1.2 Pool-reservoir properties.
Reservoirs are rocks, capable to contain and to give fluids under the conditions of pressure gradient (with the appliance of modern technologies). The reservoirs are characterized by filtration and capacitive properties.
Capacitive properties of terrigenous rocks are characterized by the porosity factor. An existence of porosity means the presence of pores (interstices), which contain fluids (gas, oil, produced water).
The total porosity coefficient is determined by the ratio
Here Vобр - sample volume, Vпор - the volume of the pore space, Vзак - the volume of closed pores, Vот - volume of open pores.
The open porosity coefficient is defined as
Dynamic (effective) porosity:
,
где Vпордин
–pore volume with a moving fluid
Thus, the definition shows
mдин ≤ mот ≤ m.
Capacitive properties of fractured rocks are characterized by the crack formation factor, defined as the ratio of the cracks volume to the sample volume. If the reservoir is porous-fractured then its capacitive properties are characterized by the coefficient sum of porosity and fracturing.
The coefficients of saturation are determined by the ratio of pore volume occupied by the fluid, to the volume of open pores:
-
oil
saturation factor
-
water saturation factor
For two-phase medium the following ratio is run:
Vн - pore volume, filled with oil, Vв - pore volume, filled with water.
Similarly, we can consider the presence of three phases in the pore space of a layer, they are water, oil and gas [1].
Permeability is the rocks’ property to pass fluids through under the condition of pressure gradient. Permeability is a filtration parameter.
There are various kinds of filtration flows in the formation conditions that can be met during the process of deposits’ development. They are separate movement of oil, water, gas or their collaborative filtration: two or three phases. Depending on the quantitative and qualitative composition of the phases, permeability of the porous medium can be different. Therefore, to characterize the reservoir filtration capacity there are introduced the notions of absolute, phase and relative permeability.
Absolute permeability (kабс) is the filtration ability of rock (core) for inert gases (air, nitrogen). It is believed that the inert gas molecules do not interact with particles of rock.
Phase permeability is the permeability of the reservoir for the given phase of oil kн, water kв, gas kг when there are some other phases in the porous space, whether they are dynamic or static.
Relative permeability is determined by the ratio of phase permeability to the absolute:
,
,
0≤kфаз*
≤1
The dimension of the permeability coefficients
k = m2, µm 2, D, mD; 1 µm 2 = 10-12 m2 1 D =10-3 mD.
Relative and phase permeability depends on water saturation factor.
1.3. Heterogeneity and anisotropy of reservoirs
Oil fields as natural objects have various characteristics. Oil can be in sandstones, siltstones with intergranular porosity, and also in limestones, dolomites, clay rocks with cracks and cavities.
The difference of pool-reservoir properties of the separate areas of a layer is called lithologic heterogeneity.
Fracturing and lithologic heterogeneity of reservoir rocks have a significant impact on the processes of oil and gas extraction.
Under the anisotropy of the reservoir it is considered the diversity of physical properties in three mutually perpendicular directions.
For homogeneous anisotropic layer there is introduced the concept of the anisotropy coefficient
Where - kг = kх = ky - permeability coefficients along stratification zone;
kв = kz - permeability coefficient perpendicular to the stratification zone [2].
CHAPTER 2. NATURAL MODES DEVELOPMENT OF DEPOSITS.
The development of oil fields usually starts on the natural modes, without stimulation, as injection wells are not built yet, water injection is not happening. For oil extraction, its inflow to the bottoms of producing wells, the energy of the reservoir is used. Before talking about the types of reservoir energy, it is necessary to observe natural condition of a reservoir before its initial drilling-in.