CN-116244938-B - Shale stratum drilling fluid density optimization design method

CN116244938BCN 116244938 BCN116244938 BCN 116244938BCN-116244938-B

Abstract

The invention provides an optimization design method of drilling fluid density of a shale stratum, which comprises the steps of obtaining physical parameters of a shale rock core, establishing a well wall instability mechanical model containing a weak surface structure according to a well wall instability mechanism, judging collapse conditions of surrounding rocks of a well according to a molar coulomb failure criterion and a weak surface failure criterion, determining minimum drilling fluid density and maximum drilling fluid density for maintaining stability of the well wall of the shale stratum according to collapse allowable degree of engineering, fitting hydrostatic column pressure relation between mechanical drilling speeds of the shale stratum and drilling fluid at different depths, calculating to obtain a drilling fluid density upper limit value at a critical mechanical drilling speed, and obtaining a design range of a drilling fluid density window of the shale stratum considering well wall stability and quick drilling according to the minimum drilling fluid density, the maximum drilling fluid density and the drilling fluid density upper limit value at the critical mechanical drilling speed. The optimization design method can obtain the lower limit of the drilling fluid density meeting the allowable collapse degree of engineering and the upper limit of the drilling fluid density of the optimal mechanical drilling rate.

Inventors

CHEN HAODONG
JIN YAN
LIU SHUJIE
LUO MING
LU YUNHU
LIANG JIWEN
WANG SHIGUO

Assignees

中海石油(中国)有限公司海南分公司
中国石油大学(北京)

Dates

Publication Date: 20260512
Application Date: 20230222

Claims (8)

1. The method for optimally designing the drilling fluid density of the shale stratum is characterized by comprising the following steps of: Obtaining physical parameters of a shale core; establishing a well wall instability mechanical model containing a weak surface structure according to a well wall instability mechanism; Judging the collapse condition of rock around the well according to the molar coulomb damage criterion and the weak surface damage criterion, and determining the minimum drilling fluid density and the maximum drilling fluid density for maintaining the stability of the well wall of the shale stratum by combining the collapse allowable degree of engineering; Fitting the relation between the mechanical drilling speed of the shale stratum and the hydrostatic column pressure of the drilling fluid under different depths of the drilled well, and calculating to obtain the upper limit value of the drilling fluid density under the critical mechanical drilling speed; obtaining a design range of a drilling fluid density window of the shale stratum considering well wall stability and quick drilling according to the minimum drilling fluid density, the maximum drilling fluid density and the drilling fluid density upper limit value under the critical mechanical drilling speed; the step of fitting the relation between the mechanical drilling speed of the shale stratum and the hydrostatic column pressure of the drilling fluid under different depths of the drilled well and calculating the drilling fluid density upper limit value under the critical mechanical drilling speed comprises the following steps: acquiring hydrostatic column pressures at different depths, and recording scattered points of the relation between the mechanical drilling speed and the hydrostatic column pressure under the same coordinate system; Performing linear fitting on a plurality of scattered points under the same coordinate system to obtain the upper limit value of the drilling fluid density under the critical mechanical drilling speed; The relation between the mechanical drilling rate of the shale stratum and the hydrostatic column pressure of the drilling fluid at different depths can be fitted through the following formula: P=0.00981ρH Wherein P is the hydrostatic column pressure of the drilling fluid, ρ is the upper limit value of the drilling fluid density at the critical drilling speed, and H is the well depth; the step of obtaining the design range of the drilling fluid density window of the shale stratum considering the well wall stability and the quick drilling according to the minimum drilling fluid density, the maximum drilling fluid density and the drilling fluid density upper limit value under the critical mechanical drilling speed comprises the following steps: Taking the minimum value of the maximum drilling fluid density meeting the well wall stability and the maximum drilling fluid density meeting the fastest drilling as the upper limit value of a drilling fluid density window; And taking the minimum drilling fluid density meeting the stability of the well wall as the lower limit value of the drilling fluid density window.
2. The method for optimizing design of drilling fluid density of a shale formation according to claim 1, further comprising the steps of, before the step of obtaining physical parameters of the shale core: collecting exploratory well data of an oilfield block; Judging the shale stratum area of the exploratory well according to gamma logging; coring the well wall at the shale formation to obtain shale cores.
3. The method for optimally designing the drilling fluid density of the shale formation according to claim 1, wherein the step of judging the collapse condition of the rock around the well according to the molar coulomb damage criterion and the weak surface damage criterion and determining the minimum drilling fluid density and the maximum drilling fluid density for maintaining the stability of the wall of the shale formation according to the engineering allowable collapse degree comprises the following steps: Acquiring the ground stress distribution of the shale stratum; respectively obtaining stress distribution values of the shale stratum and stress distribution values of the weak surface structure under a specific angle through coordinate transformation; respectively bringing the stress distribution value of the converted shale stratum and the stress distribution value of the weak surface structure under a specific angle into a molar coulomb damage criterion and a weak surface damage criterion to obtain collapse distribution around the well bore; and under the allowable collapse degree of engineering, obtaining the minimum drilling fluid density and the maximum drilling fluid density for keeping the stability of the well wall.
4. The method for optimizing design of drilling fluid density of shale formation according to claim 3, wherein the step of obtaining stress distribution values of the shale formation through coordinate transformation respectively comprises the steps of: acquiring stress distribution values of the shale stratum under a main stress coordinate system; converting the stress distribution value of the shale stratum from a main stress coordinate system to a geodetic coordinate system; And converting the stress distribution value under the geodetic coordinate system into a shaft coordinate system, wherein the stress distribution value under the shaft coordinate system is the current stress distribution value of the shale stratum.
5. The method for optimizing design of drilling fluid density of shale formation according to claim 3, wherein the step of bringing the stress distribution value of the shale formation after conversion and the stress distribution value of the weak surface structure under a specific angle into a molar coulomb failure criterion and a weak surface failure criterion respectively to obtain the collapse distribution around the borehole comprises the following steps: judging whether the shale stratum is damaged or not under the stress distribution value of the shale stratum according to a molar coulomb damage criterion; judging whether the weak surface structure is damaged or not under the stress distribution value of the weak surface structure according to the weak surface damage criterion.
6. The method for optimizing design of drilling fluid density of shale formation according to claim 5, wherein the step of judging whether the weak surface structure is damaged or not under the stress distribution value of the weak surface structure according to the weak surface damage criterion comprises the following steps: establishing a weak face coordinate system; Converting stress distribution values of the shale stratum from a shaft coordinate system to a geodetic coordinate system; converting the weak surface stress under the geodetic coordinate system into a weak surface coordinate system to obtain a stress distribution value of a weak surface structure under the weak surface coordinate system, and substituting the stress distribution value of the weak surface structure into a weak surface damage criterion; If the stress distribution value of the weak face structure meets the weak face damage criterion, the weak face structure is damaged under the stress distribution value of the current shale stratum.
7. The method for optimizing design of drilling fluid density of a shale formation according to claim 5, wherein the step of judging damage to the shale formation under the stress distribution value of the shale formation according to the molar coulomb damage criterion comprises the following steps: if the stress distribution value of the shale stratum meets the molar coulomb failure criterion, the shale stratum is indicated to be damaged under the stress distribution value of the current shale stratum.
8. The method for optimizing design of drilling fluid density of a shale formation according to any one of claims 1 to 7, further comprising, between the step of obtaining physical parameters of a shale core and the step of establishing a mechanical model of borehole instability according to a mechanism of borehole instability: and carrying out a rock triaxial compression test on the shale core, and acquiring physical parameters of the shale core.

