CN112651113A - Safe mud density window judgment method suitable for east China sea pressure failure stratum - Google Patents
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Abstract
The invention discloses a safe mud density window judging method suitable for an east China sea pressure failure stratum, which integrates various subjects and methods such as rock mechanics, drilling engineering, continuous medium mechanics, statistics, well logging interpretation methods and the like, fully utilizes original stratum data before reservoir non-use, such as logging data such as density logging, acoustic wave time difference logging and the like, establishes a ground stress calculation model by combining rock mechanics parameters, obtains a change rule of ground stress along with formation pressure failure, and finally obtains collapse and fracture pressure of the east China sea pressure failure stratum through model calculation so as to obtain a safe mud density window suitable for the pressure failure stratum.
Description
Technical Field
The invention relates to the research on the stability of a pressure well wall in the rock mechanics of petroleum engineering and oil-gas well engineering, in particular to a safe mud density window judgment method suitable for an east-sea pressure failure stratum.
Background
In the long-term exploitation process of oil and gas fields such as east-sea West lake and Pinghu lake, formation fluid is continuously extracted, formation energy is reduced, and formation pressure is caused to be exhausted. Accidents such as well leakage, pressure difference drill sticking and the like are easily caused in the drilling process of the failure stratum. After the X gas field sunken in the West lake of the east China sea is mined for years, the actual formation pressure failure degree of part of gas wells exceeds 50 percent. In order to meet the requirements of production plans, the gas field is additionally provided with a plurality of adjusting wells, and accidents such as well leakage, drill sticking and the like of a pressure failure well section account for more than 40 percent of the whole well section in the drilling process of the adjusting wells. If the well wall stability of the pressure failure stratum is not scientifically corrected, the well wall stability research is carried out by using the pressure coefficient of the original stratum so as to determine the safe mud density window, and phenomena such as well leakage, differential pressure drill sticking and the like are possibly caused. For this phenomenon, the collapse and fracture pressure of the east sea pressure failure formation need to be accurately predicted, so as to determine a safe mud density window suitable for the east sea pressure failure formation.
Disclosure of Invention
The invention aims to provide a method for determining a safe mud density window suitable for a pressure failure stratum by accurately predicting collapse and fracture pressure of the pressure failure stratum.
In order to solve the technical problems, the invention provides the following technical scheme: a safe mud density window judgment method suitable for an east China sea pressure failure stratum comprises the following steps:
1) collecting logging information of a recently drilled and used reservoir layer away from a well to be drilled, and obtaining acoustic velocity data Vp under the original formation pressure by using the following conversion relation according to acoustic time difference logging data DT:
wherein DT is sound wave time difference us/ft; vpIs the acoustic velocity, m/s.
Because the offshore oil and gas well mud line is below the sea level, the upper formation well depth-density relationship cannot be obtained through regression of lower well section density logging data, a common Gardener model can be used for calculating a shallow formation density curve, and then overburden pressure OBG is obtained through integration according to complete density data:
wherein a and b are Gardener constants, h1 is water depth, h2 is density logging data initial vertical depth, h3 is measuring point vertical depth, g is gravity acceleration, rhowIs the sea water density, ρiIs the current drilling fluid density.
Obtaining original formation pressure P according to well logging interpretation data or actual measurement datapObtaining the current formation pressure P 'after the formation pressure failure according to the steel wire test operation'pBy the Eaton method:
obtaining the acoustic velocity V under the current formation pressurep'. Wherein n is the Eton index, and the east sea stratum is generally 0.8-1.2.
2) According to the Lal empirical formula:
and calculating the internal friction angle phi and the cohesion CS of the rock mechanical strength parameters under the current formation pressure after the pressure failure occurs. In the formula, m is a correction coefficient, and the east China sea stratum is generally 1.7.
3) Collecting a drilled reservoir rock core closest to a target well position, obtaining the original three-dimensional ground stress of the stratum through an indoor Kaiser acoustic emission ground stress experiment, wherein the original three-dimensional ground stress is { sigma v, sigma H and sigma H }, and obtaining the elastic modulus E and the Poisson ratio v of the stratum rock through an indoor triaxial experiment.
