A Quantitative Evaluation Method of Anti-Sloughing Drilling Fluid Inhibition for Deep Mudstone

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.

Study of Wellbore Instability and Collapse Mechanism for a Layered Continental Shale Oil Horizontal Well

“…The maximum collapse density can reach 1.38 g/cm 3 and the minimum is 0.91 g/cm 3 under the condition of this study. The collapse density values at azimuth angles of 50°, 130°, 230° and 330° corresponding to well deviation angles of 0~90° are low, so it is relatively safe to drill along this direction.…”

“…Figure 9 shows the variation law of collapse pressure equivalent density with well inclination and azimuth under strike-slip fault condition (σ H > σ V > σ h ). The maximum collapse density can reach 1.38 g/cm 3 and the minimum is 0.91 g/cm 3 under the condition of this study. The collapse density values at azimuth angles of 50 • , 130 • , 230 • and 330 • corresponding to well deviation angles of 0~90 • are low, so it is relatively safe to drill along this direction.…”

“…Figure 8 shows the variation law of collapse pressure equivalent density with well inclination and azimuth under normal fault condition (σ V > σ H > σ h ). Under the conditions of this study, the collapse density can reach up to 1.41 g/cm 3 and the lowest is 0.83 g/cm 3 . When the azimuth angle is constant, the collapse pressure shows an increasing trend with the increase in the inclination angle.…”

“…Under the conditions of this study, the collapse density can reach up to 1.41 g/cm 3 and the lowest is 0.83 g/cm 3 . When the azimuth angle is constant, the collapse pressure shows an increasing trend with the increase in the inclination angle.…”

“…With the increasing importance of unconventional oil and gas resources in the global energy structure, shale oil has become a hot area of energy exploration and development [1][2][3]. The development of continental shale oil in China is still in its infancy.…”