2.1. Rock pressure and effective pressure.

Let’s consider the layer, located at depths H. The mass of the overlying rocks pressures on the layer. It means that the layer is in deformed, compressed condition.

Let the average density of the overlying rocks be - ρ_г , average reservoir pressure is - p₀. If we approximate the layer plate thickness h, lying on elastic foundation (bottom boundary), rock pressure, caused by the weight of the overlying rocks, can be identified with evenly distributed load _г acting on the layer, figure 2.1.

Fig.2.1. Rock pressure- _г, average reservoir pressure - p₀,

H – producing depth, h – net pay.

Rock pressure is determined by the formula

(2.1)

Reservoir, pore pressure is determined by the ratio

(2.2)

where ρ_в – water density, g – acceleration of gravity.

Effective reservoir pressure is determined by the ratio

(2.3)

Let H=2500m , g=9,8m/s², ρ_г =2400kg/m³ , ρ_в =1000kg/m³.

The values of rock, reservoir and effective pressure will be equal to:

_г=58,8MPa, p₀=24,5 MPа, _эф=33,3MPа.

Thus, the oil stratum is compressed by rock pressure, the value of which depends on the depth and density of the overlying rocks. Effective pressure, that is perceived by the particles of hard rock of porous reservoir, depends on the rock pressure and reservoir pressure. During the process of wells’ operation the pressure in the bottom-hole formation zone is reduced, the effective pressure is increased, that can lead to the formation of micro-cracks, the reservoir destruction, mechanical impurities withdrawal and changing of pool-reservoir properties of the bottom-hole formation zone, and consequently, reduction of well efficiency.

Thus, in natural, original position the stratum is compressed, deformed, and its pool-reservoir properties depend on the stress - strain state.

Due to various geological features of the formation and bedding of deposits, there can be observed an abnormally low (ALRP) and an abnormally high (AHRP) reservoir pressure.

Dense non-reservoirs take a part of the load of the overlying rocks in the fields with abnormally low reservoir pressure. In this case reservoir pressure is determined directly by measuring, rock pressure must be determined considering the difficult conditions of occurrence. The load of the overlying rocks is not determined by the formula (2.1) but as for multi-layer plate with different thicknesses of the layers and the stress distribution between the layers.

In deposits with abnormally high reservoir pressure, the pressure is much more than it is determined by the formula (2.2). So at the Salym field reservoir pressure of the Bazhenov formation deposit, stratum U₀, equals 46-48 MPa, the producing depth is 2700-2800m. Effective pressure in this case is equal to 16.5 MPa.

Therefore, the geological conditions of deposit accumulation, its structure, determine the stress - deformed state of the reservoir and influence upon the choice of the future development system and ways of wells’ operation.

2.2. Reservoir energy types.

There are two types of reservoir energy. They are potential and energy of elastic deformation [3].

The potential energy of the position is determined by the known classical physics formula

Е_п = МgH, M = V_ф_ф, Е_п = V_ф_фgH = V_ф р,

where M is the mass of the oil, water or gas reservoir; g is the acceleration of gravity; V_ф - the volume of fluid in the reservoir; ρ_ф is the density of the fluid; H is producing depth; p - pore pressure. Thus, the potential energy depends on the volume of fluid and reservoir pressure.

Energy of elastic deformation according to Hooke's law is defined as

Е_д = Fl,

Where F - the force that causes deformation, l – linear deformation; the force equals :

F = рS, where р – pressure, S – area; deformation energy will be equal Е_д = рS l,

Neglecting areal changes in volume, and assuming that the change in volume is associated with a linear deformation ratio:

V=Sl, we will have the following

Е_д = рV,

Where V -volume increment of porous layer containing fluids. If we don’t take into consideration the compressibility of particles of solid phase then we will have the following: ΔV=ΔV_пор where ΔV_пор – pores’ volume change. Compressibility factor, elastic reservoir capacity β* is defined as [4]

β^* - elastic reservoir capacity factor,

Е_д = ^*Vpp. (2.4)

Thus, the deformation energy depends on the pressure, the pressure changes, depression Δp, the volume of the reservoir and elastic reservoir capacity factor, describing the elastic reservoir properties. The elastic reservoir capacity factor describes the elastic properties of the deformed, compressed fluid-rich layer.