CN107169248B

CN107169248B - Special stratum safe mud density window determination method

Info

Publication number: CN107169248B
Application number: CN201710542811.8A
Authority: CN
Inventors: 袁俊亮; 周建良; 罗洪斌; 许亮斌; 殷志明; 郝希宁; 张玉亭; 高飞
Original assignee: China National Offshore Oil Corp CNOOC; Beijing Research Center of CNOOC China Ltd
Current assignee: China National Offshore Oil Corp CNOOC; Beijing Research Center of CNOOC China Ltd
Priority date: 2017-07-05
Filing date: 2017-07-05
Publication date: 2020-06-30
Anticipated expiration: 2037-07-05
Also published as: CN107169248A

Abstract

The invention relates to a method for determining a safe mud density window of a special stratum, which comprises the following steps: 1) determining the mechanical parameters of sedimentary rock stratum rocks according to the acoustic time difference, density and natural gamma data obtained by logging of a target well and an indoor rock mechanical experiment; 2) determining geomechanical parameters according to natural gamma logging information and a measured value of the stratum pressure-bearing capacity of a drilling site; 3) calculating the stress state around the well by using geomechanical parameters; 4) judging whether the stratum has an upper collapse pressure limit; 5) according to the principle of combination of numbers and shapes, aiming at three deformation damage forms, the lower collapse pressure limit, the fracture pressure and the upper collapse pressure limit are determined, and a safe mud density window of a special stratum is obtained. By the method, the special stratum with the upper collapse pressure limit can be effectively and accurately identified, and the safe mud density window of the special stratum is determined.

Description

Special stratum safe mud density window determination method

Technical Field

The invention relates to the technical field of petroleum drilling, in particular to a method for determining a special stratum safe mud density window.

Background

In the field of petroleum industry, the problem of borehole wall instability in the process of oil and gas field exploration and development severely restricts the drilling speed and causes huge economic loss. According to the drilling industry, the economic loss caused by borehole wall instability worldwide is over $ 1 billion per year. During actual drilling, formations in which borehole wall instability problems typically occur include: hard brittle shale formations, bedding formations, fractured formations, highly steep complex formations, and high temperature high pressure formations. There are two main forms of damage to borehole wall instability: borehole wall collapse and borehole wall rupture. The former is mainly caused by that the mud density is too low, so that the structural compression stress borne by the well wall exceeds the compressive strength of rock, and the well wall is subjected to shear failure, and the mud density at the moment is called as collapse pressure P_t|^lower(ii) a The latter is mainly caused by the fact that the mud density is too high, so that the tensile stress brought by the liquid column born by the well wall exceeds the tensile strength of the stratum, and the mud density at this moment is called as the fracture pressure P_f。

The method for analyzing the stability of the well wall generally comprises the step of calculating the density of slurry for maintaining the stability of the well wall by simulating the stress state bonding strength criterion around the well, wherein the minimum density of the slurry required for ensuring that the well wall is not collapsed and damaged is the collapse pressure. It is therefore common to maintain the borehole wall stable during drilling with an appropriate mud density and it is believed that the borehole wall will not experience shear nor tension damage as long as the mud density is greater than the collapse pressure and does not exceed the fracture pressure. However, the actual drilling phenomenon of the part of the formation ahead of the Tarim mountain with the field reflects: there is a special formation and there is an upper limit P of the collapse pressure during drilling_t|^upOnce the mud density is too high, the upper limit of the collapse pressure is exceededP_t|^upSevere borehole wall collapse can also occur at this time. The safe mud density window for this particular type of formation should not be limited by the fracture pressure curve alone. There is no current method to identify such specific formations and calculate a safe mud density window for such specific formations.

Disclosure of Invention

In view of the above problems, the present invention provides a method for determining a safe mud density window of a special formation, by which the special formation with an upper collapse pressure limit can be effectively and accurately identified, and the safe mud density window of the special formation can be determined.

In order to achieve the purpose, the invention adopts the following technical scheme: a method for determining a safe mud density window of a special stratum is characterized by comprising the following steps:

1) determining the mechanical parameters of sedimentary rock stratum rocks according to the acoustic time difference, density and natural gamma data obtained by logging of a target well and an indoor rock mechanical experiment;

2) determining geomechanical parameters according to the natural gamma logging data obtained in the step 1) and the measured value of the stratum pressure-bearing capacity of the drilling site;

3) calculating the stress state around the well by using the geomechanical parameters obtained in the step 2);

4) determining three deformation failure modes generated in the process of bearing different mud liquid column pressures by the well wall according to Mohr-Coulomb yield criterion and combining the well peripheral stress state and geomechanical parameters, and judging whether the stratum has an upper limit of collapse pressure;

5) determining the lower limit P of the collapse pressure aiming at three deformation damage forms according to the principle of combination of numbers and shapes_t|^lowerRupture pressure P_fAnd upper limit of collapse pressure P_t|^upAnd obtaining a safe mud density window of the special stratum as follows:

P_t|^lower＜ρ_mud＜min(P_t|^up,P_f) (1)

in the above formula, ρ_mudIs the mud density.

