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Stress sensitivity is a key factor affecting the productivity of single wells in low permeability gas reservoirs. A well test model for heterogeneous composite gas reservoirs under the influence of stress-sensitive effects was established. Based on the theoretical model, the well test was designed by gradually increasing the pressure difference. The relationship between abnormal high pressure and reservoir stress sensitivity was analyzed. Theoretical research shows that stress sensitivity will cause permeability damage during the production process, and the pressure drop test curve shows that the physical properties of the reservoir have gradually deteriorated. The pressure recovery test curve shows that the physical properties of the reservoir are getting better. Field practice shows that stress sensitivity is related to the formation of abnormally high pressure in the formation without considering the micro-cracks in the formation. Stress-sensitive reservoirs are generally unbalanced and compacted due to overpressure, for fluid expansion/conduction overpressure in Ledong Area. For these reservoirs, there is almost no stress sensitivity. The research results have significance for guiding the design and data interpretation of stress-sensitive reservoir.

In view of the development of an ultra-high pressure gas reservoir, the stress sensitivity of the reservoir is the most discussed in China [

According to the pressure dynamic characteristics of a well in the center of a circular composite gas reservoir, the corresponding physical model is established, as shown in

There is a well in the center of the composite gas reservoir with fixed production, the radius of the inner boundary is R1, and the outer boundary is infinite; 2) The reservoir is of equal thickness, with good interlayer at the upper and lower parts; under the original conditions, the formation pressure is evenly distributed, and the internal and external regions have different physical properties; 3) The reservoir rock is slightly compressible, and the formation permeability changes with the change of effective stress; 4) The flow of fluid in the formation is isothermal; 5) The wellbore storage effect and skin factor are considered; 6) The gravity and capillary force are ignored.

Considering the influence of stress sensitivity, the seepage differential equation of composite gas reservoir can be obtained from the motion equation, the continuity equation and the state equation of natural gas [

Seepage equation in inner region:

1 r ∂ ∂ r [ r p 1 M Z R T k 01 e − γ ( p i − p 1 ) μ 1 ( ∂ p 1 ∂ r ) ] = ϕ 1 ∂ ∂ t ( p 1 M Z R T ) (1)

where r is the radial coordinate, m, Z is deviation factor of gas, R is perfect gas constant, T is temperature of gas, K, k_{01} is permeability of inner region, μm^{2}, M is molar mass of gas, g/mol, p_{1} is the instantaneous formation pressure of inner region, MPa, p_{i} is primary formation pressure, MPa, γ is the stress sensitivity index, MPa^{2}/(mPa·s), μ_{1} is gas viscosity of inner region, mPa·s.

Seepage equation of outer zone:

1 r ∂ ∂ r [ r p 2 M Z R T k 02 e − γ ( p i − p 2 ) μ 2 ( ∂ p 2 ∂ r ) ] = ϕ 2 ∂ ∂ t ( p 2 M Z R T ) (2)

where k_{02} is permeability of outer region, μm^{2}, p_{2} is the instantaneous formation pressure of outer region, MPa, μ_{2} is gas viscosity of outer region, mPa·s.

The definition of pseudo pressure is introduced:

ψ = 2 ∫ p 0 p p μ Z d p (3)

where p is formation pressure, MPa, p_{0} is initial pressure, MPa, μ is gas viscosity, mPa·s.

By introducing Equation (3) into Equation (1) and Equation (2), the comprehensive seepage differential equation of composite gas reservoir considering stress sensitivity can be obtained.

Differential equation of seepage in inner region:

1 r ∂ ψ 1 ∂ r + ∂ 2 ψ 1 ∂ r 2 + γ * ( ∂ ψ 1 ∂ r ) 2 = ϕ 1 μ 1 C 2 ( p ) k 01 e γ * ( ψ i − ψ 1 ) ∂ ψ 1 ∂ t (4)

where ψ_{1} is pseudo pressure of inner region, C_{2} is outer region isothermal compression coefficient of gas, MPa^{−}^{1}, γ* is the stress sensitivity index of pseudo pressure.

Differential equation of seepage in outer region:

1 r ∂ ψ 2 ∂ r + ∂ 2 ψ 2 ∂ r 2 + γ * ( ∂ ψ 2 ∂ r ) 2 = ϕ 2 μ 2 C 2 ( p ) k 02 e γ * ( ψ i − ψ 2 ) ∂ ψ 2 ∂ t (5)

where ψ_{2} is pseudo pressure of outer region.

