CN101392647B - Borehole wall stability prediction method suitable for gas drilling - Google Patents

Abstract

The invention relates to a well-wall stability prediction method for gas drilling, which comprises the following steps: when a well wall achieves a critical plastic state, a corresponding borehole inner supporting force Pi is obtained according to complete stress-strain experiment data of rocks and mechanical property parameters of the rocks; whether the value of the borehole inner supporting force Pi is more than zero is judged; if the borehole inner supporting force Pi is less than zero, the well wall maintains the stability; if the borehole inner supporting force Pi is more than zero, the well wall destabilizes; and if the borehole inner supporting force Pi is equal to zero, the result shows that the plasticity of the well wall achieves the critical state during the gas drilling. The method is provided based on the failure criterion of rock strength, is applicable to the prediction of well-wall stability under gas drilling conditions, and has good prediction effect.

Description

A kind of borehole wall stability prediction method that is applicable to gas drilling

Technical field

The present invention relates to the under balance pressure drilling/gas drilling technical field in the oil/gas drilling, be specifically related to a kind of borehole wall stability prediction method that is applicable to gas drilling.

Background technology

The gas drilling borehole wall stability is to solve gas drilling Adaptability Evaluation key of problem technology; Borehole wall stability studies just that well Zhou Yanshi state is whether stable still to be destroyed; Present drilling well form mainly is DWM and gas drilling; Gas drilling is to replace mud to carry the landwaste under boring as medium with gas; Mostly propose in the theoretical research result that obtains aspect the wellbore stability both at home and abroad at present, and seldom relate to the gas drilling borehole wall stability problem under the Zhou Yanshi state under the DWM condition.

DWM drilling shaft lining stability analysis based on well Zhou Yanshi state is the mechanics analysis model of continuing to use at present; Mainly carry out according to well flexibility analysis result; Reaching elasticity limit state with Sidewall Surrounding Rock is that criterion is calculated caving pressure; Its assumed condition is exactly that rock reaches after the peak strength caving pressure greater than zero and then destruction has just been taken place, and is difficult to explain the actual conditions that still can implement gas drilling at caving pressure under greater than zero situation.Prediction does not form a cover system and efficient ways as yet for the gas drilling borehole wall stability at present, does not set up the borehole wall stability forecast model that is suitable for the gas drilling condition.

Summary of the invention

The purpose of this invention is to provide a kind of Forecasting Methodology that is applicable to the borehole wall stability under the gas drilling condition, utilize this method can predict the stability of the borehole wall under the gas drilling condition, prediction effect is good.

Utilize above-mentioned purpose, the present invention adopts following technical scheme:

A kind of borehole wall stability prediction method that is applicable to gas drilling, the method comprising the steps of:

Support force P in the corresponding well when obtaining the borehole wall and reach critical plastic state according to the mechanics parameters of rock resultant stress strain-gauge test data and rock _i

Judge support force P in the said well _iValue whether greater than zero;

If support force P in the said well _iLess than zero, then draw the borehole wall and keep stable;

If support force P in the well _iGreater than zero, then draw borehole well instability.

Wherein, in this method, if support force P in the said well _iEqual zero, borehole wall plasticity reaches critical condition when then showing gas drilling.

Wherein, obtain P in this method _iBe based on rock strength failure criterion and critical damage zone scope r _cObtain.

Wherein, in this method based on rock strength failure criterion and critical damage zone scope r _cObtain support force P in the said well by formula (1) _i:

$P_i=\frac{2}{1-N}[\frac{\mathrm{AE}\′}{h+N}+\frac{(1-N){\mathrm{\σ}}_O-{\mathrm{\σ}}_q}{1+N}]{\left(\frac{r_c}{a}\right)}^{1-N}+\frac{{\mathrm{\σ}}_q(1+h)-2\mathrm{AE}\′}{(1-N)(h+N)}-\frac{{\mathrm{\σ}}_{\mathrm{qr}}}{h+N}---\left(1\right)$

Wherein, r _cBe critical damage zone scope, obtain by formula (2):

