Fiber-containing sweep fluids validated for ultra-deepwater drilling applications

JAMES M. PAPPAS, P.E., RPSEA Ultra-Deepwater Program

While using fiber sweeps to improve borehole cleaning in the drilling process seems possible, the oil and gas industry has, thus far, failed to grasp a comprehensive understanding of the physics behind the use of fibrous particles in a field setting. For this to be achieved, a series of tests must first be conducted, prior to scale-up, to test hypotheses in a controlled setting. Such a project was recently completed through the Research Partnership to Secure Energy for America (RPSEA) Section 999 Program by The University of Oklahoma with support from MI-SWACO.

The overall objectives of this 2008 project, RPSEA Project 08121-2902-07, were to:

• Improve the understanding of fiber-containing drilling sweeps (fiber sweeps)
• Develop fiber sweeps, which improve hole-cleaning in ultra-deepwater drilling operations
• Evaluate fiber sweep variables to avoid lost circulation, stuck pipe and other hole-cleaning related problems
• Develop models and correlations that can be used to predict fiber sweep performance, to ease designing procedures and optimize sweep applications
• Recommend the best practices to apply fiber sweeps
• Demonstrate the potential for fiber sweeps to reduce drilling costs, improve operational safety, and minimize the impacts of drilling on the natural environment.

DISCUSSION

The project was conducted in several stages. It began with extensive literature review on fiber-containing fluid systems. Literature review and theoretical investigations on rheology, hydraulics, stability, and carrying capacity of fiber-containing drilling sweeps, were undertaken. The outcomes of this task were used to develop a mechanistic model to predict the rising velocity of fiber particles suspended in fluid. Sensitivity analysis was conducted to examine the effects of fluid properties on the rising behavior of buoyant fiber particles. Figure 1 is an example of rising velocity versus yield stress for one fluid. The following conclusions were reached, based on the results of the analysis:

• In highly shear-thinning fluids with no measurable yield stress, the rising velocity of the fiber was sensitive to the consistency index, “K” 
• As the flow behavior index, “n”, decreased, the fluid became increasingly shear-thinning. The decrease in “n” allowed for a greater influence of “K” on the rising velocity of the fiber for a given yield stress, “τy” 
• Fluid density had a greater magnitude of influence on rising velocity at high-yield stress values
• The model developed for a vertically oriented fiber particle over-predicted the rising velocity, while the model formulated for a horizontal particle provided reasonable predictions.

Fig. 1. Rising velocity vs. yield stress varying fluid density for horizontally-oriented fiber particle (n = 1.0 and K = 2.1 lbfsn/100 ft2).

Bench-top experiments were aimed at developing fiber sweep formulations that have superior stability and optimal fiber concentration at ambient and high temperature conditions. The experiments used different base fluids (water-based and oil-based fluids) with varying fiber concentrations. Fibers were extracted from the samples after each test and weighed to determine the final fiber concentration in the top layer. These data were then used to determine whether the fiber had risen to the top or remained consistent in concentration, in the fluid sample throughout the length of the experiment. These measurements were compared with stability predictions obtained from the mathematical model. The following inferences were made:

• Horizontally oriented particle model predictions generally concurred with the experimental data. Therefore, reasonable, real-time application performance could be predicted using the model.
• The vertically oriented particle model overestimated the rising velocity of fibers in all tested fluids. It did not reflect experimental results and could not provide accurate predictions.
• Despite the dominant effect of fluid rheology on the stability of fiber sweeps, other fluid properties, such as polymer type, presence of fluid structure, or hindering effect of other particles, considerably influenced stability.
• The polymer type selected for drilling sweep applications was critical in designing fluids that remain stable under downhole conditions.
• Oil-based and synthetic-based fluids had high fiber stability, which was attributed to the high yield stress that they exhibited, and the presence of emulsion structure in the two-phase system.

The effects of temperature and fiber concentration on rheology of fiber-containing sweeps were then evaluated. Rheology experiments were conducted, using rotational viscometers to measure the rheology of base fluids and fiber-containing fluids at ambient temperature and 170°F. Figure 2 is a graph of shear stress versus rate for several fiber concentrations. The shear viscosity profiles of fiber sweep fluids were analyzed, and the following conclusions were drawn:

• The addition of fiber up to 0.08% by weight slightly affected the fluid’s shear viscosity profile, whether at ambient temperature or 170°F. Some instances indicated increases in viscosity, and others showed a decrease with increasing fiber concentration.
• Fluid temperature increase resulted in a decrease in the non-Newtonian behavior of the fiber fluid, and a decrease in the viscosity throughout the shear rate range of 2 to 1,000,s-1
• In most cases, as fiber concentration increased, the viscosity showed increasingly non-Newtonian behavior: in the Yield Power Law model, “n” decreased, while “K” and “τy” increased.
• Neither oil-based nor synthetic-based fluids exhibited any significant shear viscosity sensitivity to fiber concentration at ambient temperature or at 170°F. It may be possible for oil-based or synthetic-based mud sweeps to be utilized in the field, with no increase in equivalent circulating density (ECD).

Fig. 2. Rheology of weighted (12.2-ppg) oil-based fluids at 72°F and 170°F varying fiber concentrations: a) OBM; and b) SBM.