4) Building a ground stress calculation model
The stress-strain relation before formation pressure failure is as follows according to generalized Hooke's law, wherein the original three-dimensional ground strain of the formation is { Epsilon v, Epsilon H, Epsilon }:
after a period of production, the formation pressure is depleted to p'pAt this time, the three-dimensional ground stress is { σ'v，σ′H，σ′hIs strained three-dimensionally to { ε'v，ε′H，ε′hThe stress-strain relationship after formation pressure failure is as follows:
assuming that the target formation is buried deep enough and the formation height is limited, the formation extends transversely indefinitely, which can be: changes in formation pressure cause changes in vertical strain only, and not horizontal strain; only a change in horizontal stress is induced, but not a change in vertical stress, i.e.:
in the formula:△εh＝εh-ε′h，△εH＝εH-ε′H，△σν＝σν-σ′ν。
the simultaneous (4a), (4b) and (4c) establishes a ground stress calculation model as follows:
wherein sigmahIs the original level minimum ground stress, σHMaximum stress for the original level; sigma'hIs the minimum post-failure level ground stress, σ'HMaximum stress for post-failure level; delta pp＝pp-p′pThe formation pressure failure value is obtained; alpha is an effective stress coefficient, and the east sea stratum is generally 0.85-0.95; from this model, it is known that the collapse of the formation pressure causes a decrease in horizontal stress, and the decrease in horizontal stress is equal.
5) Construction of borehole wall stable model
According to the well-to-well stress distribution and molar coulomb criterion of the pressure failure stratum well section, combining the rock mechanical parameter value obtained in the step 2) and the ground stress value obtained in the step 4), and finally determining the collapse pressure, the leakage pressure and the fracture pressure of the pressure failure stratum; according to the collapse pressure, calculating the minimum density of the drilling fluid which can not cause the borehole wall collapse when the pressure failure stratum is drilled; according to the leakage and the fracture pressure, the maximum density of the drilling fluid which can not cause the well wall fracture when the pressure failure stratum is drilled is calculated; a safe density window for mud drilling from the pressure depleted formation is determined.
5.1) if the target well is a vertical well, the borehole wall collapse pressure and the fracture pressure of the vertical well section are calculated as follows:
Pf＝3σh-σH-2△σh-αp′p+St (5b)
wherein p isr，pfCollapse pressure equivalent (g/cm) and rupture pressure equivalent (g/cm) respectively3) H is vertical depth (m);
eta is a stress nonlinear correction coefficient, StThe tensile strength of the rock.
5.2) if the target well is a directional well, calculating the collapse pressure and the fracture pressure of the well wall of the directional well section in the following mode:
order:
where ψ is the angle of the well-dip, Ω is the azimuth angle of the well-dip relative to the horizontal maximum stress azimuth, θ is the well-perimeter angle, ν is the Poisson's ratio, σ is the angle of the well-diphTo level minimum ground stress, σHTo level maximum ground stress, σvIs the overburden ground stress.
The three-directional main stress on the wall of the inclined shaft can be expressed as follows:
wherein p isiIs the drilling fluid column pressure; k is a well wall permeability coefficient, and the value of K is 0-1.
In the formula: max represents taking the maximum value; min represents taking the minimum value.
Directional well wall shearing and stretching failure criteria:
the rock is subjected to shear failure:
(σ1-σ3)-sinφ(σ1+σ3-2αpp)-2CS cosφ＝0 (5f)
tensile failure of the rock occurs:
σk-αpp+|St|＝0 (5g)
in the case of a directional well, the principal stress in three directions on the well wall can be found by equation (5 d). The collapse pressure and the rupture pressure are jointly determined by the three-way main stress state and the failure criterion, when the three-way main stress state meets the shear failure criterion (5f), the well wall is collapsed and damaged, and the density of the drilling fluid is the equivalent of the collapse stress at the moment; when the three-dimensional principal stress state meets the tensile failure criterion (5g), the well wall is broken in a tensile mode, and the density of the drilling fluid is equivalent to the breaking pressure. From the formulas (5 d-5 g), the collapse pressure and the fracture pressure of the directional well are functions of three-dimensional ground stress, rock mechanical strength (internal friction angle and cohesion), formation pressure, well circumferential angle, well inclination angle and well inclination azimuth angle.
In the case of a failure pressure stratum, changes in the stratum pressure cause the rock mechanical strength parameters and the three-dimensional ground stress to change simultaneously, and finally cause changes in the well-circumferential stress. At this point, the drilling fluid density p satisfies (5f) the shear failure criterioniComprises the following steps:
pi＝F1(σ′v,σ′H,σ′h,φ,CS,P′p,α,Ω,Ψ,θ) (5h)
drilling fluid density p meeting (5g) tensile failure criteriaiComprises the following steps:
pi＝F2(σ′v,σ′H,σ′h,St,P′p,α,Ω,Ψ,θ) (5i)
in formulae (5h), (5 i):
F1、F2representing a functional relationship;
σ′v,σ′H,σ′hvertical ground stress, horizontal maximum ground stress, horizontal minimum ground stress after formation pressure failure;
wherein the value of the well circumferential angle theta is (0-2 pi).