$r_c=a{\left\{\frac{\frac{2}{1-N}[\frac{\mathrm{AE}\′}{h+N}+\frac{(1-N){\mathrm{\σ}}_o-{\mathrm{\σ}}_q}{1+N}]}{p_i-\frac{{\mathrm{\σ}}_q(1+h)-2\mathrm{AE}\′}{(1-N)(h+N)}+\frac{{\mathrm{\σ}}_{\mathrm{qr}}}{h+N}}\right\}}^{\frac{1}{N-1}}---\left(2\right)$

Wherein, N is confirmed by unified angle of internal friction

according to formula (3):

In formula (1), formula (2) and the formula (3), A is the elastic limit strain value that rock elasticity and plasticity are softened intersection around the borehole wall; E ' is the softening modulus of rock; σ ₀Be far field stress equably; σ _qPeak strength for the uniaxial compression of rock; σ _QrResidual strength for rock; H is the softening stress-displacement stage poisson's ratio of rock; α is the well radius.

Borehole wall stability prediction method provided by the invention; Be suitable for the borehole wall stability prediction under the gas drilling condition; This method is the new method that on the basis of rock strength failure criterion, combines on-the-spot practical application effect to propose, and through the on-site actual situations check, has good effect.

Description of drawings

Fig. 1 is applicable to the borehole wall stability prediction method flow chart of gas drilling for the present invention.

The specific embodiment

The borehole wall stability prediction method that is applicable to gas drilling that the present invention proposes is explained as follows in conjunction with accompanying drawing and embodiment.

Based on theoretical research, gas drilling well Zhou Yanshi state is divided into failure zone, damage zone and is undisturbed the district.Well is bored and is opened the back because stress is concentrated, and after stress surpassed the elasticity limit state of the borehole wall, damage zone appearred in wellbore, and the micro-crack that simultaneous is a large amount of is damage zone; There are a large amount of cracks in the damage zone,, have only when the scope of damage zone surpasses a certain threshold the borehole wall just can cave in and destroy and form the failure zone because rock has residual strength.Damage zone threshold performance form is the destruction of whether caving in, and the factors such as residual strength of the strain softening characteristic behind its size and the rock material peak, shearing expansion characteristics, rock are relevant.

The present invention has set up the Forecasting Methodology that the gas drilling borehole wall stability is new that is applicable to based on the damage criterion.The theoretical foundation of institute's foundation is: well is bored and is opened the back because stress is concentrated, and produces damage in wellbore, and damage zone is gradual expansion and is divided into two kinds of situation.The firstth, the situation of stable expansion along with the increase of border upper stress, can settle out after the damage zone expansion, up to reaching critical faulted condition; Second kind of situation, if the border upper stress surpasses the ultimate bearing capacity of well, the de-stabilise expansion just takes place in damage zone, wellbore damage on a large scale occurs and loses stable state.

Embodiment

The present invention is support force P in the well corresponding when reaching critical plastic state according to the borehole wall _iJudge that the gas drilling borehole wall is whether stable, as shown in Figure 1, the method comprising the steps of: support force P in the corresponding well when obtaining the borehole wall and reach critical plastic state according to the mechanics parameters of rock resultant stress strain-gauge test data and rock _iJudge support force P in the said well _iValue whether greater than zero; If P _iLess than zero, show that then borehole wall damage zone does not reach critical condition, it is stable that the borehole wall keeps; If P _iGreater than zero, show that then the borehole wall has formed the residual impairment district on every side, the borehole wall is with caving in borehole well instability; Because support force is P in the well the when borehole wall reaches critical condition _{I, Critical}Be zero, promptly Sidewall Surrounding Rock reaches the critical condition that the remaining state of plasticity is defined as gas drilling just in borehole wall position, is the ultimate limit state of wellbore stability, the interior support force P of well that this ultimate limit state is corresponding _{I, Critical}Be zero, if support force P in the well _iEqual zero, borehole wall plasticity reaches critical condition when then showing gas drilling.

