New Low-Impact Drilling Fluid for Deepwater Applications
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This paper describes a low-impact, nonaqueous drilling fluid (LIDF) designed to minimize equivalent circulating density (ECD) increases and associated risks in deep water by reducing the effect of cold temperature on fluid viscosity. The fluid offers a superior low-viscosity profile and rapid-set, easy-break gel strengths while maintaining low shear-rate viscosity at high temperatures with optimal-weight material suspension. Field application demonstrated that the LIDF reduced the effect of temperature on the fluid rheological properties and minimized the risk of induced formation losses. These same rheological features reduced nonproductive time (NPT) associated with cement displacement and barite sagging.
The extreme conditions to which an offshore drilling fluid is subjected pose numerous challenges to achieving appropriate formulation. Among these is minimizing the effect of temperature on rheological properties. The low water temperature in deepwater environments cools the drilling fluid, producing an increase in fluid viscosity and gel strengths. The higher viscosity and gels require higher pump pressures to initiate and maintain circulation, which translates into higher ECD, higher spike pressures during pump initiation, and higher surge pressures while running casing. The increase in ECD could induce formation fractures with loss-of-circulation consequences.
Another rheological challenge results from the low shear rates the drilling fluid experiences at several sections of the well. There is a high risk of barite sagging during low shear if the rheological properties of the mud are not adequate to keep barite and cuttings suspended, or to allow cuttings removal for adequate hole cleaning. Therefore, the drilling fluid’s rheological properties must be low enough to address ECD/equivalent-static-density-window control and hole cleaning, and high enough to address barite suspension in a wide range of temperatures. For these reasons, controlling mud-rheology properties and temperature dependency of drilling fluids is crucial to ensuring successful offshore drilling operations.
Conventional invert emulsion drilling fluids use organophilic clays and rheological modifiers, together with the internal aqueous phase of the fluid, to create viscosity and suspension characteristics. These systems are the product of the optimal combination of emulsifiers, rheology modifiers, filtration-control agents, and viscosifiers. However, because they generate excessive progressive gel strengths in static conditions, high viscosity and high gel strength at low temperatures, and insufficient yield point and low shear values at high temperatures, their application offshore is limited.
Offshore applications require a drilling-fluid system that features minimal temperature dependence, builds low shear viscosity for an optimal-weight material suspension under dynamic and static conditions, and develops low and fragile gel strength over the entire low-/high-temperature range. These characteristics mitigate surge pressures when running pipe or initiating circulation, thus reducing the risk of fracturing the wellbore and the resultant mud losses, cement misplacement, and related NPT.
Conventional drilling fluids require a wide range of additives because emulsifiers and wetting agents often complicate field operations and compromise operational performance of the fluids. Additionally, conventional drilling fluids derive rheological properties primarily from organophilic clay. The LIDF system described in the complete paper uses unique components, each with a specific function. The combination generates a simple and versatile drilling-fluid system, maintains ease of use during drilling applications, and keeps optimal properties across a range of oil/water ratios (OWR) and a wide range of densities from 10 to 14 lb/gal. This approach simplifies engineering and logistics without compromising the system’s performance at the limit of its operational range.
Experimental Methods and Materials
The complete paper presents a detailed discussion of various laboratory fluid formulations and testing protocols to evaluate and compare the ability of additives to achieve required specifications. The base oil is an isomerized olefin, typically used in operations in the Gulf of Mexico and Brazil. Commercial additives, including alkalinity activators, 25% calcium chloride brine, rheology modifiers, filtration-control agents, and emulsifiers, were used to prepare samples.
Following a typical testing protocol for the LIDF, additional testing was conducted to evaluate acceptable formulations for the new LIDF at different OWR, densities, and temperatures, and varying contaminations.
