Abstract
Wellbore instability resulting from deep mudstone hydration severely restricts the development of oil and gas resources from deep reservoir in western China. Accurate evaluation of drilling fluid inhibition properties plays an important role in selecting drilling fluid that can control deep mudstone hydration and then sustain wellbore stability. The previous evaluations are conducted by qualitative analysis and cannot consider the influence of complex hydration conditions of deep mudstone (high temperature, high pressure and flushing action). The study proposes a quantitative method to evaluate drilling fluid’s inhibition property for deep mudstone under natural drilling conditions. In this method, the cohesive strength of mudstone after hydration is adopted as the inhibition index of the tested drilling fluid. An experimental platform containing a newly designed HPHT (High pressure and high temperature) hydration experiment apparatus and mechanics characterization of mudstone after hydration based on scratch test is proposed to obtain the current inhibition index of tested drilling fluid under deep well drilling environments. Based on the mechanical–chemical wellbore stability model considering strength weakening characteristics of deep mudstone after hydration, a cross-correlation between drilling fluid density (collapse pressure) and required inhibition index (cohesive strength) for deep mudstone is provided as the quantitative evaluation criterion. Once the density of tested mud is known, one can confirm whether the inhibition property of tested mud is sufficient. In this study, the JDK mudstone of a K block in western China is selected as the application object of the proposed evaluation method. Firstly, the evaluation chart, which can demonstrate the required inhibition indexes of the tested fluids quantitatively with various densities for JDK mudstone, is constructed. Furthermore, the experimental evaluations of inhibition indexes of drilling fluids taken from two wells in K block are conducted under ambient and deep-well drilling conditions, respectively. In order to show the validity and advantage of the proposed method, a comparison between the laboratory evaluation results and field data is made. Results show that the laboratory evaluation results under deep-well drilling conditions are consistent with the field data. However, the evaluation under ambient conditions overestimates the inhibition property of the tested fluid and brings a risk of wellbore instability. The developed quantitative method can be a new way to evaluate and optimize the inhibition property of drilling fluid for deep mudstone.
Highlights
The exploration and development of oil and gas resources from a deep reservoir in western China is showing great potential
The developed HPHT experimental apparatus is utilized to conduct the hydration of deep mudstone for different times under deep-well drilling conditions
The mechanical properties of deep mudstone after hydration are estimated based on scratch test
Summary
The exploration and development of oil and gas resources from a deep reservoir in western China is showing great potential. Wellbore instability resulting from deep mudstone hydration is still a troublesome problem to drilling engineers, which causes tremendous economic losses. The capability of drilling fluid to inhibit deep mudstone hydration is a key indicator of wellbore stability. To accurately evaluate drilling fluid inhibition property is a prerequisite for choosing the drilling fluid with good performance and sustaining wellbore stability. (1) Methods to evaluate the physical and mechanical properties of solid medium after hydration (swelling, dispersion and strength): 1 hot rolling dispersion test [1,2,3]; 2 slake durability evaluation [2,4,5]; Based on the analysis of evaluation principles, these evaluation methods can be clarified into two types. (1) Methods to evaluate the physical and mechanical properties of solid medium after hydration (swelling, dispersion and strength): 1 hot rolling dispersion test [1,2,3]; 2 slake durability evaluation [2,4,5];
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