Optimizing Pressure Differential to Improve the Recovery of Viscous Oil
Authors: Zhao Chunming , Lizhen Ge , Su Yanchun , Fang Liwen , Yang QingHong , Li Tingli 
Published: May 2011

Abstract
Using big pump to enhanced liquid is a important workover in the developing of the complicated fluvial facies viscosity reservoirs, especially in medium-high water cut period, the reasonable magnitude of enhanced liquid and the regularity of increasing oil production are probed for different type reservoirs by numerical simulations. Based on this study, some wells are selected to do field tests, which have resulted in good effects of increasing oil production, providing a theoretical guidance and abundant experiences for effectively developing offshore heavy oilfields with different types of reservoirs.
On the base of the numerical simulation and the actual field tests, others oil field with bottom water / edge water / injection water/rigid water can use the pressure differential optimization conclusions to improve the oil production.
A number of medium-high water cut wells were optimized to enhanced liquid in QHD32-6 field after identifying the using big pump conditions. These wells display significant higher oil production with big pump and slower increase of water cut. Now the success fate is 92%, the incremental oil rate from 10m3 to 65m3, the cumulative oil is 15×104m3.
The simulation and the field test verified the pressure differential is different in different oil type reservoir, for QHD 32-6 oil field, the pressure differential is about 2MPa in injection water reservoirs, 2.5MPa in edge water reservoirs, 2.5MPa to 3MPa in bottom water reservoir, and possible pressure differential under capable disposal in rigid water reservoirs.
Both simulations and field data demonstrate that the medium-high water cut wells can improve the increasing oil by applicable big pump under the pressure differential optimization, the OOIP, the energy, the oil viscosity and the heterogeneity are also have great impact on the pressure differentia optimization. Now the QHD32-6 oil field big pump workover have significantly improved oil production, which provides the theoretical and production data for development optimization of different oil type reservoirs in Bohai Bay.

1 Introduction
QHD32-6 [1] oil field is located in the center area of the Bohai Bay, with 20m average water depth. It is a joint venture between China National Offshore Oil Company (CNOOC) and Chevron. The Oil Field geographically is divided into 3 areas: North, South, and West area. The main producing formations are Ming (Nm) and Guantao (Ng). Nm formation is fluvial depositional reservoirs with meandering channels, multiple sand systems and complex oil/water systems, while Ng is a fluvial sand deposition with braided channels and strong bottom water.
Fluid type is typical heavy oil characterized by the following properties. In the reservoir condition oil has high density (0.903-0.926 g/cm3), and high viscosity (43-260mpa.s), and low soluble gas/oil ratio (13-24m3/m3). Reservoir porosity of Nm is 0.25-0.45, and permeability of Nm is 100-11487× 10-3µm2, and porosity of Ng is 0.25-0.43, and permeability of Ng is 500-18443×10-3µm2. The difference of formation pressure and bubble pressure is about 2-5MPa. There are complex oil water contacts in the reservoir.
QHD32-6 Oil Field was put into the production in 2001-2002 by stages. The average water cut of the whole field is 81% by the end of 2008, with average 7.6% of recovery factor. Now the field has edge water and injects water, and some zones have bottom water problems. How to get good production in medium-high water cut period is very important, in fact, there are many workover that we can do, including water flooding, infill wells, water-shut off and so on, except for those methods.