In the step 1), the national standard engineering rock mass test method standard (GB/T50266-99) is adopted in the indoor rock mechanics experiment for determining the sedimentary rock formation rock mechanics parameters; the sedimentary rock stratum lithology is sandstone-mudstone, the mechanical property is an ideal linear elastomer with homogeneous isotropy, and a borehole is drilled to meet the Mohr-Coulomb yield criterion:

τ＝C+σ·tgφ (2)

wherein tau is the shear stress of sedimentary rock; c is cohesive force of sedimentary rock; phi is the internal friction angle of the sedimentary rock; σ is the positive stress of the sedimentary rock.

In step 2) above, the geomechanical parameter comprises overburden pressure σ_VHorizontal maximum stress σ_HAnd horizontal minimum ground stress σ_h：

Where rho is the density of sedimentary rock, α is the effective stress coefficient, omega₁,ω₂To construct the stress coefficient; 0-H is a depth integration interval; h is the well depth of the target well; g is the acceleration of gravity; p_pIs the pore pressure; v is the Poisson's ratio.

In the step 3), according to the effective stress theory of the porous medium, the thick-wall cylinder stressed unevenly has the well-periphery stress state expression as follows:

in the formula, theta is an included angle between a well circumferential radial vector and a horizontal maximum principal stress direction; sigma'_ZAxial effective stress of the well wall; sigma'_θIs the tangential effective stress of the well wall; sigma'_rRadial effective stress of the well wall; p_iThe pressure of the mud-liquid column, R the well-to-well distance, R the radius of the well bore, f the porosity, η the nonlinear correction coefficient, 0.95, delta the seepage coefficient, ξ the intermediate variable,

in the step 4), for the hard and brittle shale stratum, the well wall has the following three forms of deformation damage:

(1) when the density of the slurry is lower than the conventional lower limit P of the collapse pressure_t|^lowerWhen the most dangerous azimuth around the well is the azimuth theta of 90 degrees or 270 degrees, the difference value between the maximum principal stress and the minimum principal stress around the well is sigma'_θ-σ′_rThe well wall will be damaged by lower limit shearing;

(2) when the mud density is higher than the fracture pressure P_fIn this case, the most dangerous azimuth around the well is the azimuth of θ ═ 0 ° or 180 °, and the stress state around the well is σ'_θIf the well wall is less than 0, the well wall will be subjected to tensile damage;

(3) for a particular type of formation, at mud column pressure P_iReaches a rupture pressure P_fPreviously, the difference between the maximum principal stress and the minimum principal stress of the well wall at the positions of 0 DEG and 180 DEG is sigma DEG'_Z-σ′_θIf the difference is too high, the upper limit shearing damage will occur to the well wall, and the pressure P of the mud-liquid column at the moment_iIs the upper limit P of the collapse pressure_t|^up。

In the step 5), the lower limit of collapse pressure P in the three deformation damage forms existing on the well wall_t|^lowerRupture pressure P_fUpper limit of collapse pressure P_t|^upThe relative size relationship of the three is as follows:

(1) the tangential to radial stress difference curve for the borehole wall represents the maximum principal stress σ 'on the borehole wall at the orientation of θ -90 ° or 270 ° as the mud column pressure increases'_θAnd minimum principal stress σ'_rThe difference between the two values and the equation expression of the tangential and radial stress difference curve of the well wall are as follows:

y＝3σ_H-σ_h-2x (5)

in the formula, x is the pressure of a mud slurry column; y is the stress difference;

the abscissa of the intersection point of the tangential and radial stress difference curve of the well wall and the stress difference curve required by shear failure is collapse pressureLower limit of force P_t|^lowerThe calculation expression is as follows:

wherein UCS is the uniaxial compressive strength of sedimentary rock, and UCS is 2. C.K; k is an intermediate variable, K ═ tan (pi/4 + phi/2);

