1. A method for measuring the safety period of an extended reach well comprises the following steps:

s1, taking an exploratory well drilling core of a target stratum, processing 2m standard rock samples, and measuring physical parameters of the standard rock samples after saturated simulated formation water; m is a natural number greater than or equal to 5;

s2, taking two standard rock samples, and respectively carrying out uniaxial compressive strength test and triaxial compressive strength test to obtain the uniaxial compressive strength and the triaxial compressive strength of the standard rock samples;

s3, respectively carrying out long-term uniaxial compressive strength test and triaxial compressive strength test on the rest standard rock samples under constant load, and recording the failure period of each standard rock sample to obtain a logarithmic function of the uniaxial compressive strength along with time and a logarithmic function of the triaxial compressive strength along with time;

s4, setting a regularization period, and obtaining uniaxial compressive strength and triaxial compressive strength in a corresponding period according to the logarithmic function of the axial compressive strength along with time and the logarithmic function of the triaxial compressive strength along with time;

s5, obtaining an internal friction angle and a cohesive force under a regularization period according to the uniaxial compressive strength and the triaxial compressive strength obtained in the step S4, and accordingly obtaining a logarithmic function of the internal friction angle with time and a logarithmic function of the cohesive force with time;

and S6, obtaining the change rule of the collapse pressure of the extended reach well along the horizontal maximum ground stress direction and the horizontal minimum ground stress direction along with the time according to the calculation model of the collapse pressure of the well wall.

2. The method for measuring according to claim 1, wherein: in the step S2, the confining pressure adopted in the triaxial compression strength test is 5-15 MPa.

3. The assay method according to claim 1 or 2, characterized in that: in step S3, the constant load is 95% to 75% of the uniaxial compressive strength or the triaxial compressive strength.

4. The assay method according to any one of claims 1 to 3, wherein: in step S3, the logarithmic function of the uniaxial compressive strength with time is shown in formula (1);

the logarithmic function of the triaxial compressive strength along with time is shown as a formula (2);

UCS(t)＝a1-b1·ln(t) (1)

TCS(t)＝a2-b2·ln(t) (2)

wherein t is the time for bearing the load, d; ucs (t) uniaxial compressive strength at different times, MPa; TCS (t) is the triaxial compressive strength at different times, MPa; a1, b1, a2 and b2 are all empirical coefficients.

5. The assay method according to any one of claims 1 to 4, wherein: in step S4, the regularization period is 0-60 d.

6. The assay method according to any one of claims 1 to 5, wherein: in step S5, the internal friction angle and the cohesion are obtained according to formula (3) and formula (4), respectively;

in the formula (I), the compound is shown in the specification,internal friction angle at different periods, °; c (t) is cohesion in different periods, MPa; ucs (t) is uniaxial compressive strength at different periods, MPa; TCS (t) is the triaxial compressive strength in MPa at different periods.

7. The assay method according to any one of claims 1 to 6, wherein: in step S5, the logarithmic function of the internal friction angle with time and the logarithmic function of the cohesion force with time are respectively expressed by the formulas (5) and (6);

φ(t)＝a3-b3·ln(t) (5)

C(t)＝a4-b4·ln(t) (6)

wherein t is the time for bearing the load, d;the internal friction angle at different time; c (t) is cohesion at different times, MPa; a3, b3, a4 and b4 are all empirical coefficients.

8. The assay method according to any one of claims 1 to 7, wherein: in step S6, the borehole wall collapse pressure calculation model is represented by the following formulas (7) and (8);

in the formula, Pt-_HThe collapse pressure of the large-displacement well along the horizontal maximum ground stress direction is MPa, Pt-_hThe collapse pressure of the extended reach well along the direction of horizontal minimum ground stress, MPa, the internal friction angle at different time; c (t) is cohesion at different times, MPa; ζ is 0.95; α is 0.85; pp, σ_V、σ_H、σ_hRespectively pore pressure, overburden rock pressure, maximum horizontal stress and minimum horizontal ground stress (MPa) which are measured through a field floor drain experiment; h is the well depth, m.

9. The assay method according to any one of claims 1 to 8, wherein: the extended reach well refers to a long open hole extended reach well with a well deviation angle of more than 70 degrees and an open hole section length of more than 2000 m.