The initial condition is:

ψ 1 ( r , t = 0 ) = ψ 2 ( r , t = 0 ) = ψ i (6)

The inner boundary condition is:

[ r e − γ * ( ψ i − ψ 1 ) ∂ ψ 1 ∂ r ] | r = r we = q sc p sc T π k 01 h T sc Z sc + C μ 2π k 01 h ∂ ψ 1 ∂ t (7)

where, r we = r w e − S is the effective well diameter, ψ_{1} is initial pseudo pressure, q_{sc} is pressure in standard conditions, MPa, T_{sc} is temperature in standard conditions, K, Z_{sc} is deviation factor in standard conditions, h is thickness of reservoir, m.

Set connection conditions between inner and outer areas:

k 01 e − γ * ( ψ i − ψ 1 ) μ 1 ∂ ψ 1 ∂ r | r = r 1 = k 02 e − γ * ( ψ i − ψ 2 ) μ 2 ∂ ψ 2 ∂ r | r = r 1 (8)

where r_{1} is radius of inner region, m.

ψ 1 ( r 1 , t ) = ψ 2 ( r 1 , t ) (9)

The outer boundary condition is:

ψ 2 ( r → ∞ , t ) = ψ i (10)

The mathematical model of well test interpretation for composite gas reservoir with deformed medium consists of Equations (4)-(10).

The dimensionless well test model of deformable medium composite gas reservoir is obtained by dimensionless treatment:

{ 1 r D ∂ ψ 1 D ∂ r D + ∂ 2 ψ 1 D ∂ r D 2 − γ D * ( ∂ ψ 1 D ∂ r D ) 2 = e γ D * ψ 1 D e 2 S ∂ ψ 1 D ∂ t D 1 r D ∂ ψ 2 D ∂ r D + ∂ 2 ψ 2 D ∂ r D 2 − γ D * ( ∂ ψ 2D ∂ r D ) 2 = ω M e γ D * ψ 2 D e 2 S ∂ ψ 2 D ∂ t D ψ 1 D ( r D , t D ) | t D = 0 = ψ 2 D ( r D , t D ) | t D = 0 = 0 r D e − γ D * ψ 1 D ∂ ψ 1 D ∂ r D | r D = 1 = − 1 + C D ∂ ψ 1 D ∂ t D ∂ ψ 1 D ∂ r D | r = r 1 D = 1 M ∂ ψ 2 D ∂ r D | r = r 1 D ψ 1 D ( r 1D , t D ) = ψ 2 D ( r 1D , t D ) lim r D → ∞ ψ 2 D ( r D , t D ) = 0 (11)

The definition of dimensionless variables is shown in

Variables | Expression | Variables | Expression |
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Dimensionless internal formation pressure | ψ 1D = π k 01 h T sc q sc p sc T ( ψ i − ψ 1 ) | Dimensionless outer zone formation pressure | ψ 2 D = π k 01 h T sc q sc p sc T ( ψ i − ψ 2 ) |

Dimensionless bottom pressure | ψ wD = π k 01 h T sc q sc p sc T ( ψ i − ψ w ) | Dimensionless time | t D = k 01 ϕ 1 μ 1 C t 1 r w 2 t |

Dimensionless radius | r D = r r we = r w e − S , r 1D = r 1 r we | Mobility ratio | M = k 01 / μ 1 k 02 / μ 2 |

Storage capacity ratio | ω = ϕ 1 C t 1 ϕ 2 C t2 | Dimensionless stress sensitivity coefficient | γ D * = γ μ i Z i 2 p i q sc p sc T π k 0 h T sc |

Dimensionless wellbore survival factor | C D = C 2 π h ϕ 0 C t r w 2 |

where subscript D is dimensionless variable.

In order to solve the system of Equations (11), firstly, let:

ψ 1 D = − 1 γ D * ln [ 1 − γ D * ξ 1 D ( r D , t D ) ] (12)

ψ 2 D = − 1 γ D * ln [ 1 − γ D * ξ 2 D ( r D , t D ) ] (13)

In addition, the following perturbation transformation is used:

ξ D = ξ 0 D + γ D * ξ firstD + ( γ D * ) 2 ξ secondD + ⋯ (14)

− 1 γ D * ln [ 1 − γ D * ξ D ] = ξ D + 1 2 γ D ξ D 2 + ⋯ (15)

1 1 − γ D * ξ D = 1 + γ D * ξ D + ( γ D * ξ D ) 2 + ⋯ (16)

Considering that the dimensionless stress sensitivity coefficient is generally small, as long as the zero solution is taken into account, the perturbation solution can satisfy the solution accuracy.

