Numerical Simulation Study of Water Injection Development in an Extra-low-permeability Fractured Reservoir, Xiaoguai Oilfield
Authors: Qian Genbao , Jiang Xiaohui , Zhang Hongmei 
Published: December 2007

Abstract
The Xiazijie formation oil pool in Xiaoguai oilfield is a low-porosity, extra-low-permeability, fractured conglomerate reservoir. Based on 3-D detailed geological reservoir description and production performance data, the authors investigate the factors influencing the production of the oil pool with CMG's black oil model (IMEX). Variation and sensitivity of such factors as IPR, location of water injection, production rate, well pattern, direction of well array in relation to fracture orientation and recovery method (solution gas drive vs. Water injection) are analyzed.

Introduction
Xiaoguai oilfield is located at about 35Km to the south of Karamay, Xinjiang, which lies on the northwestern slop belt of Changji sag between Chepaizi uplift and Zhongguai uplift in northwestern margin of Junggar basin. The target zone Xiazijie Formation is a series which trend north to south, sedimentary thickness average 600~700m. The reservoir lithology is dominated by medium-conglomerate then by arenaceous seriate fine conglomerate, pabbly sandstone and cobblestone. The pore types are mainly composed of micro-fracture, boundary pore, inter-granular dissolved pore and inter-crystalline pore of inter-granular laumontite. Average porosity and permeability are 5.57% and 0.16×10-3µm2, respectively. Reservoir has developed by natural fractures and dominated by the high-angle, right angle of structural genesis, the fracture strike is near EW direction. The fracture porosity and permeability are 0.058% and 36.72×10-3µm2, respectively. This reservoir is a typical low-pore, extra-low-permeability fractured pool and oil saturated with reservoir middle part temperature of 85.5°C, original formation press are of 39.72 MPa, formation press are coefficient of 1.13 and no active edge-bottom water. Since 1996, 154 development wells have been drilled and cumulative oil production is 48.5×10 4t. The main problem during the development is the formation pressure falling fast and the well production declining fast by natural solution gas drive process. Therefore, for this kind of low-pore, extreme-low-permeability fractured reservoir, using what kind of secondary oil recovery process to supplement formation energy is a new subject for oilfield recovery.

Model Development and Parameter Preparation
Based on the depositional facies belt, reservoir and productivity distribution, G2080 well group that has perfect well pattern and long production history is selected to simulate. The main purposes of study are evaluating the water-flooding effect and predicting production indexes in different recovery processes. Therefore, we selected the black oil simulation software (IMEX) which is made in CMG company Canada, to simulate the 3D 3phases and multitypes of fracture systems.

Geologic model development.
This model consists of two parts: fracture and matrix, its heterogeneity reflects two aspects: one is heterogeneity between the systems of fracture and matrix, the other is one fracture system. For these reasons, Geotools Software is used to develop the combinative model which is composed of fractures and rocks, and it has basically reflected the heterogeneity distribution of fracture and rocks on 3D space.

Grid division.
Grid division should be considered to the following several aspects:Simulation modelcontend is needed,Well site should be as on center of grid as possible,The accuracy of calculation result,The direction of reservoir major fractures and plane grid is the same as X, andCalculation speed.

Geologic model development.
This model consists of two parts: fracture and matrix, its heterogeneity reflects two aspects: one is heterogeneity between the systems of fracture and matrix, the other is one fracture system. For these reasons, Geotools Software is used to develop the combinative model which is composed of fractures and rocks, and it has basically reflected the heterogeneity distribution of fracture and rocks on 3D space.

Grid division.
Grid division should be considered to the following several aspects:Simulation modelcontend is needed,Well site should be as on center of grid as possible,The accuracy of calculation result,The direction of reservoir major fractures and plane grid is the same as X, andCalculation speed.

Grid division.
Grid division should be considered to the following several aspects:Simulation modelcontend is needed,Well site should be as on center of grid as possible,The accuracy of calculation result,The direction of reservoir major fractures and plane grid is the same as X, andCalculation speed.