Improve The Practicability Study Of The Core Drillability Evaluation

The determination and classification of rock drillability are an important basis for drilling speed in deep well.In China,the earliest classification standard(SY/T 5429-1991)of rock drillability referenced to Rollow AG's method(1962).After increased PDC microbit,which has been revise to SY/T 5429-2000,and again been revise to SY/T 5429-2016 in 2016.The currently standard has four major problems that hinder its efficient application.Firstly,the drillability test is carried out at normal temperature and pressure,and it fails to simulate the actual physical environment of the formation rock.This results in a serious discrepancy between the measured results and the actual drilling results.Secondly,the single point test method leads to a large amount of core,high cost and low evaluation efficiency.Thirdly,drillability(or drilling rate)measurement results only reflect the effects of rock material properties.Because of the differential pressure,bit pressure,rotational speed and so on are not considered,so this results can not comprehensively and effectively guide the drilling speed.Fourthly,not consider the influence of saturated fluid core and pressure penetration on the rock drillability.In 2010,Yingfeng Meng et al.invented a microdrilling test device that can simulate the actual physical environment of the formation rock,and solved the first problem.In this paper,the above two,three and four problems are carried out research by using this test device.The concrete research content is as follows:(1)Analysis of petrophysical characteristics.The petrophysical characteristics of the horizontal(longitudinal)wave speed,mineral composition and content,microstructure,porosity and permeability were tested,which can provide data support for selection of rock samples and theoretical analysis.In order to ensure that the test samples are as homogeneous as possible,there are no microcracks,concretions and other internal defects.(2)Study on the experimental method to improve the evaluation efficiency of core drillability.On the basis of fully analyzing the shortcomings of the existing experimental methods,the structural parameters of the cone microbit and the rock sample size.Through the cone microbit and experimental method were improved.This paper deduces the calculation method of the improved drillability test,and sets the comparison experiment to verify the accuracy of this experimental method.Finally,a set of simulating rock-breaking tools and physical environment of the core experimental methods to improve the evaluation efficiency of core drillability.(3)Research and application on drilling speed equation of the cone microbit.This model is based on the research of drilling speed equation as background,and the drillability experimental data of retouching bottom tested by the improved bit,and then based on the mechanical energy of the unit diameter to establish the drilling speed equation model,which is verified by experimental data.Finally,it is necessary to set up a drilling speed equation of which considering multiple drilling parameters(bit pressure,rotational speed and wellbore pressure)with as few cores as possible,it aims to quantify the influence of each parameter on the drilling speed(or drillability),and provide theoretical basis for guiding drilling speed.And,using the drilling rate equation corrected drilling rate of the continuous drillability experiment,so the entire experimental results can be accurate to use,which can greatly enhance the efficiency of the evaluation of rock drillability.(4)Analyze on the influence of pressure penetration on the drillability of sandstone in different saturated mediums.Based on the theory of rock capillary pressure theory,we analyze the stress state of the rock surface before and after penetrating through the core throat by hydraulic oil.This paper systematically analyzes the reasons for the drillability variation trend of different saturated mediums(gas,oil,water)sandstone using different types of microbits tests under different wellbore pressure.We put forward the concept of three types of compaction effect(surface compaction effect,areal compaction effect,framework compaction effect),which provides a new analysis idea for the reason that the rock drillability variation trends with increasing the wellbore pressure.