{ 1 r D ∂ ξ 1D ∂ r D + ∂ 2 ξ 1 D ∂ r D 2 = e − 2 S ∂ ξ 1 D ∂ t D 1 r D ∂ ξ 2 D ∂ r D + ∂ 2 ξ 2 D ∂ r D 2 = ω 12 M e − 2 S ∂ ξ 2 D ∂ t D r D ∂ ξ 1 D ∂ r D = − 1 + C D ∂ ξ 1D ∂ t D ξ 1 D ( r D , t D ) | t D = 0 = ξ 2 D ( r D , t D ) | t D = 0 = 0 ∂ ξ 1 D ∂ r D | r = r 1 D = 1 M ∂ ξ 2 D ∂ r D | r = r 1D ξ 1 D ( r 1 D , t D ) = ξ 2 D ( r 1D , t D ) lim r D → ∞ ξ 2 D ( r D , t D ) = 0 (17)

By Laplace transformation of the Equation (15), we can get:

{ 1 r D ∂ ξ ¯ 1D ∂ r D + ∂ 2 ξ ¯ 1 D ∂ r D 2 = u e − 2 S ξ ¯ 1 D 1 r D ∂ ξ ¯ 2 D ∂ r D + ∂ 2 ξ ¯ 2 D ∂ r D 2 = u ω 12 M e − 2 S ξ ¯ 2 D r D ∂ ξ ¯ 1 D ∂ r D = − 1 u + u C D ξ 1D ∂ ξ ¯ 1 D ∂ r D | r = r 1D = 1 M ∂ ξ ¯ 2D ∂ r D | r = r 1D ξ ¯ 1 D ( r 1 D , t D ) = ξ ¯ 2 D ( r 1D , t D ) lim r D → ∞ ξ ¯ 2 D ( r D , u ) = 0 (18)

The solution of Equation (16) is as follows:

{ 1 r D ∂ ξ ¯ 1D ∂ r D + ∂ 2 ξ ¯ 1 D ∂ r D 2 = u e − 2 S ξ ¯ 1 D 1 r D ∂ ξ ¯ 2 D ∂ r D + ∂ 2 ξ ¯ 2 D ∂ r D 2 = u ω 12 M e − 2 S ξ ¯ 2 D r D ∂ ξ ¯ 1 D ∂ r D = − 1 u + u C D ξ 1D ∂ ξ ¯ 1 D ∂ r D | r = r 1D = 1 M ∂ ξ ¯ 2D ∂ r D | r = r 1D ξ ¯ 1 D ( r 1 D , t D ) = ξ ¯ 2 D ( r 1D , t D ) lim r D → ∞ ξ ¯ 2 D ( r D , u ) = 0 (19)

In Equation (19), M TP = u f I ( u ) f KI 1 ( u ) − F f KI0 ( u ) + F .

where, F = f KI0 ( r 1 D u ) + f KI1 ( r 1D u ) R TP R TP − 1 ,

f KI 0 ( x ) = K 0 ( x ) I 0 ( x ) ,

f KI 1 ( x ) = K 1 ( x ) I 1 ( x ) ,

where K_{0}(x) is the modified Bessel function second kind (zero order), K_{1}(x) is the modified Bessel function of second kind (first order), I_{0}(x) is the modified Bessel function of first kind (zero order), and I_{1}(x) is the modified Bessel function of first kind (first order).

R TP = − f I ( r 1D N u ) f I ( r 1 D u ) f KI0 ( r 1D N u ) f KI1 ( r 1D N u ) M N ,

f I ( x ) = I 0 ( x ) I 1 ( x ) ,

N = ω 12 M .

In order to obtain the solution of composite gas reservoir considering stress sensitivity, Solution (17) is sorted out as follows:

ψ 1 D = − 1 γ D * ln [ 1 − γ D * ξ wD ( r D , t D ) ] (20)

The typical layout of pressure drop test for heterogeneous composite gas reservoir considering stress sensitivity is shown in _{D} will cause the curve of pressure and pressure derivative to be upwarped in the middle and later stages. The pressure derivative curve in the radial flow stage in the inner region deviates from the 0.5 horizontal line under Darcy seepage condition and the pressure derivative curve in the radial flow stage in the outer region also warps. It can be seen from _{D}, the more obvious the curve of pressure and pressure derivative will be warped, which shows the characteristics of gradual deterioration of peripheral physical properties.