The complete paper includes a discussion of the test results illustrated by charts and tables. Among the results is that the new LIDF provides acceptable low-profile viscosity at different densities and OWR. Small plastic viscosity (PV) ratios between low and high temperature are a consequence of reducing the influence of cold temperature in the fluid viscosity. Additionally, the 3- and 6-rev/min readings at 40°F are consistently lower than those values at 150°F. This fluid’s characteristic is achieved with a combination of surfactants adsorbed onto the clay surface and inhibited from creating structures at low temperature. These additives also reduce gel-strength formation, resulting in low, fragile values.
Low rheological profiles translate into low ECD at the bottom of the hole, low spike pressures, and low surge pressures while running casing. To complement the low rheological profile, yield point (YP), 6- and 3-rev/min readings, and gel strengths remain optimal over a wide temperature range. This produces a minimal effect on pump pressures and maintains excellent hole-cleaning suspension properties. Dynamic and static barite sagging results demonstrate the superior barite-suspension effectiveness.
A more-detailed analysis of low-end rheological properties showed the temperature independence of the new LIDF, remaining almost invariable at low shear rate. Unlike conventional fluids, the new fluid does not exhibit progressive gels at low temperature, so shear stress remains constant. According to the authors, the shear rate constancy is also likely to improve hole cleaning and cuttings/barite suspension.
Test results also concluded that rheological properties of the LIDF exhibited minimal variation with respect to temperature and pressure compared with conventional fluid.
An operator in Brazil successfully used LIDF synthetic-based fluid in an offshore well in 5,088-ft water depth. The operator’s objective was to drill 2,148 ft in a 14¾-in. section using an LIDF system. This section contained an area of reactive shale and final inclination at 88°. The mud weight was set at 9.3 lb/gal.
Among the challenges of this project was drilling a very soft formation with a narrow mud window. Collapse vs. fracture gradient of 0.7 lb/gal caused demanding hole cleaning. High dogleg severity and significant losses while drilling and running casing were encountered in previous wells.
Hydraulic simulation was required to evaluate wellbore-cleaning efficiency and pressure surge for the casing run, using rheological readings of this system. Calculated ECD at bottom depth for the new LIDF was 9.59 lb/gal, compared with conventional fluid, which showed an ECD of 9.73 lb/gal—a potential improvement of 0.14 lb/gal and a 32% reduction in annulus pressure. This substantial ECD reduction when narrow-pressure-window wells are drilled could mark the difference between finishing or not finishing a project.
The rheological properties of this fluid (Fig. 1) enabled flow rates to increase from 650 to 700 gal/min, which improved average rate of penetration (ROP) from 14 to 27 m/h without increasing operational risks. ROP values were increased safely and gradually, showing stable behavior of ECD values. The casing run and circulation before the cementing phase were executed successfully without a whole fluid loss in the formation. This represented a 100% mud-loss reduction compared with offset wells. Additionally, cementing operations were conducted at 200-psi-lower pressure than required for an offset well. The overall performance of the system exceeded the operator’s expectation, provided a new level of ECD management, and optimized overall drilling performance and wellbore construction.
- The new LIDF is produced by combining specialized organophilic clay, emulsifiers, dispersant, and polymers. The formulation offers an optimal low-shear-rate viscosity, maintains constant rheology across temperatures and pressures, and creates a nonprogressive gel structure that reduces hydraulic effect with a rapid-set/easy-break profile.
- Low-impact drilling-fluid components are environmentally acceptable for offshore use and meet environmental requirements for the Gulf of Mexico.
- The new drilling fluid is designed to manage hydraulic impact by maintaining the right viscosity in the right areas of the well. This contributes to optimal hole cleaning without putting excess pressure on the formation while remaining sag-resistant and very stable.
- The new LIDF requires a very small amount of treatment and does not lose fluid integrity even after long periods in static condition. It mitigates pressure spikes to reduce or eliminate mud-loss risk, and protects the formation from surge pressures, which increases the ability to run casing faster.
New Low-Impact Drilling Fluid for Deepwater Applications
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