A combination of divalent base brine and high wellbore temperature presents significant challenges for high density aqueous reservoir drilling fluids. Such systems traditionally use biopolymers as viscosifiers; however, they are subject to degradation at elevated temperatures. Non-aqueous drilling fluids are thermally stable but complete removal of the filtercake is challenging and this can lead to formation damage. This paper describes the qualification and first deepwater drilling application of a unique aqueous reservoir drilling fluid at temperatures above 320°F. A high-temperature divalent brine-based reservoir drilling fluid (HT-RDF) and a solids-free screen running fluid (SF-SRF) were designed, both utilizing the same novel synthetic polymer technology. Calcium bromide brine was selected for use to minimize the total amount of acid-soluble solids in the drilling fluid. A comprehensive qualification was undertaken examining parameters such as rheology performance across a range of temperatures, long-term stability, fluid loss under expected and stress conditions (16 hours at 356°F), production screen test (PST), and various fluid-fluid compatibility tests. Return permeability tests were conducted on the final formulations to validate their suitability for use. The synthetic polymer technology provided excellent rheology, suspension, and fluid loss control in the fluid systems designed in the laboratory. To prepare for field execution multiple yard mixes were performed to verify the laboratory results on a larger scale. Additionally, a flow loop system was utilized to evaluate fluid performance under simulated downhole temperature and pressure conditions before field deployment. The final high temperature drilling fluid as designed provided rheological properties that met the necessary equivalent circulating density (ECD) requirements while drilling the reservoir. The fluid loss remained extremely stable and there were no downhole losses despite the depleted nature of the wellbore. Production screens were run straight to total depth (TD) with no wellbore stability issues after a three-day logging campaign. High temperature aqueous reservoir drilling fluids have historically been limited by the lack of suitable viscosifiers and fluid loss control additives. This paper outlines the design, mixing and logistical considerations and field execution of a novel polymer-based reservoir drilling fluid.

Open hole wireline logging, due to its lower overpull capacity relative to logging-while-drilling, is subject to increased stuck tool risk and can lead to costly fishing operations, lost-in-hole charges, and the need to sidetrack the borehole. The risk increases in higher overbalance environments typical for depleted reservoirs but there are other controls on sticking risk that can be managed. This paper describes the measures taken in an open hole wireline logging operation in a Deepwater Gulf of Mexico well under close to 7000 psi overbalance in a high temperature-high pressure (HTHP) slim-hole environment using water-based reservoir drilling fluid (RDF) to achieve a successfully efficient logging operation without incurring lost time attributed to conveyance related issues. A wireline program was designed that paid close attention to the RDF properties to ensure excellent fluid loss performance and a thin filter cake, a recipe to keep differential sticking in check. The target reservoir was an aeolian sandstone with a porosity of 19% and permeability in the tens of millidarcy range. Additional steps taken to mitigate sticking risks included good borehole cleaning practices, sticking assessment during drilling, use of high-pull capacity cable, minimization of tool contact area with the borehole through reduced tool string length and use of rollers and standoffs, minimization of stationary time within depleted reservoir, tension monitoring during wireline operation, and inclusion of a jar in the tool string to increase the chance of pulling free when stuck. A high temperature and pressure rated sampling tool was run in a 7″ borehole with a 25° open-hole deviation to acquire the pressure profile in the reservoir. Correlation was performed inside the casing such that the only stationary time the tool string experienced was during the pressure sampling operation. The sampling itself was limited to no more than ten minutes. As a result of the mitigation measures implemented, a total of five good pressure points were obtained in approximately 150 ft of reservoir interval. Negligible tension drop-off was observed when moving down from one pressure station to another and no overpull was observed when picking up the tools. The tool string was successfully retrieved at the surface. From the authors’ knowledge, this is the first time logging under such high overbalance has been performed with a water-based RDF. The fluid mentioned in this paper has been engineered to overcome the typical shortcomings of water-based RDF when it comes to differential sticking. Learnings from stuck tool experiences during previous wireline logging attempts in the same field will be described, providing the rationale for some of the mitigation steps pursued in this paper. The success of differential sticking mitigation during this logging program opens the door to possibilities for performing future open hole wireline logging under extreme conditions.