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.

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Oil-based mud (OBM) has strong inhibitive ability, which can prevent shale formations from hydration and swelling, and thus is often used in the development of shale gas reservoirs in China. Due to the low formation temperature (under 90 °C), high pressure (with pressure coefficient up to 2.2), long barefoot interval (up to 2138 m), borehole instability problem is easily encountered in dealing with the drilling of horizontal intervals of shale gas wells, which brings great challenges to drilling fluid technology. Maintaining wellbore stability is the most critical aspect of drilling operations of horizontal well intervals. However, most of OBM system can not prevent filtrate invasion into nano-sized pores and micro-fractures on shale surface, and subsequent cause borehole instability problems. The main reason is that conventional drilling fluid particles are too large to seal the nano-sized pore throats or micro-fractures in shales, and thus can not stop fluid invasion. A new type of composite particles is developed, and the mean particle diameter of composite particles is about 105.6 nm. These micro-nano particles (MNP) are added to OBM, and their influence on shale stability is evaluated. The core displacement experiment indicates that these micro-nano particles can reduce the shale permeability by a significant amount, and thus stop fluid invasion and inprove wellbore stability. The reason is that these particles are small enough to penetrate and seal the pore throats in shale, and built an effective internal mud cake possessing low permeability on shale surface, resulting in the reduction of filtrate penetration into the shale. The experimental results show that these particles can prevent pressure transmission and improve wellbore strength greatly. In addition, MNP have no effect on rheological properties of drilling fluids, and they are environmentally friendly, too. As a result, MNP can reduce fluid losses to the formation significantly, and consequently provide an excellent sealing effect, so MNP are an effective plugging agent for strengthening wellbore stability. Reduction of fluid losses and enhancement of wellbore strength can be achieved by addition of these micro-nano particles to oil-based drilling fluids. MNP-based drilling fluids have been successfully run for dozens of wells in Weiyuan-Changning and Shaotong shale block. The field application indicates that no borehole instability problems are encountered in the drilling process of horizontal well intervals. Using these MNP-based fluids containing micro-nano particles to seal the shale is a very powerful and economical approach to address borehole instability problem in troublesome shale formations. These MNP-based fluids are suitable for the drilling of long sections of horizontal laterals. In the future, the MNP-based drilling fluids might hold great promise to resolve shale instability problem.

Global energy demand has driven the petroleum industry to develop hydrocarbon resources from extremely harsh formations which contain ultra-high pressure and temperature (HPHT) reservoirs. Ultra-high density drilling fluids are critical to successful drilling and completion practices in all of these wells. In this paper, potential weighting materials were systematically evaluated and screened to accomplish an ultra-high density oil-based drilling fluid system (19.62 to 22.12lb/gal) aimed to utilize in ultra HPHT conditions (>30000psi and >410°F). Several potential high-density weighting materials were evaluated in the laboratory conditions. Basic properties (pure density, particle size/distribution, surface area etc.) were evaluated and compared. Special treatments were conducted to optimize the properties of weighting materials. HPHT filtration tests under static and dynamic conditions were conducted at higher than 410°F and 300 psi. Real cores with an average porosity of 19% and an average permeability of 50 mD were used in the filtration tests. Rheological properties, sag tendency, the volume of filtrate, and the filtrate cake characterization of oil-based drilling fluids were measured before and after heating at 410°F for 16 hours. Results revealed that ultra-micro manganese and ilmenite complex after suitable surface treatment could act as an ideal weighting material than ultra-pure barite or other materials, which could fail in rheology and sag controlling measurement with such high temperature and density. The viscosity and filtration analysis confirmed the stability and reliability of this novel ultra-high density oil-based drilling fluid. This study developed a challenged drilling fluid system under critical testing states, as well as established a systematical laboratory evaluation and screening procedure of weighting materials for ultra-deep wells and contributed recommendations on how to utilize it in the fields.