Settling behavior of spherical particles in fiber-containing fluids, ranging from 0% to 0.08% by weight, fiber concentration, was investigated. There was a significant reduction in settling velocity in fiber-containing fluids. Figure 3 depicts settling velocity curves for varying fiber concentrations in a given fluid. As a result of the investigation, the following were concluded:

• Fiber particles that were uniformly dispersed in fluids hindered motion and reduced the settling velocity of suspended particles.
• Fiber concentration (up to 0.08% w/w) had a negligible effect on the rheological properties of the fluid.
• Fiber drag was related to: fiber concentration, fluid properties, size, density, and settling velocity of the particle.
• Particles suspended in fiber sweep exhibited an additional drag force under both static and dynamic conditions.
• The correlations developed in this study provided reasonable settling velocity predictions in fiber suspensions.

Fig. 3. Predicted and measured settling velocity vs. fiber concentration for 0.25% xanthan gum

An experimental study was conducted next to determine the wellbore cleaning efficiency of fiber-containing, water-based and synthetic-based fluids. The two fluids, un-weighted xanthan gum (XG)-based fluid and weighted synthetic-based mud, were each mixed with varying fiber concentrations. A flow loop test apparatus (Fig. 4) was utilized to conduct the cleaning experiments, as well as to measure pressure drop in a pipe viscometer. By varying flowrate, inclination angle, fiber concentration and pipe rotation, a wide array of data was gathered, which was then used to evaluate the fiber sweep optimal usage conditions. The following were concluded from these experiments:

• Very dilute concentrations of fiber in the XG-based fluid had no significant influence on measureable pressure loss, when compared to the fiber-free base fluid.
• Introducing fiber into synthetic-based fluid resulted in an increase in (i.e., a larger) pressure loss at low flowrates. However, this disparity disappeared as the flowrate increased, and the pressure loss versus flowrate curves converged at high flowrates.
• For a non-rotating pipe case, a noticeable increase in cuttings removal was not observed when increasing fiber concentration. It was further surmised that the inertia of the weighted fluid was great enough to mask any slight improvement that might be gained by adding fiber to the system.
• With pipe rotation, cuttings removal increased with an introduction of fiber to the system. Only a slight amount of fiber was necessary to observe the improved wellbore cleaning performance of the fiber sweep over the base fluid sweep. 
• Maximum wellbore cleaning performance with a very diluted concentration of fiber was achieved at maximum flowrate and pipe rotation.

Fig. 4. Flow loop schematic diagram

Then, the hole-cleaning performance of fiber-containing sweep fluid was investigated in flow loop experiments by varying inclination angle, fiber concentrations, pipe rotation and flowrate. Adding fiber to the fluid had a significant effect on hole-cleaning efficiency, when applied in conjunction with inner pipe rotation. It was found that:

• Fiber sweeps provided better borehole cleaning than the base fluid in horizontal and highly inclined configurations.
• In the presence of the pipe rotation, adding fiber substantially improved sweep fluid efficiency.
• Increasing the fiber concentration with pipe rotation considerably improved cuttings transport.
• Pipe rotation also had a substantial effect on bed erosion and fiber sweep applications.
• Based on pipe viscometer and rotational viscometer measurements, the quantity of fiber added to the sweep fluid had only minor effects on rheological and hydraulic characteristics of the fluid. Mechanistic modeling provided better prediction than the existing model.

Cuttings transport model predictions agreed with the experimental data at low flowrates for both base fluid and fiber-containing fluid.

SUMMARY

A stability model for fiber sweeps was developed and theoretically analyzed, which bracketed the selected ranges of base fluid properties that are suitable for fiber sweep applications. The stabilities of commonly used base fluids were tested, and stable formulations were identified. A rheology study was conducted, which determined that the rheologies of test samples remained roughly the same, even as the fiber concentration increased. Extensive settling experiments were subsequently carried out to assess fiber sweep carrying capacities, and a mathematical model to predict the settling behavior of particles in fiber sweeps was developed. This model was applied to formulate a mechanistic hole-cleaning model. Results from flow loop tests showed the hole cleaning performance of the fiber sweep under different conditions, and validated the mechanistic model. They could be used to calibrate models that are based on the generalized conservation laws, and are applicable for both field and lab-scale measurements. The full report can be found online at http://www.rpsea.org/attachments/contentmanagers/4898/08121-2902-07-FR-Fiber_Containing_Sweep_Fluids_UDW-03-03-12_P.pdf 

ACKNOWLEDGEMENT
The author thanks Dr. Ramadan Ahmed and Matthew George, The University of Oklahoma, as well as representatives from MI-SWACO. RPSEA is a 501(c)(3) organization that is entrusted with managing research and development projects for the U.S. Department of Energy’s National Energy Technology Laboratory, which was established pursuant to Title IX, Subtitle J, Section 999, of the Energy Policy Act of 2005. RPSEA is a multi-purpose entity that facilitates a cooperative effort to identify and develop new methods and integrated systems for exploring, producing and transporting to market, energy or other derivative products from ultra-deepwater and unconventional natural gas and other petroleum resources, and to ensure that small producers continue to have access to the technical and knowledge resources necessary to continue their important contribution to energy production in the U.S.