Support force P in the corresponding well when obtaining the borehole wall and reach critical plastic state according to the mechanics parameters of rock resultant stress strain-gauge test data and rock in the present embodiment _i, be based on rock strength failure criterion and critical damage zone scope r _cObtain.Support force P in the corresponding well when preferably obtaining the borehole wall and reach critical plastic state by formula (1) _i:

$P_i=\frac{2}{1-N}[\frac{\mathrm{AE}\′}{h+N}+\frac{(1-N){\mathrm{\σ}}_O-{\mathrm{\σ}}_q}{1+N}]{\left(\frac{r_c}{a}\right)}^{1-N}+\frac{{\mathrm{\σ}}_q(1+h)-2\mathrm{AE}\′}{(1-N)(h+N)}-\frac{{\mathrm{\σ}}_{\mathrm{qr}}}{h+N}---\left(1\right)$

Wherein, r _cBe critical damage zone scope, obtain by formula (2):

$r_c=a{\left\{\frac{\frac{2}{1-N}[\frac{\mathrm{AE}\′}{h+N}+\frac{(1-N){\mathrm{\σ}}_o-{\mathrm{\σ}}_q}{1+N}]}{p_i-\frac{{\mathrm{\σ}}_q(1+h)-2\mathrm{AE}\′}{(1-N)(h+N)}+\frac{{\mathrm{\σ}}_{\mathrm{qr}}}{h+N}}\right\}}^{\frac{1}{N-1}}---\left(2\right)$

Wherein, N is an intermediate variable, is confirmed by the unified angle of internal friction

of rock according to formula (3):

In formula (1), formula (2) and the formula (3), A is the elastic limit strain value that rock elasticity and plasticity are softened intersection around the borehole wall; E ' is the softening modulus of rock; σ ₀Be far field stress equably; σ _qPeak strength for the uniaxial compression of rock; σ _QrResidual strength for rock; H is the softening stress-displacement stage poisson's ratio of rock; α is the well radius.

Wherein,

Be one of rock intrinsic basic parameter, but the elastic limit strain value A of the softening intersection of rock elasticity and plasticity around the borehole wall, that the softening modulus E ' of rock can utilize prior art to pass through rock mechanics experiment is definite, the far field is stress σ equably ₀, rock the peak strength σ of uniaxial compression _q, rock residual strength σ _QrThe softening stress-displacement stage poisson's ratio h that reaches rock all can obtain through prior art, no longer details here.

Above embodiment only is used to explain the present invention; And be not limitation of the present invention; The those of ordinary skill in relevant technologies field under the situation that does not break away from the spirit and scope of the present invention, can also be made various variations and modification; Therefore all technical schemes that are equal to also belong to category of the present invention, and scope of patent protection of the present invention should be defined by the claims.

Claims (1)

1. a borehole wall stability prediction method that is applicable to gas drilling is characterized in that, this method may further comprise the steps:

Support force P in the corresponding well when obtaining the borehole wall and reach critical plastic state according to the mechanics parameters of rock resultant stress strain-gauge test data and rock _i

Judge support force P in the said well _iValue whether greater than zero;

If support force P in the said well _iLess than zero, then draw the borehole wall and keep stable;

If support force P in the said well _iEqual zero, borehole wall plasticity reaches critical condition when then showing gas drilling;

If support force P in the well _iGreater than zero, then draw borehole well instability;

Wherein, obtain P in this method _iBe based on rock strength failure criterion and critical damage zone scope r _cObtain;

In this method based on rock strength failure criterion and critical damage zone scope r _cObtain support force P in the said well by formula (1) _i:

Wherein, r _cBe critical damage zone scope, obtain by formula (2):

Wherein, N is confirmed by unified angle of internal friction according to formula (3):

In formula (1), formula (2) and the formula (3), A is the elastic limit strain value that rock elasticity and plasticity are softened intersection around the borehole wall; E ' is the softening modulus of rock; σ ₀Be far field stress equably; σ _qPeak strength for the uniaxial compression of rock; σ _QrResidual strength for rock; H is the softening stress-displacement stage poisson's ratio of rock; α is the well radius.

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