Drilling/completion fluids

Novel, Thermally Stable Fluid-Loss Pill Performs Better Than Guar-Based Gels

A novel solids-free fluid-loss pill for higher-temperature reservoirs has been formulated.

A novel solids-free fluid-loss pill for higher-temperature reservoirs has been formulated. This pill can be used effectively in reservoirs with temperatures up to 350°F. Static thermal aging at 320°F demonstrated no noticeable loss of gel strength for at least 20 days. With regard to thermal stability and fluid loss, the synthetic-polymer-based gel outperforms guar-/borate-based gels tested under similar conditions.

Introduction

High downhole temperatures put a strain not only on equipment but also on the complex chemistry of drilling muds, drill-in fluids, and viscous pills. Some conventional materials, such as polysaccharide gums and sodium tetraborate crosslinking agents, are demonstrably unstable or simply rendered useless under high thermal loads for extended periods. This paper describes the development of a material for one such challenge—replacement of biopolymer-based gels for high-temperature applications. Research produced a novel, workable aqueous monovalent brine-based gel derived from a synthetic water-soluble polymer. The gel, which can be used for a variety of downhole applications including well kills, perforations, and other fluid-loss applications, is thermally stable and can be applied at temperatures up to 350°F, conditions where both polysaccharides and borate crosslinking begin to fail.

The gel was specifically developed to be used in a 320°F gas reservoir. Gas regained permeability was used to assess any formation-damage issues of the gel in downhole applications. However, leakage of brine into the core during the hold-off period alters the water saturation of the core, adding complication in the regained permeability experiments because of water blockage.

Experimental Section

Gel Formulation. Gel was formulated with a high-density brine solution of a synthetic water-soluble polymer and a metal-based crosslinker. Further additives include pH buffers to lower the pH to an appropriate range for crosslinking to occur (pH of 4 to 5) and thermal stabilizers to mitigate gel decomposition by radical reactions and other processes. Concentration of polymer, the polymer/crosslinker ratio, and the pH of the formulation were tuned carefully to provide optimized gelling and fluid-loss properties. The formulations presented in this paper were based on 10.0-lbm/gal brine prepared by diluting stock 12.5-lbm/gal NaBr brine with water.

Kinetics of Gel Formation. To help ensure the rate of cure is sufficiently slow to allow spotting under reasonable rig conditions, a series of experiments was conducted to evaluate gel time of this system at different temperatures. A number of samples derived from the same master batch were placed in aging cells and pressurized to 500 psi. Next, the samples were placed individually in ovens at 350, 320, 300, 275, and 225°F, respectively, and were removed at predetermined time intervals. These samples were qualitatively assessed for cure, which is indicated by the resistance of the fluid to flow when held at a 45° angle.

Static Fluid Loss. Static fluid-loss testing was performed on gels using a high-pressure/high-temperature (HP/HT) filter press. First, the uncured formulation was added to fill a standard HP/HT cell with a 20-µm disk. The filter press was sealed, pressurized to 500 psi, and heated to 320°F for a previously determined time to set the gel. Next, flow was initiated through the gel with a 200-psi differential pressure and the volume of fluid was collected and recorded as a function of time at 320°F. Fluid-loss tests were run for 6 hours. The same experimental procedure was used to measure the fluid loss at 225, 275, 300, and 350°F.

Regained Permeability. Regained-permeability testing was performed to assess any formation-damage issues that could result when using the novel perforation pill in downhole applications. Berea sandstone core plugs (1-in. diameter and 2.5-in. length) were used in the regained-permeability experiments. The apparatus used in the regained-permeability measurements is depicted in Fig. 1. First, the core was mounted in a Hassler-type core holder. Approximately 2,000 psi of confining pressure was applied on the core. Approximately 500 psi of backpressure was maintained during the experiment. First, 3% KCl (30,000 ppm) was injected into the core from the reservoir side at room temperature at three different flow rates (1, 3, and 5 cm3/min). Stabilized differential pressure at each flow rate was measured, and pressure readings and fluid-injection rates were used to calculate the brine permeability at room temperature (70°F). The same procedure was repeated to examine the accuracy of the brine-permeability measurement at room temperature. Next, the core holder and the brine-transfer vessel were heated overnight to the targeted reservoir temperature (320°F).

jpt-2013-11-novelthermalfig1.jpg
Fig. 1—Schematic of regained-permeability apparatus.

Brine permeability of the core at 320°F was measured by injecting brine (3% KCl) at three different flow rates (1, 3, and 5 cm3/min) from the reservoir side. The same procedure was repeated until two consecutive permeability values with less than 5% deviation were observed. Measured brine permeability at 320°F was compared with the brine permeability at room temperature. Permeability values should be similar at both temperatures; however, different permeability values would be observed if the brine temperature (while it is passing through the core) is different from the targeted 320°F. This step can be used to measure the temperature of the brine when it is flowing through the core. Next, stabilized differential pressure along the core was measured while air was injected into the core at three different flow rates (10, 20, and 30 cm3/min) from the reservoir side. Air was injected into the core for a -longer period of time to establish the initial water saturation (Swi), and then gas permeability was measured at the Swi. Brine- and air-permeability measurements were repeated until two consecutive permeability values with less than 5% variation for both fluids were observed.

