain significant amount of oil reserves. Because of the unique of a NFR, there are lots of uncertainties in exploration and field development of these complex reservoirs.
White Tiger is the biggest fractured basement reservoir up to now on the continental shelf of Viet Nam and even the world. This reservoir has a very complicated geological structure; high temperature (more than 2840F) and closure stress (more than 6,000 psi). The total OIIP of this field reached nearly 4 billion barrels with 6,561 feet of the oil bearing thickness in granite basement and has been produced by more than 200 wells, with production rate of 180,000bbls/d. Due to the challenges of geology, these are essential to have successful operations as well as reducing uncertainties and improving the efficiency of oil field management. With a large database collected from initial production stages, the authors developed an integrated static and dynamic workflow to forecast oil production under several production scenarios for this reservoir. A successful assisted automatic history matching approach was proposed by combining global and local optimization which could be effectively accelerate the convergent problem and minimize computation mass of inverse problem, to construct a reliable model for complex reservoirs. Then, a series of compositional reservoir model were performed and analyzed by CMGTM simulator in order to evaluate the possibility of polymer flooding in fractured basement reservoir. The results showed polymer flooding would become an excellent candidate for enhanced oil recovery. Additionally, polymer retention phenomenon by adsorption on the rock surface and precipitation by high salinity was deeply investigated. An optimum range of important factors were determined to reduce the effect of chemical adsorption, it helps minimize mass of chemical loss and improve economic efficiency of chemical flooding process.

Introduction
A reservoir fracture is a naturally occurring macroscopic planar discontinuity in rock due to deformation or physical diagenesis (Nelson, 1985). A great portion of the world's oil reserves is contained in naturally fractured reservoirs (NFR). In NFRs, fluids exist in two interconnected systems: the rock matrix, which usually provides the bulk of the reservoir volume, and the highly permeable rock fractures. Fractured basement reservoir is a special type of NFR which is commonly very thick and the distribution of porosity and permeability is irregular. In the past, fractured basement reservoirs were often considered uneconomic. Recently, due to the increasing knowledge of basement plays and the demonstration of successful cases around the world, fractured basement reservoirs are becoming increasing attractive for exploration. Although oil production from basement rocks is not a common occurrence worldwide; but, there is significant oil production from such reservoirs in a number of countries