6) Calculating collapse and burst pressures
6.1) when σj>σk>σiWhen the well circumferential angle value satisfies
Then, p is calculated from the formula (5h)iMaximum value along the well circumference
And corresponding well peripheral angle, here
Namely the required collapse pressure value:
6.2) when σj>σi>σkWhen the well circumferential angle value satisfies
Then, p is calculated from the formula (5h)iMinimum value along well circumference, note
6.3) or σi>σj>σkWhen the well circumferential angle value satisfies
Then, p is calculated from the formula (5h)iMinimum value along well circumference, note
6.4) when the well circumferential angle value satisfies
Then, p is calculated from the formula (5i)iMinimum value along well circumference, note
6.5) the desired rupture pressure is
7) Calculating the leakage pressure of well wall
The leakage pressure of the well wall is calculated according to the minimum ground stress in the following way:
pl＝σh
8) calculating a safe mud density window from the collapse, burst and loss pressures:
let pm＝min(pl,pf) Then drilling fluid density piHas a safety interval range of (k)1pr,k2pm) Namely:
k1pr<pi<k2 min(pl,pf)
wherein k is1，k2For safety factor, the east China sea generally takes k1＝1.1，k2＝1。
The invention has the beneficial effects that:
1. the method can practically solve the problem of density change of the mud window caused by pressure failure of the east-sea oil-gas reservoir, and the collapse pressure calculation model established from two aspects of influences of formation pressure failure on rock mechanical strength and ground stress has higher prediction precision on collapse pressure of failure formation, so that a more accurate drilling fluid safety density window is provided.
2. The invention can accurately predict the collapse and fracture pressure of the directional well in the pressure failure stratum, thereby accurately judging the safe density window of the drilling fluid and having universal significance in practical application.
3. According to the method, the influence of formation pressure failure on the mechanical strength of the rock is innovatively considered, and then the collapse pressure calculation method under the influence is deduced.
Drawings
Fig. 1 is a schematic cross-sectional view of rock mechanics parameters of an A4H well in this example.
FIG. 2 is a schematic diagram of the ground stress profile of the A4H well in this example.
FIG. 3 is a schematic of four stress profiles and a safe mud density window for the A4H well in this example.
Detailed Description
The following describes embodiments of the present invention in further detail with reference to the accompanying drawings. These embodiments are merely illustrative of the present invention and are not intended to limit the present invention.
The K gas field is put into production in 2015 in the sunken west slope zone of the east-sea west lake, the geology of the gas field is divided into an east-sea group, a three-pool group, a Liulang group, a Longjing group, a Huagang group, a Pinghu group and a gem group from top to bottom, and the reservoir is the Huagang group and the Pinghu group. Over the years of production, the field has exhibited varying degrees of formation pressure failure. A1H well was put into production in 2015, reservoir p2a pressure was depleted to 0.74sg, A4H well was a new adjusted well in 2020, with two wells closer and drilled to meet p2a layers.
The first step is as follows: collecting stratum original data as a stratum original state; and collecting the information of the formation pressure after the pressure failure as an actual value of the formation pressure after the formation pressure failure.
The second step is that: A4H well formation acoustic velocity data, adjacent A1H well density log data, are collected, and overburden pressure is calculated. The K gas field is positioned in a depression of the West lake of the east sea, and shallow data is lacked in formation density logging, so that a widely-applied shallow formation density fitting model and a Gardener model are adopted to fit the shallow formation density. The density of the seawater is 1.07g/cm3. And (4) solving the overburden pressure of the stratum by adopting a mathematical integration method.
According to the Eton method, obtaining the current formation acoustic velocity when the formation pressure is depleted to 0.74 sg; and obtaining the cohesion CS and the internal friction angle phi of the rock mechanics parameters under the current formation pressure according to the Lal empirical method, which is shown in figure 1.
The third step: and (3) applying a calculation formula of the ground stress of the pressure failure stratum deduced by the generalized Hooke's law to obtain a ground stress change rule after pressure failure, which is shown in figure 2. Where SHG, SHG are the horizontal maximum stress and the horizontal minimum stress, respectively.
The fourth step: because the A4H well drilling failure stratum well section is a directional well, a directional well wall stress analysis method is adopted, three-way main stress obtained by well circumferential stress distribution is substituted into a shearing and tensile failure judgment criterion, and stratum collapse, leakage and fracture pressure after pressure failure is obtained, as shown in figure 3. In fig. 3, a curve a represents the burst pressure, a curve b represents the leak pressure, and a curve e represents the collapse pressure.