After the initial-permeability measurements were obtained, the novel fluid-loss pill was injected into the core from the production side with 1,000-psi constant pressure (500-psi differential pressure) for 72 hours. Fluid that leaked from the core was collected, and the total fluid loss was measured at the end of the 72-hour hold-off period. After 72 hours, humidified nitrogen was injected into the core from the reservoir direction at a constant differential pressure starting at 0.5 psi and increasing in 0.1-psi increments until a visual flow increase was established or a pressure spike was observed. This behavior could correspond to pressure decrease and confirm the breakthrough event. The observed pressure spike was used to estimate the liftoff pressure.

Regained permeability after the treatment was measured by injecting gas from the reservoir side and measuring the differential pressure drop of the core. First, gas was injected into the core at 10 cm3/min for a longer time period while the differential pressure was recorded continuously. Differential pressure decreased very slowly with time because of a decrease in water saturation of the core. Gas was injected into the core at three different flow rates (10, 20, and 30 cm3/min), and the stabilized differential pressure drop across the core was measured at each flow rate. Air-permeability measurement was repeated to check the accuracy of the measured permeability value. This procedure was repeated until two consecutive permeability values with less than 5% variation were observed. Finally, the core was dismounted from the core holder and visually inspected for any fractures or deformations.

Results and Discussion

The solids-free-gel formulation flows as a viscous liquid at room temperature (Fig. 2a). Viscosity of the pill will increase with increase in downhole temperature because of polymer crosslinking at elevated temperatures. The formulation will be gelled completely when the crosslinking of the polymer reaches a certain value (Fig. 2). At relatively lower temperatures, the formulation will require more time to form the gel because of the lower kinetics of the crosslinking reaction. As the temperature increases, time required to form the gel will decrease.

jpt-2013-11-novelthermalfig2.jpg
Fig. 2—Fluid-loss-gel formulation before and after aging at 320°F.

Fluid loss of this novel gel at different temperatures shows better performance compared with crosslinked guar gum. This can be attributed to the higher thermal stability of the synthetic-copolymer backbone in the formulation. At 320°F, (targeted reservoir temperature) 8.3 mL of fluid loss was observed in the first 30 minutes. After 6 hours, approximately 21.5 mL of fluid loss was observed at 320°F. Fluid-loss testing at other temperatures showed that, in general, an increase in fluid-loss volume was observed with an increase in experimental temperature. This can be explained by reduction in the gel stability with increasing temperature. In all the experimental temperatures, the novel fluid-loss pill performance is much superior to that of the crosslinked guar gum.

However, less than 10 mL of fluid was lost over a 72-hour coreflooding experiment at 320°F and 500-psi differential pressure using a Berea core. This is because of the permeability difference between the disk used in the fluid-loss test and the Berea core. Berea cores used in the coreflooding experiments have approximately 200-md permeability compared with the 20-µm pore-throat size in the disks used in the high-temperature fluid-loss experiments. The average pore size in the Berea rock is much smaller than the 20 µm in the disks.

Approximately 10 mL of fluid loss was observed during the hold-off time (72 hours). The pressure spike from liftoff was observed to be approximately 2.2 psi. Liftoff pressure measurement was repeated through several experiments; each time, liftoff pressure was observed to be between 2 and 3 psi. Lower liftoff pressure was observed because of the solids-free nature of the pill. Because of the lower liftoff pressure needed in the experiments, it could be concluded that this particular pill can be removed easily by the formation pressure upon well completion. Therefore, the pill can be used without any external breakers, which, in turn, reduces costs and other problems associated with using breakers such as corrosion, formation damage, and fluid incompatibility.

Considerably lower gas permeability was observed at the beginning of gas injection. This behavior was expected because of the higher water saturation (water blockage) of the core at the beginning of the experiment. Approximately 10 mL of fluid (approximately 2 pore volumes) was passed through the core during a 3-day hold-off period, which would increase the water saturation of the core. It was expected that, by the end of the test (hold-off period), the core would have been saturated completely with the brine. Gas-permeability value increased slowly with gas injection because of displacement of water by the gas. However, because of the unfavorable mobility ratio between gas and water, it took longer to displace additional water and re-establish Swi in the core sample. Finally, nitrogen permeability was stabilized at approximately 86 md. This observed value is approximately 91% of the initial gas permeability before the injection of the formulation.

Finally, the core was cooled to room temperature and removed from the sleeve. The core remained intact, and no fractures were observed. Some gel remained on top of the core. However, the remaining gel was not completely attached to the core surface.

This article, written by Special Publications Editor Adam Wilson, contains highlights of paper SPE 165096, “Performance and Formation-Damage Assessment of a Novel, Thermally Stable Solids-Free Fluid-Loss Gel,” by Pubudu Gamage, Jay P. Deville, SPE, and Bill Shumway, SPE, Halliburton, prepared for the 2013 SPE European Formation Damage Conference and Exhibition, Noordwijk, The Netherlands, 5–7 June. The paper has not been peer reviewed.