Flat-rheology invert emulsion drilling fluids have made strides in maintaining a fluid that is resistant to sag.
When flat-rheology drilling fluids were introduced a decade ago, they brought the promise of reduced drilling equivalent circulating densities (ECDs), break circulation pressures, and surge pressures while tripping and running casing. Primarily intended for drilling narrow pressure windows in deepwater environments, these benefits spanned into other critical well conditions such as extended-reach drilling and HP/HT drilling.
However, these drilling fluid systems have fallen short of their promises in some cases. Furthermore, questions have been raised with regard to maintaining the highest level of wellbore integrity when using these drilling fluid systems.
Improving flat-rheology systems
Flat rheology as it relates to drilling fluids generally refers to the characteristics of a particular fluid to demonstrate a minimal variance in a few key rheological properties across a wide temperature range. Those properties that relate most directly to annular hydraulics are used to define whether a fluid is "flat" or not. In most cases, these properties are the 6-rpm viscometer reading, the 10-minute gel strength, and to a lesser degree the yield point or low-shear yield point.
FIGURE 1. This figure represents the typical approach to flat-rheology invert emulsion systems. (Images courtesy of M-I SWACO)
The American Petroleum Institute (API) standard of measuring the rheology of oil-based drilling fluids at 66°C (150°F) has been expanded for evaluation of flat-rheology systems. Commonly, the rheology is also measured at two additional temperatures. For most deepwater wells, the typical temperatures are 4.5°C (40°F), 38°C (100°F), and API standard 66°C.
An additional measure to flat-rheology fluid systems is the evaluation of the gel strength progression over time. The inclusion of measured 30-minute gel strengths has added breadth to the API standard 10-second/10-minute test regime. The ratio of gel strength progression across the time interval indicates the degree to which break-circulation pressure and surge/swab effects will continue to increase following extended static periods. Due to the wide shift in temperature on deepwater wells, this effect can be exaggerated.
Using organophilic clay as viscosifier
With conventional invert emulsion drilling fluids, the use of organophilic clay as a viscosifier results in rheological properties that change greatly with the circulating temperature. This results in a drilling fluid that is thinner near the bit and becomes more viscous as it cools while traveling up the annulus to the flowline. During extended periods of drilling or circulating, a system will find a level of equilibrium that is greatly defined by the temperature gradient of a given drilling environment. Land-based or shallow-water offshore wells often exhibit flowline temperatures elevated significantly above ambient surface temperature.
However, in deepwater wells the cold temperature often associated with the surrounding sea conditions, coupled with the reduced annular velocity in the riser, cool the drilling fluid significantly. As a result, flowline temperatures are frequently below ambient surface conditions.
During extended static periods such as bit trips, casing operations, and operational upsets due to inclement weather, the drilling fluid in the hole equalizes to the surrounding environment. Thus, the observed temperature range for the fluid in the hole can range from less than 4.5°C to more than 177°C (350°F). The apparent viscosity of a conventional invert emulsion drilling fluid will also have a dramatic range. Gel strengths in a conventional system can be 200% to 300% greater at cold temperatures compared to the same fluid at a higher temperature.
FIGURE 2. Real-time processing of automated drilling fluids data with hydraulics simulations is shown here.
Due to this natural tendency, the typical approach to designing flat-rheology systems has been to reduce or eliminate organophilic clay in an effort to minimize cold temperature gelation (Figure 1). A polymeric viscosifier is added that becomes more active as the temperature increases. The combination of these two results in a fluid system that compensates for the normal viscosity shift at varied temperatures.
Addressing barite sag
This approach has led to some undesirable consequences. Most notable – and experienced throughout the industry – is the occurrence of barite sag that has become more frequent with the increased use of flat-rheology systems. The implications of barite sag extend into both dynamic and static conditions. While drilling, imbalances in circulating pressures can occur, and equivalent downhole densities fluctuate.
On wells with critical limits between pore pressure and fracture gradient, these small fluctuations can make the difference between inducing fractures and staying within the narrow operating window. Under static conditions, there is an ever-present risk of a well control event if a drilling fluid cannot resist a change in density. Varied mitigations have been made to compensate for this deficiency.
In some cases, nonorganophilic clays have been used in an effort to supplement the gel strengths provided previously by organophilic clay. However, a common practice throughout the drilling fluids industry has been to engineer flat but thick fluid systems. By doing so, the original intended benefits of improved control over lost circulation, the ability to drill within narrower margins, and improved tripping speeds have been marginalized.
By design, M-I SWACO has approached flat-rheology invert emulsion systems with the intention of keeping organophilic clay in the formulation. This requires manipulation of the normal organophilic clay chemistry. The RHELIANT Plus flat-rheology drilling fluid system, which is a second-generation flat-rheology system, has made strides in improving the balance between maintaining a fluid that is resistant to dynamic or static sag while at the same time reclaiming the original intended benefits of flat-rheology systems. One of the key improvements with the second-generation system is evident with the ability to increase the organophilic clay content by approximately 50% compared to previous-generation systems. Despite the increase in organophilic clay, gel strength progression has been notably reduced. Table 1 compares the gel strength progression of a typical conventional invert emulsion system to similarly specified flat-rheology systems.
The primary emulsifier and rheological modifier in the second-generation system work along with organophilic clay to deliver the flat-rheology profile. The emulsifier reduces organophilic clay gelation at low temperature and limits the progression of gel strength over time. The rheological modifier offsets the natural tendency of the organophilic clay to lose activity and become thinner at higher temperatures.
By doing this, a robust concentration of organophilic clay can be included in the fluid formulation that provides excellent suspension of weighting materials and cuttings, resilience against sag, and thermal stability of the fluid
under extended static periods exposed to downhole temperatures. Historically, the concentrations of organophilic clay required to provide these benefits would come with the undesirable effect of high gel strength progression.
TABLE 1. This table compares organophilic clay concentration vs. rheological properties in mineral oil-based formulations.
Ensuring wellbore integrity
Several new technologies are being employed to provide continuous improvement to ensure wellbore integrity while capturing the intended benefits of flat-rheology invert emulsion drilling fluids. Micronized mineral weighting agents can be used in place of normal barite. Consistent with principles of Stokes Law, smaller suspended solids require a less viscous fluid to maintain suspension.
Additionally, considerable resources are being dedicated to automated fluids measurement (Figure 2). Simple properties such as density, rheology, and particle size distribution are accurately monitored on a continuously updated basis using new sensors and equipment. More complex apparatuses are emerging for determining chemical composition, including critical concentrations of emulsifiers, rheological modifiers, and organophilic clay, among other parameters. This information is fed back to specialized software that performs real-time hydraulics simulations while drilling, tripping, and running casing.
Ryan Shursen, M-I SWACO, a Schlumberger company
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