Improve Offshore Heavy Oil Recovery by Compound Stimulation Technology Involved Thermal, Gas and Chemical Methods

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.

The Effects of Multi-component Thermal Fluids on the Viscosity of Offshore Heavy Oil

“…By injecting a compound fluid of steam-flue gas or CO 2 /N 2 -chemical agents, multi-component thermal fluids stimulation technology can improve the recovery of heavy oil efficiently (Hornbrook et al 1991;Liu et al, 2010;Sun et al, 2011;Tang et al, 2011;Gu et al, 2012;Jiang et al, 2012). Because the needed equipments and apparatus have small volume and can be handled easily, multicomponent thermal fluids stimulation technology are a feasible way to recover offshore heavy oil reservoirs.…”

Multi-thermal fluid assisted gravity drainage process: A new improved-oil-recovery technique for thick heavy oil reservoir

“…1 cycle N 2 injecting volume is 21 Â 10 4 m 3 ; the cycle CO 2 injecting volume is 3.7 Â 10 4 m 3 ; the cycle steam injecting volume is 480 t. After operation, the average water cut of this well is reduced by about 27.2%, and the oil daily rate is increased from 2.8 t to 10.1 t. The cumulative oil increment is about 1009 t. Considering the successful operation of multi-thermal fluid stimulation technique in this well, two well groups in C20 block of Shengli oilfield have been operated as the new testing sites. In 2010, multi-thermal fluid stimulation technique is also introduced into the development of NB35-2S heavy oil block of Bohai offshore oilfield in China (Liu et al, 2010(Liu et al, , 2011Sun et al, 2011;Yu et al 2014), as shown in Fig. 1.…”

Roles of Flue Gas in Promoting Steam Flow and Heat Transfer in Multithermal Fluid Flooding

“…At present, the multithermal fluid technology has been applied in oilfield. In Bohai Oilfield, China, due to the limitation of the offshore platform area, the conventional technology based on steam injection is unsuitable and the multithermal fluid technology was introduced [11][12][13]. The field results show high oil recovery and good economic benefit.…”