The fifth step: combining the calculated collapse, loss, and burst pressures, a safe window of drilling fluid density for the A4H well is obtained, see fig. 3, where the range between dashed lines c and d is the safe mud density window for each formation.
The above description is only a preferred embodiment of the present invention, and it should be noted that, for those skilled in the art, various modifications and substitutions can be made without departing from the technical principle of the present invention, and these modifications and substitutions should also be regarded as the protection scope of the present invention.
Claims (5)
1. A safe mud density window judgment method suitable for an east China sea pressure failure stratum comprises the following steps:
1) collecting the acoustic time difference logging information and the density logging information of the drilled well closest to the target well position, obtaining overburden pressure according to rock density logging data integration, calculating the effective stress of the original stratum rock and the effective stress of the current stratum rock according to the failure degree of the original stratum pressure and the current stratum pressure, and then calculating the acoustic velocity V of the current stratum according to the Eaton methodp′：
Wherein OBG is overburden pressure, PpIs the original formation pressure, Pp' is the current formation pressure, VpIs the acoustic velocity of the original formation, Vp' is the current formation acoustic velocity, n is the Eton index, east-sea formationGenerally taking 0.8-1.2;
2) based on the Lal empirical formula, the current formation acoustic velocity V obtained according to the step 1p' calculating the current stratum rock mechanical parameter value after the pressure failure occurs:
wherein CS is cohesion; phi is an internal friction angle; vp' is the current formation acoustic velocity; m is a correction coefficient, and the east sea stratum is generally 1.7;
3) collecting a drilled reservoir rock core closest to a target well position, obtaining original three-dimensional ground stress of a stratum through an indoor Kaiser acoustic emission ground stress experiment, wherein the original three-dimensional ground stress is { sigma v, sigma H and sigma H }, and obtaining an elastic modulus E and a Poisson ratio v of the stratum rock through an indoor triaxial experiment;
4) establishing a ground stress calculation model:
a. the stress-strain relation before formation pressure failure is as follows according to generalized Hooke's law, wherein the original three-dimensional ground strain of the formation is { Epsilon v, Epsilon H, Epsilon }:
after a period of production, the formation pressure is depleted to p'pAt this time, the three-dimensional ground stress is { σ'v，σ′H，σ′hThree-dimensionally strained to { ε }v'，ε′H，εh', the stress-strain relationship after formation pressure failure is:
b. assuming that the target formation is buried deep enough and the formation height is limited, the formation extends transversely indefinitely, which can be: changes in formation pressure cause changes in vertical strain only, and not horizontal strain; only a change in horizontal stress is induced, but not a change in vertical stress, i.e.:
the simultaneous (4a), (4b) and (4c) establishes a ground stress calculation model as follows:
wherein sigmahIs the original level minimum ground stress, σHMaximum stress for the original level; sigma'hIs the minimum post-failure level ground stress, σ'HMaximum stress for post-failure level; delta pp＝pp-p′pThe formation pressure failure value is obtained; alpha is an effective stress coefficient, and the east sea stratum is generally 0.85-0.95; the model can know that the collapse of the formation pressure causes the horizontal stress to be reduced, and the reduction values of the horizontal stresses are equal;
5) according to the well-to-well stress distribution and molar coulomb criterion of the pressure failure stratum well section, combining the rock mechanical parameter value obtained in the step 2) and the ground stress value obtained in the step 4), and finally determining the collapse pressure, the leakage pressure and the fracture pressure of the pressure failure stratum; according to the collapse pressure, calculating the minimum density of the drilling fluid which can not cause the borehole wall collapse when the pressure failure stratum is drilled; according to the leakage and the fracture pressure, the maximum density of the drilling fluid which can not cause the well wall fracture when the pressure failure stratum is drilled is calculated; a safe density window for mud drilling from the pressure depleted formation is determined.
2. A safe mud density window decision method according to claim 1, characterized by: in the step 1), a shallow formation density curve is calculated by using a Gardener model, and then according to the complete density data, the shallow formation density curve is integrated to obtain overburden pressure OBG, wherein the calculation method is as follows:
wherein VpThe formation layer velocity or inversion velocity, m/s; a and b are Gardener constants, h1 is water depth, h2 is density logging data initial vertical depth, h3 is measuring point vertical depth, g is gravity acceleration, rhowIs the sea water density, ρiIs the current drilling fluid density.
