Evaluation method of degree of shale strength reduction in research on shale wellbore instability

Abstract

The invention discloses an evaluation method of degree of shale strength reduction in research on shale wellbore instability. The evaluation method comprises the steps that (1) a shale core or outcropwith one flat surface is adopted, the shale sample is placed in a rock strength continuous marking test system to be marked, strength values in all marked positions are acquired; (2) the whole shalesample is immersed into a working fluid to be soaked; (3) in the same position, the shale sample is marked again in the same direction as the first marking, and strength values in all marking positions after soaking are acquired; and (4) difference of the strength values between the same position before soaking and after being soaked is the degree of shale strength reduction, degree of reduction is divided by the strength values before soaking, and thus strength reduction rate is obtained. According to the evaluation method of degree of shale strength reduction in the research on shale wellbore instability, errors due to core taking damage in a traditional pressure resistance method are avoided, the evaluation method has the characteristics that accuracy, fastness and convenience are achieved, and damage degree of the shale sample is small, the heterogeneous characteristic of the shale can be effectively reflected, and influence on the shale strength and strength reduction degree of mineral components and structures is finely evaluated.

Description

technical field [0001] The invention relates to the technical field of shale, and more specifically relates to a method for evaluating the strength reduction degree of shale in the study of shale well wall instability. Background technique [0002] Different from wellbore instability caused by conventional expansive shale with strong water sensitivity, shale reservoirs usually have low water-sensitive clay mineral content, developed micro-fractures and bedding, and are prone to collapse and block , leading to the occurrence of wellbore instability. In addition, the current shale oil and gas wells are all developed in the form of horizontal wells with long horizontal sections, which makes the problem of wellbore instability more prominent. Therefore, it is very necessary to carry out research on the stability of shale wellbore. Accurately and comprehensively evaluating the strength characteristics of shale before and after contact with drilling fluid is the key to the stabil...

AI Technical Summary

Problems solved by technology

In the process of rock sample processing, it will cause a certain degree of damage to the rock.

Especially for shale with strong bedding and developed micro-cracks, it is very easy to fracture along the bedding plane during processing, which greatly reduces the accuracy of the compression test results; and the compression test can only obtain data at a certain depth point. Therefore, it is difficult to comprehensively and effectively evaluate the wellbore stability of shale

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