Method for determining safe drilling fluid density of coal bed based on structural element analysis model

A technology of drilling fluid density and analysis model, which is applied to the drilling composition, chemical instruments and methods, and determines the specific gravity by measuring the pressure difference. Work and other issues

Inactive Publication Date: 2011-11-02
CHINA UNIV OF PETROLEUM (BEIJING)
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Problems solved by technology

[0003] The coal seam has cleats, well-developed fractures, high brittleness, low strength, significant anisotropy and discontinuity, and its wellbore instability characteristics are ...

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  • Method for determining safe drilling fluid density of coal bed based on structural element analysis model
  • Method for determining safe drilling fluid density of coal bed based on structural element analysis model
  • Method for determining safe drilling fluid density of coal bed based on structural element analysis model

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Embodiment Construction

[0046] In order to have a clearer understanding of the technical features, purposes and effects of the present invention, specific implementations of the present invention will now be described with examples.

[0047] 1. Determine the relevant geological parameters of the target coal seam, such as the distance between cleats a / r i =0.1, end cleat spacing b / r i =0.1, cleat dip angle θ 1 = θ 2 =45°, ground stress σ 2 / σ 1 =0.9, cleat surface friction angle

[0048] 2. p in judgment formula (10) c The sign of the coefficient c. E.g Corresponding to c>0, Corresponding to c The corresponding value of c changes to a plus or minus sign.

[0049] 3. Determine the safe drilling fluid density. Figure 4 middle Corresponding to c>0, the safe mud pressure should be greater than the critical value (maximum value of the curve); Corresponding to c Figure 5 Medium beta 1 In the intervals of 11-20 degrees and 63-78 degrees, the value of c is close to 0, indicating that the st...

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Abstract

The invention discloses a method for determining the safe drilling fluid density of a coal bed based on a structural element analysis model. The method has the advantages of coal bed cleat and fracture development, big rock brittleness, low strength and obvious anisotropy and discontinuity. The borehole wall instability characteristics of the method are different from those of the common sandstone stratum. The phenomenon that the traditional borehole wall stability model based on the mechanics of continuous media is difficult to be successful for solving coal bed instability problems can be eliminated. In the method, the coal bed is composed of discrete blocks. A coal briquette which has the biggest possibility of generating spallation between surface cleats is analyzed to carry out force analysis. A structural element model of collapse pressure is built by the stressed balance condition among pressure stress, shearing strength and shaft pressure. A coefficient c is introduced to judge the stable relationship of borehole pressure and borehole wall so as to judge the range of the safe drilling fluid density.

Description

technical field [0001] The invention relates to a method for determining the density of a safe drilling fluid for coal seam drilling, so as to ensure the quality of the drilling borehole. Meanwhile, the invention is also applicable to the determination of the density of a safe drilling fluid for fractured formations of other lithologies. Background technique [0002] Coalbed methane is an important direction for the development of unconventional natural gas at present. With the promotion and deepening of coalbed methane exploration and development, various deep wells, horizontal wells and wells with complex structures have been widely used, and the problem of wellbore stability in corresponding drilling has become increasingly prominent. There are two groups of nearly vertical cleats in coal rocks. The surface cleats in the main fracture group are relatively well developed and can extend up to hundreds of meters. The end cleat group develops between the surface cleats and com...

Claims

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Application Information

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IPC IPC(8): E21B47/00G01N9/26C09K8/04
Inventor 赵海峰陈勉郑力会
Owner CHINA UNIV OF PETROLEUM (BEIJING)
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