As shown in

higher the mobility ratio is, the higher the pressure derivative curve and pressure curve of the outer zone radial flow stage are, and the greater the bottom pressure drop is.

The pressure build-up well test curve of composite gas reservoir considering stress sensitivity is shown in

This shows that the permeability damage is caused by stress sensitivity in the production process of gas wells, and the damage range and degree increase with the increase of production time, resulting in the obvious difference between pressure drop curve and pressure recovery curve, which is consistent with the experimental phenomenon. Therefore, for this kind of reservoir, if the conventional reservoir test theory is used to interpret the test data, the pressure drop well test interpretation and the pressure build-up well test interpretation will get the opposite conclusion.

Well ID-X is the first exploration well successfully tested in the field of ultra-high temperature and ultra-high pressure in Ledong District, Yinggehai

basin, western South China Sea. The formation pressure coefficient of the well is 2.28. According to previous research results, abnormal high pressure gas reservoirs are generally stress sensitive. In order to fully understand whether the reservoir has stress sensitivity, the productivity test under the condition of step-by-step amplification of differential pressure is designed. The test results of this well are shown in

Choke size | Bottom pressure | Bottom temperature | Daily production |
---|---|---|---|

(mm) | (MPa) | (˚C) | m^{3}/d |

2.38 | 59.768 | 194.14 | 2.82 |

3.18 | 49.251 | 194.09 | 4.82 |

3.97 | 44.353 | 193.49 | 6.98 |

5.56 | 33.819 | 190.09 | 10.65 |

7.14 | 26.904 | 185.64 | 12.26 |

According to the interpretation of the established well test model, the permeability in the inner zone is K = 0.73 mD, the radius is 20 m, the outer zone is K = 1.04 mD, the skin factor is 20.5, and the stress sensitivity coefficient is very small, which is 0.0004, indicating that the reservoir almost has no stress sensitivity.

It is found that: for the abnormal high pressure gas reservoirs with stress sensitive characteristics, the formation rocks generally show the characteristics of under compaction (without discussing the formation with micro fractures), that is, the abnormal formation pressure is caused by unbalanced compaction, such as Kela 2 and the east of Yingqiong basin. According to the pore structure of the formation, the formation pore is composed of “storage pore” and “connected pore”, corresponding to “pore” and “throat” in rock. The “storage hole” has high aspect ratio and large volume, which basically represents the size of the total pore. However, the “connected hole” has low aspect ratio and small volume, which controls the flow of fluid in the pore. Compared with the “storage hole”, the “connected hole” is more flexible. On the one hand, it is easy to be compressed, on the other hand, it is easy to be opened. Because under compaction delays or prevents normal compaction, the “storage hole” and “connecting hole” are larger than the normal compaction stratum with the same depth, so it is easier to deform and show stress sensitivity. It is found that the formation of overpressure in Ledong area of Yinggehai basin is due to fluid expansion/conduction, and the mechanism of fluid expansion (such as hydrocarbon generation) occurs in deep buried strata. The rapid increase of fluid volume makes the relatively flexible “connecting hole” open, while the rigid “storage hole” is basically unchanged. Therefore, the rigid “storage hole” is basically the same in the process of productivity test and pressure recovery However, only the “connected hole” changes slightly, showing weak or no stress sensitivity. Therefore, not all abnormal high pressure gas reservoirs have stress sensitivity. Except for the influence of micro fractures, the stress sensitivity is related to the formation of abnormal high pressure.

1) The well test analysis model of heterogeneous composite natural gas reservoir considering the effect of stress sensitivity is established. In the production process, stress sensitivity will cause permeability damage. The pressure drop well test curve shows that the physical properties of the reservoir periphery gradually become worse, and the pressure build-up well test curve shows that the physical properties of the reservoir periphery gradually become better.

2) The field practice of testing reservoir stress sensitivity is carried out. The results show that the stress sensitivity is related to the cause of formation of abnormal high pressure without considering the development of microfractures. The reservoirs with stress sensitivity are generally over pressured due to unbalanced compaction. For the fluid swelling/conductive overpressure reservoirs in Ledong District, they are almost not stressed sensitive.

The authors declare no conflicts of interest regarding the publication of this paper.

Wu, M.W. and Liang, H. (2020) Application of Heterogeneous Composite Model with Consideration of Stress Sensitive in Low Permeability Gas Reservoir. International Journal of Geosciences, 11, 756-767. https://doi.org/10.4236/ijg.2020.1111038