Improve Offshore Heavy Oil Recovery by Compound Stimulation Technology Involved Thermal, Gas and Chemical Methods
Authors: Liu Yigang , Yang Hailin , Zhao Lichang , Sun Yongtao , Cui Gang , Zhao Mingguo , Hu Shaobin , Zhong Liguo 
Published: July 2010

Abstract
Heavy oil resource is three times of conventional oil in Bohai oilfield - the largest offshore oilfield in China. The heavy oil reservoirs are deposited at depth from 900m to 1900m with high permeability (up to 5000mD), large thickness (5 to 30 meters), conventional heavy oil viscosity(usually below 2000 mPa·s under reservoir conditions). Some heavy oil reservoirs are produced by electrical submersible pumps (ESP) in past years, and very low oil production is obtained after short considerable oil production peak at the beginning. In summary, most of the heavy oil reservoirs are not effectively produced. The main reason is that effective thermal production technology such as steam injection is not practical under offshore production conditions, for example, conventional steam generator is too weight and large to lift and install onto offshore production platform.
A new effective compound stimulation technology is developed to produce offshore heavy oil which involves thermal, gas and chemical methods. First of all, a portable and convenient gas generator is established to produce high temperature and high pressure steam, CO2 and N2 on production platform using diesel oil or light oil as fuel. The temperature of produced steam could be regulated, usually 150? to 300?, and the pressure is up to 20MPa. The exhausted CO2 and N2 could be injected with steam together, sequentially, or separately (flue gas is injected through annulus between casing and tubing, stesm through tubing). The principles of such compound technology to enhance heavy oil recovery include reduction in oil viscosity by heating and gas solution, enlargement of heat sweeping efficiency by coinjected gas, and expansion of heated porous rock and saturated fluid. At the same time, CO2 and N2 could decrease heat loss of steam to some extent. Alternately, some surfactant could be injected to increase sweep efficiency by controlling injected fluid mobility.
The compound stimulation has been successfully applied to two offshore horizontal wells in a typical heavy oil reservoir. As a result, the gas generator is operated soundly, the engineering design and related implement techniques feasible, and oil production increased obviously (over 2 times than the oil production rate before treatment). Primarily, it shows that compound stimulation technology combined thermal, gas and chemical methods is feasible and potential for offshore heavy oil production.
Preliminary results of laboratory experiment, numerical simulation, EOR principles and field application of the compound stimulation technology involved thermal, gas and chemical methods are presented in this paper.

Introduction
There are abundant heavy oil resource in Bohai Oilfield, proven reserve is up to 4 billion tons, recoverable reserve is about 0.8 billion tons if 20% oil recovery were obtained. Most of the heavy oil reservoirs are conventional for lower heavy oil viscosity (usually below 2000 mPa·s under reservoir conditions), and typical blocks of heavy oil reservoirs include Lvda, Nanpu (NB), Suizhong, Chengbei, and so on. Properties of these typical reservoirs are summarized in Table 1. Most heavy oils are usually mobile under these reservoir conditions and most can be produced cold production methods such as electrical submersible pumps (ESP) and progressing cavity pump (PCP) in past years, but limited primary recovery is obtained (lower than 5%~10% OOIP), and oil production after short considerable oil production peak at the beginning is much lower than expected. In summary, most of the heavy oil reservoirs are not effectively produced. It is challenging to recover sufficient heavy oil at economical rate within limited lifetime of the production platform.