(2) the axial and tangential stress difference curves of the borehole wall represent the maximum principal stress σ 'on the borehole wall at an orientation of 0 ° or 180 ° with increasing mud column pressure'_ZAnd minimum principal stress σ'_θThe difference between the two, axial and tangential stress difference curve equation expression of the well wall is as follows:

y＝σ_V-(2v-1)σ_H+(2v-3)σ_h+x (7)

the abscissa of the intersection point of the axial and tangential stress difference curves of the well wall and the stress difference curve required by shear failure is the upper limit P of collapse pressure_t|^upSubstituting the formula (4) into the Mohr-Coulomb yield criterion, and finishing to obtain the upper limit P of the collapse pressure_t|^upThe calculation expression of (a) is as follows:

the expression of the transition point (Xo, Yo) between the lower limit shear failure and the upper limit shear failure is as follows:

(3) the tangential stress curve of the borehole wall represents the tangential effective stress σ 'at an orientation of 0 or 180 as the mud slurry column pressure increases'_θWhen the pressure is reduced to be less than 0, the well wall is subjected to tensile failure, and the abscissa of the intersection point of the tangential stress curve of the well wall and the x axis is fracture pressure P_fThe calculation expression is as follows:

P_f＝3σ_h-σ_H-αP_p+S_t(10)

wherein St is the tensile strength of the well wall rock.

Less than critical uniaxial compressive strength UCS for uniaxial compressive strength UCS_pThe formation of (1), which is a special formation that must consider the upper collapse pressure limit, the critical uniaxial compressive strength UCS_pCalculated by equation (11):

UCS_P＝σ_V-2v(σ_H-σ_h)-αP_p(11)。

due to the adoption of the technical scheme, the invention has the following advantages: 1. the invention provides a concept and a calculation method of an upper collapse pressure limit, and considers that a safety density window is limited by a lower collapse pressure limit and a fracture pressure, and the upper collapse pressure limit also exists for a special stratum, and a real safety density window is determined by the three. Compared with the prior art, the invention expands the knowledge of the safe density window. 2. The invention provides a method for judging a special stratum with a collapse pressure upper limit, which can calculate the critical Uniaxial Compressive Strength (UCS) through overburden pressure, horizontal ground stress and stratum pore pressure_p) The stratum with the uniaxial strength lower than the critical value belongs to a special stratum with the collapse pressure upper limit needing to be considered, so that the invention also provides the applicable conditions while obtaining the determination method.

Drawings

FIG. 1 is a schematic diagram of a borehole wall shear failure at a lower collapse pressure limit;

FIG. 2 is a schematic view of a borehole wall in tension failure;

FIG. 3 is a schematic diagram of borehole wall shear failure at an upper collapse pressure limit;

FIG. 4 is a graph showing the pressure profile of a slurry column in which lower limit shear failure, upper limit shear failure and tension failure occur, respectively;

FIG. 5 is a flow chart of the present invention.

Detailed Description

The invention is described in detail below with reference to the figures and examples. It is to be understood, however, that the drawings are provided solely for the purposes of promoting an understanding of the invention and that they are not to be construed as limiting the invention.

As shown in fig. 5, the method for determining the safe mud density window of the special formation provided by the invention comprises the following steps:

1) determining mechanical parameters of sedimentary rock stratum rocks including uniaxial compressive strength, cohesive force and internal friction angle of the sedimentary rock stratum rocks according to acoustic time difference, density and natural gamma data obtained by logging of a target well and an indoor rock mechanical experiment;

in the embodiment, the indoor rock mechanics experiment for determining the sedimentary rock formation rock mechanics parameters adopts the national standard engineering rock mass test method standard (GB/T50266-99). Wherein, the sedimentary rock stratum lithology is sand shale, the mechanical property is an ideal line elastomer with homogeneous isotropy, and the borehole is drilled to meet the Mohr-Coulomb (Mohr-Coulomb) yield criterion:

τ＝C+σ·tgφ (1)

2) Determining geomechanical parameters according to the natural gamma logging data obtained in the step 1) and the measured value of the stratum pressure-bearing capacity of the drilling site; wherein the geomechanical parameter comprises overburden pressure sigma_VHorizontal maximum stress σ_HAnd horizontal minimum ground stress σ_h：

Where rho is the density of sedimentary rock, α is the effective stress coefficient, P_pIs the pore pressure; omega₁,ω₂The formation stress coefficient is determined by a drilling site stratum bearing capacity test; 0-H is a depth integration interval; h is the well depth of the target well; g is the acceleration of gravity; v is the Poisson's ratio.