Background

The large-displacement drilling technology can economically and effectively explore and develop oil and gas fields on the sea, on the coast, in islands and in regions with severe ground conditions, reduce the number of platforms and artificial islands and reduce the number of oil and gas wells drilled. For old oil and gas fields, the original infrastructure can be used for drilling extended reach wells, the edge detection and development of the oil fields are accelerated, the oil production period is shortened, the oil drainage radius is enlarged, the single well yield is improved, the service life of the wells is prolonged, the yield and the final recovery ratio of the whole oil field are increased, and the investment is greatly saved.

However, the large-displacement well drilling period is long, the stratum and the shaft are exposed for a long time, how to accurately obtain the well wall safety period (namely the collapse period) of the large-displacement well section and determine the reasonable drilling fluid density to support the well wall so that the well wall is not collapsed is a key step for successfully implementing the large-displacement well drilling. Therefore, there is a need for a method of determining the safe period of extended reach wells.

Disclosure of Invention

The invention aims to provide a method for measuring the safety period of a large-displacement well in consideration of the fatigue strength of rocks, which is suitable for large-displacement drilling engineering in limited areas such as oceans, beaches, artificial islands and the like, and has the functions of obtaining the change rule of the collapse pressure of a well wall along with the exposed time of the well wall, and timely adjusting the performance of drilling fluid to maintain the stability of the well wall so as not to collapse to cause well hole landfill.

The invention provides a method for measuring the safety period of a large-displacement well considering the fatigue strength of rocks, which comprises the following steps:

s1, taking an exploratory well drilling core of a target stratum, processing 2m standard rock samples, saturating simulated formation water, and measuring physical parameters of the standard rock samples, such as parameters of actual diameter, length, density and the like; m is a natural number more than or equal to 5 (if less than 5, no rule can be established);

s2, taking two standard rock samples, and respectively carrying out uniaxial compressive strength test and triaxial compressive strength test to obtain the uniaxial compressive strength and the triaxial compressive strength of the standard rock samples;

s3, respectively carrying out long-term uniaxial compressive strength test and triaxial compressive strength test on the rest standard rock samples under constant load, and recording the failure period of each standard rock sample to obtain a logarithmic function of the uniaxial compressive strength along with time and a logarithmic function of the triaxial compressive strength along with time;

s4, setting a regularization period, and obtaining uniaxial compressive strength and triaxial compressive strength in a corresponding period according to the logarithmic function of the axial compressive strength along with time and the logarithmic function of the triaxial compressive strength along with time;

s5, obtaining an internal friction angle and a cohesive force under a regularization period according to the uniaxial compressive strength and the triaxial compressive strength obtained in the step S4, and accordingly obtaining a logarithmic function of the internal friction angle with time and a logarithmic function of the cohesive force with time;

and S6, obtaining the change rule of the collapse pressure of the extended reach well along the horizontal maximum ground stress direction and the horizontal minimum ground stress direction along with the time according to the calculation model of the collapse pressure of the well wall.

In the above determination method, in step S2, the confining pressure used in the triaxial compression strength test is 5 to 15 MPa.

In the above measuring method, in step S3, the constant load is 95% to 75% of the uniaxial compressive strength or the triaxial compressive strength, and the degree of decrease may be set according to the number of cores, for example, the degree of decrease is 5%: 95%, 90%, 85%, 80%, 75%.

In the above measuring method, in step S3, the logarithmic function of the uniaxial compressive strength with time is represented by formula (1);

the logarithmic function of the triaxial compressive strength along with time is shown as a formula (2);

UCS(t)＝a1-b1·ln(t) (1)

TCS(t)＝a2-b2·ln(t) (2)

wherein t is the time for bearing the load, d; ucs (t) uniaxial compressive strength at different times, MPa; TCS (t) is the triaxial compressive strength at different times, MPa; a1, b1, a2 and b2 are all empirical coefficients.

In the above measurement method, in step S4, the regularization period is 0 to 60d, and the number of days between intervals may be set according to the number of cores, for example, 0d, 1d, 5d, 10d, 30d, and 60 d.

