An oil and gas reservoir is the oil and gas accumulation in an independent trap with a single pressure system and the unitary gas–oil interface and oil–water interface. In the light of development geology, an oil and gas reservoir has its geometric configuration and boundary conditions, storage and flow characteristics, and fluid properties. >> Oil & Gas Books Store <<
Classification of Reservoirs on the Basis of Storage and Flow Characteristics of Reservoirs.
In accordance with different storage spaces and main flow channels of formation fluids, oil and gas reservoirs can be divided into the following types, shown in Table 1-1.
1. Porous reservoir. The storage space and percolation channels are the intergranular pores.Therefore, the flow is called flow in porous media such as sandstone, conglomerate, bioclastic limestone, and oolitic limestone reservoirs.
2. Fractured reservoir. Natural fractures are not only the main storage space, but also the flow channels. There may be no primary pores or be disconnected pores. Generally, the fracture porosity is not more than 6%. Tight carbonatite and metamorphic rock reservoirs, and mud shale gas reservoirs, are of this type of reservoir.
3. Fracture porosity reservoir. Intergranular pores are the main storage space, whereas fractures are the main flow channels. The flow is called flow in dual-porosity single-permeability media. Usually, the fractures extend for a long distance, but the pore permeability is very low. The Renqiu carbonatite oil field of China and the Spraberry Trend oil field of the United States are of this type of reservoir.
4. Porous fractured reservoir. Both intergranular pores and fractures are the storage space and the flow channels. The flow is called flow in dual-porosity dual-permeability media. Fractures grow fine, but extend for a short distance. The matrix porosity is low.
5. Combined fracture–vug–pore reservoir. Fractures, vugs, and pores are both the storage space and the flow channels. All reservoirs of this type are soluble salinastone. The secondary pores are the main. This type of reservoir is also called a triple-porosity reservoir.
Classification of Reservoirs on the Basis of Reservoir Geometry.
Reservoirs can be divided into massive, stratified, fault block, and lenticular reservoirs in accordance with the geometry.
1. Massive reservoir. The reservoir has a large effective thickness (more than 10 m). The oil reservoir may have a gas cap and bottom water while the gas reservoir may have bottom water. The reservoir has a unitized hydrodynamic system and good connectivity. The bottom water has rechargeablity. When choosing completion modes, whether gas cap and bottom water exist for oil reservoir and whether bottom water is active for gas reservoir should be considered. Generally, perforating or open hole completion is selected.
2. Stratified reservoir. Most reservoirs of this type belong to an anticlinal trap with a complete structure and a unitized oil–water interface. It has good stratification and a number of beds along a vertical section. Every individual bed has a small thickness. A bed with a thickness of 5–10, 1–5, and less than 1 m is called thick, medium thick, and thin, respectively. These beds differ greatly in permeability. The drive energy of edge water is weaker. Separate zone water injection, separate zone fracturing, and separate zone water shutoff should be applied during waterflooding in order to control injection and production profiles and increase flooding efficiency, thus the perforated completion mode is generally adopted. Reservoirs in the Daqing Shaertu, Shengli Shengtuo, and Changqing Malin oil fields in China are of this type of reservoir.
3. Fault block reservoir. Fractures well developed in reservoirs of this type. The structure is cut into many fault blocks of different sizes. The areas of some fault blocks are less than 0.5 km2. Vertically, there are many oil-bearing series of strata with a long total oil-bearing section. In each fault block or even various reservoir groups in the same fault block, different oil–water interfaces and obvious differences in degrees of oil and gas enrichment, physical property of reservoir, and natural drive energy exist. For a seal-type fault block, elastic energy is used in the early stage, whereas selective scattered flooding is used in the late stage. Peripheral or outer edge waterflooding is applicable to the open-type fault block. Generally, the perforated completion mode is adopted for this type of reservoir due to their numerous oil-bearing series of strata and the great interlayer differences. The Shengli Dongxin and Zhongyuan Wenminzhai oil fields are of this type of reservoir.
4. Lenticular reservoir. The geologic description of geometry for a sand body depends on the length-to-width ratio. A sand body with a ratio less than or equal to 3 is called lens. Lenses are scattered, and the major area is occupied by the pinchout region. When lenses alternately overlap each other, multiple reservoirs may appear on the vertical oil- and gas-bearing well section.
Classification of Reservoirs on the Basis of Properties of Reservoir Fluids. Reservoirs can be divided into gas, condensate gas, volatile oil, conventional oil, high pour-point oil, and heavy oil reservoirs in accordance with the properties of reservoir fluids.
Classification of Reservoirs on the Basis of Main Characteristics of Development Geology. The continental reservoirs in China are divided intomultilayer sandstone oil reservoirs, gas cap sandstone oil reservoir, low-permeability sandstone oil reservoirs, complex fault-block sandstone oil reservoirs, glutenite oil reservoirs, fractured buried-hill basement rock reservoirs, conventional heavy oil reservoirs, high pour-point oil reservoirs, and condensate gas reservoirs in accordance with the main characteristics of development geology and the development mode of the continental reservoirs in China.