3) Calculating the stress state (sigma ') around the well by using the geomechanical parameters obtained in the step 2)'_Z、σ′_r、σ′_θ)：

According to the effective stress theory of porous media, the thick-wall cylinder under the action of non-uniform ground stress has the following expression of the stress state around the well:

4) according to Mohr-Coulomb yield criterion, three deformation failure modes generated in the process of bearing different mud liquid column pressures on the well wall are determined by combining the stress state around the well and geomechanical parameters:

for a hard and brittle shale stratum, the seepage effect and the nonlinear characteristics can be simplified, and the well wall has the following three deformation damages:

(1) when the density of the slurry is lower than the conventional lower limit P of the collapse pressure_t|^lowerWhen the most dangerous azimuth around the well is the azimuth theta of 90 degrees or 270 degrees, the difference value between the maximum principal stress and the minimum principal stress around the well is sigma'_θ-σ′_rThe well wall will have lower limit shear failure, the failure form is shown in figure 1;

(2) when the mud density is higher than the fracture pressure P_fIn this case, the most dangerous azimuth around the well is the azimuth of θ ═ 0 ° or 180 °, and the stress state around the well is σ'_θIf the pressure is less than 0, the well wall will be subjected to tensile damage, and the damage form is shown in figure 2;

(3) for a particular type of formation, at mud column pressure P_iReaches a rupture pressure P_fPreviously, the difference between the maximum principal stress and the minimum principal stress of the well wall at the positions of 0 DEG and 180 DEG is sigma DEG'_Z-σ′_θIf the difference is too high, the upper limit shearing damage will occur to the well wall,the failure is shown in FIG. 3, where the mud column pressure P is_iIs the upper limit P of the collapse pressure_t|^up。

5) According to the principle of combination of numbers and shapes, the lower limit P of collapse pressure is given for three deformation failure forms_t|^lowerRupture pressure P_fUpper limit of collapse pressure P_t|^upAnd giving a safe mud density window:

as shown in FIG. 4, the lower limit of collapse pressure P in the three types of deformation damage existing on the well wall_t|^lowerRupture pressure P_fUpper limit of collapse pressure P_t|^upThe relative size relationship of the three is as follows:

y＝3σ_H-σ_h-2x (4)

in the formula, x is the pressure of a mud slurry column; and y is the stress difference.

The abscissa of the intersection point of the tangential and radial stress difference curve of the well wall and the stress difference curve required by shear failure is the lower limit P of collapse pressure_t|^lowerThe calculation expression is as follows:

y＝σ_V-(2v-1)σ_H+(2v-3)σ_h+x (6)

the abscissa of the intersection point of the axial and tangential stress difference curves of the well wall and the stress difference curve required by shear failure is the upper limit P of collapse pressure_t|^upSubstituting the formula (3) into the Mohr-Coulomb yield criterion, and finishing to obtain the upper limit P of the collapse pressure_t|^upThe calculation expression of (a) is as follows:

P_f＝3σ_h-σ_H-αP_p+S_t(9)

wherein St is the tensile strength of the well wall rock.

If the upper limit of the collapse pressure P_t|^upGreater than the rupture pressure P_fThen there is no upper collapse pressure limit for that type of formation (because the formation will first experience tensile failure); if the upper limit of the collapse pressure P_t|^upLess than the cracking pressure P_fAnd judging that the stratum has the upper limit of collapse pressure, namely judging the stratum to be the special stratum.

Comparing the collapse pressure upper limit with the rupture pressure, and taking the smaller value of the two, i.e. min (P)_t|^up,P_f) As an upper limit on the safe mud density window for a particular formation. In summary, the safe mud density window for a particular formation is as follows:

P_t|^lower＜ρ_mud＜min(P_t|^up,P_f) (10)

in the formula, ρ_mudIs the mud density.

6) To clarify the application range of the present invention, the critical uniaxial compressive strength UCS can be calculated by analyzing the mechanical properties of the special stratum, i.e. making formula (7) and formula (9) equal_p：

UCS_P＝σ_V-2v(σ_H-σ_h)-αP_p(11)

Less than critical uniaxial compressive strength UCS for uniaxial compressive strength UCS_pThe formation of (2) is a specific formation for which the upper collapse pressure limit must be considered.

The above embodiments are only used for illustrating the present invention, and the structure, connection mode, manufacturing process, etc. of the components may be changed, and all equivalent changes and modifications performed on the basis of the technical solution of the present invention should not be excluded from the protection scope of the present invention.