In the above measurement method, in step S5, the internal friction angle and the cohesion are obtained from the formula (3) and the formula (4), respectively;

in the formula (I), the compound is shown in the specification,internal friction angle at different periods, °; c (t) is cohesion in different periods, MPa; ucs (t) is uniaxial compressive strength at different periods, MPa; TCS (t) is the triaxial compressive strength in MPa at different periods.

In the above-described measuring method, in step S5, the logarithmic function of the internal friction angle with time and the logarithmic function of the cohesion with time are represented by formulas (5) and (6), respectively;

φ(t)＝a3-b3·ln(t) (5)

C(t)＝a4-b4·ln(t) (6)

wherein t is the time for bearing the load, d;at different timesInternal angle of friction, angle; c (t) is cohesion at different times, MPa; a3, b3, a4 and b4 are all empirical coefficients.

In the above measurement method, in step S6, the borehole wall collapse pressure calculation model is represented by the following formulas (7) and (8);

in the formula, Pt-_HThe collapse pressure of the large-displacement well along the horizontal maximum ground stress direction is MPa, Pt-_hThe collapse pressure of the extended reach well along the direction of horizontal minimum ground stress, MPa, the internal friction angle at different time; c (t) is cohesion at different times, MPa; ζ is 0.95; α is 0.85; pp, σ_V、σ_H、σ_hRespectively pore pressure, overburden rock pressure, maximum horizontal stress and minimum horizontal ground stress (MPa) which are measured through a field floor drain experiment; h is the well depth, m.

The method is suitable for the long open hole section extended reach well with the well inclination angle of more than 70 degrees and the open hole section length of more than 2000m, and has wider application range, rather than the conventional extended reach well with the water vertical ratio of more than or equal to 2.

In the method, the rock is always in a state of saturated formation water, so that a 'dry core' state which does not exist in an actual formation is avoided, and the interference of strong hydration factors of the dry core is eliminated.

The method can reflect the long-period attenuation effect of the rock strength and represent the periodic collapse of the open hole section of the extended reach well caused by the attenuation of the rock strength.

Based on an indoor rock fatigue strength test experiment, by obtaining the attenuation rule of single/three-axis compressive strength of the rock along with time under a fixed load, the change rule of the strength parameter-internal friction angle and cohesive force along with time is further calculated, and finally the change rule of borehole wall collapse pressure along with time, namely the safe drilling period is obtained.

Drawings

FIG. 1 is a flow chart of the method of the present invention.

Detailed Description

The experimental procedures used in the following examples are all conventional procedures unless otherwise specified.

Materials, reagents and the like used in the following examples are commercially available unless otherwise specified.

As shown in FIG. 1, the method for calculating the safety period of the extended reach well considering the fatigue strength of the rock provided by the invention comprises the following steps:

1) and acquiring an exploratory well drilling rock core of a target stratum, processing the rock core into 12 (namely m is 6) standard rock samples by using an indoor standard core barrel, numbering the rock samples, saturating simulated formation water, and measuring physical parameters such as actual diameter, length, density and the like of each rock sample.

2) Uniaxial compressive strength test and triaxial compressive strength test were performed on cores No. 1 and No. 7, respectively, to obtain uniaxial compressive strength UCS of 40MPa, and triaxial compressive strength TCS of 70MPa under a confining pressure condition of 10 MPa.

3) And (3) respectively carrying out long-term single/triaxial strength tests on the cores No. 2-6 and No. 8-12 under constant load, wherein the set single/triaxial constant load is respectively 95%, 90%, 85%, 80% and 75% of the compressive strength of the single/triaxial, and the failure period of each core is recorded as shown in the following tables 1 and 2.

TABLE 1 core failure period under uniaxial constant load

No	1#	2#	3#	4#	5#	6#
							Uniaxial constant load/MPa	40	38	36	34	32	30
Destruction period/d	0	0.8	3.2	9.1	27.4	62.3

Core failure period under triaxial constant load of table 210 MPa confining pressure

According to the experimental data table, establishing a logarithmic function relation of the single/three-axis compressive strength along with time:

UCS(t)＝37.15-1.56ln(t)

TCS(t)＝65.67-2.65ln(t)

wherein t is the time for bearing the load, d; ucs (t) uniaxial compressive strength at different times, MPa; TCS (t) is the triaxial compressive strength, MPa, at different times.