Description

Shale stratum drilling fluid density optimization design method Technical Field The invention belongs to the technical field of oil and gas drilling, and particularly relates to a shale stratum drilling fluid density optimization design method. Background With the development of exploration drilling technology, huge natural gas resources exist in the deep sea, but drilling and exploitation are difficult due to the limitation of high-temperature and high-pressure conditions. When drilling deep plastic shale stratum, there are engineering problems of mud bag, stable well wall, low mechanical drilling speed, etc., which severely restrict the operation time, and bring high cost and safety risk to deep sea drilling. From the aspect of influence of mechanics angle on the stability of the shale well wall, the instability of the well wall is caused by the fact that the strength of the shale is low, stress near the well wall cannot be balanced, stress concentration and unbalance near the well wall are caused, and the shale is subjected to shearing or stretching damage to cause the instability of the well wall. The density design of the drilling fluid at present mainly takes the stability of the well wall into consideration, and the influence of the density of the drilling fluid on the mechanical drilling speed is not considered, so that the defect exists in the density window design of the drilling fluid. Disclosure of Invention The invention mainly aims to provide an optimal design method for drilling fluid density of a shale stratum, and aims to solve the technical problem that the density design of the existing drilling fluid mainly starts from the stability of a well wall and does not consider the influence of the drilling fluid density on the mechanical drilling rate. In order to achieve the above purpose, the invention provides a shale stratum drilling fluid density optimization design method, which comprises the following steps: Obtaining physical parameters of a shale core; establishing a well wall instability mechanical model containing a weak surface structure according to a well wall instability mechanism; Judging the collapse condition of rock around the well according to the molar coulomb damage criterion and the weak surface damage criterion, and determining the minimum drilling fluid density and the maximum drilling fluid density for maintaining the stability of the well wall of the shale stratum by combining the collapse allowable degree of engineering; Fitting the relation between the mechanical drilling speed of the shale stratum and the hydrostatic column pressure of the drilling fluid under different depths of the drilled well, and calculating to obtain the upper limit value of the drilling fluid density under the critical mechanical drilling speed; and obtaining the design range of a drilling fluid density window of the shale stratum considering well wall stability and quick drilling according to the minimum drilling fluid density, the maximum drilling fluid density and the drilling fluid density upper limit value under the critical mechanical drilling speed. In an embodiment of the present invention, before the step of obtaining the physical property parameters of the shale core, the method further includes the steps of: collecting field data of the oilfield blocks; Judging the shale stratum area of the exploratory well according to gamma logging; coring the well wall at the shale formation to obtain shale cores. In an embodiment of the present invention, the shale formation includes a rock body and a weak surface structure, and the step of determining a collapse condition of rock around the well according to a molar coulomb damage criterion and a weak surface damage criterion, and determining a minimum drilling fluid density and a maximum drilling fluid density for maintaining stability of a wall of the shale formation in combination with an engineering allowable collapse degree includes: Acquiring the ground stress distribution of the shale stratum; respectively obtaining stress distribution values of the shale stratum and stress distribution values of the weak surface structure under a specific angle through coordinate transformation; respectively bringing the stress distribution value of the converted shale stratum and the stress distribution value of the weak surface structure under a specific angle into a molar coulomb damage criterion and a weak surface damage criterion to obtain collapse distribution around the well bore; and under the allowable collapse degree of engineering, obtaining the minimum drilling fluid density and the maximum drilling fluid density for keeping the stability of the well wall. In an embodiment of the present invention, the steps of obtaining stress distribution values of the shale formation through coordinate transformation respectively include: acquiring stress distribution values of the shale stratum under a main stress coordinate system; converting the stres