Claims

1. A method for determining a safe mud density window of a special stratum is characterized by comprising the following steps:

1) determining the mechanical parameters of sedimentary rock stratum rocks according to the acoustic time difference, density and natural gamma data obtained by logging of a target well and an indoor rock mechanical experiment; wherein, the indoor rock mechanics experiment for determining the sedimentary rock stratum rock mechanics parameters adopts the national standard of engineering rock mass test method standard GB/T50266-99; the sedimentary rock stratum lithology is sandstone-mudstone, the mechanical property is an ideal linear elastomer with homogeneous isotropy, and a borehole is drilled to meet the Mohr-Coulomb yield criterion:

τ＝C+σ·tgφ (1)

wherein tau is the shear stress of sedimentary rock; c is cohesive force of sedimentary rock; phi is the internal friction angle of the sedimentary rock; σ is the normal stress of the sedimentary rock;

Where rho is the density of sedimentary rock, α is the effective stress coefficient, omega₁,ω₂To construct the stress coefficient; 0-H is a depth integration interval; h is the well depth of the target well; g is the acceleration of gravity; p_pIs the pore pressure;vis the poisson ratio;

3) calculating the stress state around the well by using the geomechanical parameters obtained in the step 2), wherein the specific process is as follows:

4) determining three deformation failure modes generated in the process of bearing different mud liquid column pressures by the well wall according to Mohr-Coulomb yield criterion and combining the well peripheral stress state and geomechanical parameters, and judging whether the stratum has an upper collapse pressure limit:

for hard and brittle shale formations, the borehole wall has three forms of deformation damage:

(1) when the density of the slurry is lower than the conventional lower limit P of the collapse pressure_t|^lowerWhen the most dangerous orientation around the well is θ ═90 degrees or 270 degrees, and the difference value of the maximum main stress of the well wall and the minimum main stress of the well wall is sigma'_θ-σ'_rThe well wall will be damaged by lower limit shearing;

(2) when the mud density is higher than the fracture pressure P_fIn the meantime, the most dangerous azimuth around the well is the azimuth of 0 ° or 180 °, and the well wall stress state is σ'_θ<0, the well wall will be subjected to tensile damage;

(3) for a particular type of formation, at mud column pressure P_iReaches a rupture pressure P_fPreviously, the difference between the borehole wall maximum principal stress and the borehole wall minimum principal stress was σ 'at the positions of 0 ° and 180 °'_Z-σ'_θIf the difference is too high, the upper limit shearing damage will occur to the well wall, and the pressure P of the mud-liquid column at the moment_iIs the upper limit P of the collapse pressure_t|^up；

P_t|^lower<ρ_mud<min(P_t|^up,P_f) (4)

in the above formula, ρ_mudIs the slurry density;

wherein, the lower limit P of the collapse pressure in three deformation damage modes of the well wall_t|^lowerRupture pressure P_fUpper limit of collapse pressure P_t|^upThe relative size relationship of the three is as follows:

(1) the tangential and radial stress difference curve of the well wall represents the difference between the maximum main stress of the well wall and the minimum main stress of the well wall at the position of theta being 90 degrees or 270 degrees along with the increase of the pressure of the mud liquid column, and the equation expression of the tangential and radial stress difference curve of the well wall is as follows:

y＝3σ_H-σ_h-2x (5)

in the formula, x is a mud liquid column pressure value; y is the stress difference;

tangential and radial stress difference curves and shear failure of well wallsThe abscissa of the intersection point of the required stress difference curve is the lower limit P of the collapse pressure_t|^lowerThe calculation expression is as follows:

wherein UCS is the uniaxial compressive strength of sedimentary rock, UCS is 2. CK；KIs the intermediate variable(s) of the variable,K＝tan(π/4+φ/2)；

(2) the axial and tangential stress difference curve of the well wall represents the difference between the maximum main stress of the well wall and the minimum main stress of the well wall at the position of theta equal to 0 DEG or 180 DEG along with the increase of the pressure of the mud liquid column, and the equation expression of the axial and tangential stress difference curve of the well wall is as follows:

y＝σ_V-(2v-1)σ_H+(2v-3)σ_h+x (7)

P_f＝3σ_h-σ_H-αP_p+S_t(10)

wherein St is the tensile strength of the well wall rock.

2. The method of claim 1, wherein the UCS is less than the UCS for a compressive uniaxial strength_pThe formation of (1), which is a special formation that must consider the upper collapse pressure limit, the critical uniaxial compressive strength UCS_pCalculated by equation (11):

UCS_P＝σ_V-2ν(σ_H-σ_h)-αP_p(11)。