4) Setting a regularization periodic table (values of 0d, 1d, 5d, 10d, 30d and 60d), and calculating the single/triaxial compressive strength under the corresponding period according to a logarithmic function of the single/triaxial compressive strength along with time, as shown in table 3:

TABLE 3 Single triaxial compressive Strength of the core at regular periods

Regularization period/d	0	1	5	10	30	60
							Uniaxial strength UCS (t)/MPa	40.73	37.15	34.65	33.57	31.86	30.78
Three-axis intensity TCS (t))/MPa	71.77	65.67	61.40	59.57	56.66	54.82

5) The internal friction angle and cohesion values at the regularization period were calculated according to the following formula:

in the formula (I), the compound is shown in the specification,internal friction angle at different periods, °; c (t) is cohesion in different periods, MPa; ucs (t) is uniaxial compressive strength at different periods, MPa; TCS (t) is the triaxial compressive strength in MPa at different periods.

The internal friction angle and cohesion values at the regularization period were calculated as shown in table 4:

table 4 internal friction angle and cohesion of core at regular cycles

Establishing an internal friction angle according to the calculation resultAnd the cohesion force C (t) as a function of time logarithmically:

φ(t)＝28.49-1.02ln(t)

C(t)＝11.01-0.26ln(t)

wherein t is the time for bearing the load, d;the internal friction angle at different time; c (t) is the cohesion in MPa at different times.

6) According to the following borehole wall collapse pressure calculation model, calculating the change rule of the collapse pressure of the extended reach well along the horizontal maximum ground stress direction and the horizontal minimum ground stress direction along with time, wherein the formula is respectively as follows:

in the formula (I), the compound is shown in the specification,t is the borehole drilling time, d;the internal friction angle at different time; c (t) is cohesion at different times, MPa; ζ is 0.95; α is 0.85; pp, σ_V、σ_H、σ_hRespectively pore pressure, overburden rock pressure, maximum horizontal stress and minimum horizontal ground stress (MPa) which are measured through a field floor drain experiment; h is the well depth, m.

TABLE 5 rule of collapse pressure change with time under regularization period

Regular period/d	0	1	5	10	30	60
							Collapse pressure Pt-H(g/cm3)	1.211	1.241	1.282	1.295	1.318	1.331
Collapse pressure Pt-h(g/cm3)	1.25	1.28	1.321	1.334	1.357	1.37

In the prior art, through an indoor rock mechanics experiment, the strength of a dry rock core is taken as a reference point, the strength decreasing rule of the rock core after being soaked in drilling fluid for different time is measured, and a collapse period is calculated on the basis of the strength decreasing rule, but the collapse period does not accord with the shaft condition under the real geological condition. In fact, on the one hand, the majority of the rocks faced by the well are the water-forming rocks (namely sedimentary rocks) formed by depositing for millions of years in the formation water environment, and are in a long-term water-containing state rather than a dry state; on the other hand, the drilling fluid activity can be kept to be matched with the formation water activity or lower in the drilling process, and the rock strength is slightly influenced by the water content. Therefore, the actual stratum condition cannot be reflected by the existing method by taking the dry core strength as a reference point and considering the collapse period calculation method of rock hydration.

The method considers the long-period attenuation effect of the rock strength, reflects the real geological environment, ensures that the rock is always in the state of saturated formation water, avoids the dry core state which does not exist in the actual formation, and eliminates the interference of strong hydration factors of the dry core. The method reflects the influence of the long-period attenuation effect of the rock strength on the collapse pressure, and calculates the safety period of the open hole section of the extended reach well according to the influence.

The above description is only an exemplary embodiment of the present invention, and should not be taken as limiting the scope of the invention, and any person skilled in the art should understand that they can make equivalent changes and modifications without departing from the concept and principle of the present invention. It should be noted that the components of the present invention are not limited to the above-mentioned whole application, and various technical features described in the present specification can be selected to be used alone or in combination according to actual needs, so that the present invention naturally covers other combinations and specific applications related to the present invention.