3. A safe mud density window decision method according to claim 1, characterized by: in the step 5), if the target well is a vertical well, the calculation mode of the collapse pressure and the fracture pressure of the well wall of the vertical well section is as follows:
Pf＝3σh-σH-2△σh-αp′p+St (5b)
wherein p isr，pfCollapse pressure equivalent (g/cm) and rupture pressure equivalent (g/cm) respectively3) H is vertical depth (m);
eta is a stress nonlinear correction coefficient, StThe tensile strength of the rock.
4. A safe mud density window decision method according to claim 1, characterized by: in the step 5), if the target well is a directional well, the calculation modes of the borehole wall collapse pressure and the fracture pressure of the directional well section are as follows:
a. order:
where ψ is the angle of the well-dip, Ω is the azimuth angle of the well-dip relative to the horizontal maximum stress azimuth, θ is the well-perimeter angle, ν is the Poisson's ratio, σ is the angle of the well-diphTo level minimum ground stress, σHTo level maximum ground stress, σvIs overburden ground stress;
the three-directional main stress on the wall of the inclined shaft can be expressed as follows:
wherein p isiIs the drilling fluid column pressure; k is a borehole wall permeability coefficient and takes a value of 0-1;
b. directional well wall shearing and stretching failure criteria:
the rock is subjected to shear failure:
(σ1-σ3)-sinφ(σ1+σ3-2αpp)-2CScosφ＝0 (5f)
tensile failure of the rock occurs:
σk-αpp+|St|＝0 (5g)
in the case of a directional well, the three-dimensional principal stress on the well wall can be found by equation (5 f); the collapse pressure and the rupture pressure are jointly determined by the three-way main stress state and the failure criterion, when the three-way main stress state meets the shear failure criterion, the well wall is collapsed and damaged, and the density of the drilling fluid is the equivalent of the collapse pressure; when the three-dimensional main stress state meets the tensile failure criterion, the well wall is broken in a tensile mode, and the density of the drilling fluid is equivalent to the breaking pressure at the moment; from the formula (5 d-5 g), the collapse pressure and the fracture pressure of the directional well are functions of three-dimensional ground stress, rock mechanical strength (internal friction angle and cohesion), formation pressure, well circumferential angle, well inclination angle and well inclination azimuth angle;
in the declineUnder the condition of a pressure-depleted stratum, the change of the stratum pressure causes the simultaneous change of the rock mechanical strength parameters and the three-dimensional ground stress, and finally causes the change of the well-peripheral stress; at this point, the drilling fluid density p satisfies (5f) the shear failure criterioniComprises the following steps:
pi＝F1(σ′v,σ′H,σ′h,φ,CS,P′p,α,Ω,Ψ,θ) (5h)
drilling fluid density p meeting (5g) tensile failure criteriaiComprises the following steps:
pi＝F2(σ′v,σ′H,σ′h,St,P′p,α,Ω,Ψ,θ) (5i)
in the formula:
σ′v,σ′H,σ′hvertical ground stress, horizontal maximum ground stress, horizontal minimum ground stress after formation pressure failure;
wherein the value of the well circumferential angle theta is (0-2 pi);
c. collapse pressure calculation:
when sigma isj>σk>σiWhen the well circumferential angle value satisfies
Then, p is calculated from the formula (5h)iMaximum value along the well circumference
And corresponding well peripheral angle, here
I.e. the determined collapse pressure value pr；
d. Calculation of burst pressure:
(1) when sigma isj>σi>σkWhen the well circumferential angle value satisfies
Then, p is calculated from the formula (5h)iMinimum value along well circumference, note
(2) Or σi>σj>σkWhen the well circumferential angle value satisfies
Then, p is calculated from the formula (5h)iMinimum value along well circumference, note
(3) When the well circumferential angle value satisfies
Then, p is calculated from the formula (5i)iMinimum value along well circumference, note
(4) The required rupture pressure is
5. The safe mud density window determination method according to claim 4, wherein: the influence of the leakage pressure of the well wall is also considered, and the leakage pressure p of the well wall is calculated according to the minimum ground stressl＝σhLet p denotem＝min(pl,pf) Drilling fluid density piHas a safety interval range of (k)1pr,k2pm) I.e. the safe mud density window, where k1，k2For safety factor, the east China sea generally takes k1＝1.1，k2＝1。
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| CN113468646A (en) * | 2021-07-07 | 2021-10-01 | 常州大学 | Method for detecting geothermal well risk based on geomechanical model and machine learning |
| CN113468646B (en) * | 2021-07-07 | 2024-03-15 | 常州大学 | Geomechanical model and machine learning-based method for detecting geothermal well risk |
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