Displacement of synthetic mud from openhole horizontal wellbores

Tests focus on two products

Syed A. Ali, Byron C. Sketchler
Chevron USA Production
David Hines, S. K. Baijal, Warren Ray
Deep South Chemicals

Baseline for 7% solids [66,383 bytes]
Layout of gravel pack screen configuration [47,594 bytes]
Baseline for 13% solids [79,067 bytes]
Return permability [16,229 bytes]
Rheological properties of Petrofree at 77° [57,515 bytes]
Steady state pressure differential [19,936 bytes]
Openhole completions with slotted or pre-perforated liners are increasingly the completion of choice for horizontal wells requiring no sand control. Attaining maximum productivity from these wells is possible by preventing or removing mud damage to the formation before bringing the well on line.

Synthetic-based muds have proven to be excellent in terms of hole stability, cuttings integrity, lubricity, better penetration, and gauged holes. The common industry approach is to displace the synthetic or oil-based mud to a clear, aqueous fluid before production.

A breaker system is usually placed in the open hole to remove or destabilize the filter cake, thus reducing the drawdown required to initiate production. However, this approach can lead to problems.

For the synthetic fluid, displacement to completion brine can result in a thick damaging emulsion (sludge) from mud/brine incompatibility. An alternate approach is to displace the fluid with a specially formulated non-aqueous fluid and bring the well on production. This article presents results of laboratory core testing and flow model study performed to determine:

(1) The optimum solids loading of the synthetic mud that will cause minimal formation damage

(2) The efficiency of Gold Flush SA in displacing a synthetic (Petrofree - Baroid product) fluid from the wellbore.

Lab core testing

Laboratory tests were performed to determine the effect of simulated drill solids (Rev Dust) on Petrofree fluid, with respect to the permeability of fired Berea sand s tone cores. Laboratory data shows that return permeability decreased as the simulated drill solids were increased, with 13% solids loading giving the poorest return, and 2% solids loading giving the best results. In the core and flow-model testing, we have relied on Rev Dust to simulate the behavior of drill solids. While Rev Dust is readily available and possesses some of the qualities common to drill solids, the reactivity and behavior in filter-cake cleanup may represent a worse-case scenario, since not all reservoir sands may produce solids that are similar in behavior to Rev Dust.

Synthetic fluid

Various formulations of the Petrofree fluid and base ester were supplied by Baroid Drilling Fluids for testing. The rheological property data is shown in an accompanying chart.

Flow model testing

Tests were conducted in a 25-ft-long horizontal wellbore model to determine the efficiency of Gold Flush SA in displacing the Petrofree, with varying amounts of Rev Dust, from the wellbore. In these tests, the Petrofree, containing 7% or 13% Rev Dust, was displaced in a sequence of three spacers - initial base ester, Gold Flush SA, and final base ester. The amount of Rev Dust was selected on the basis of core-test data to represent a worse-case scenario. The final spacer of base ester was used to simulate the hydrocarbon production.

The flow model was designed to simulate flow conditions during the openhole and cased hole displacement. The casing (ID = 5.25 in.) and the tubing (10 = 2.25 in.) was fabricated from acrylic plastic. A 5.6-ft Halliburton Low Profile 6-gauge prepack screen with 50/70 gravel was attached to the tubing with a PVC coupling.

The tubing was connected to a centrifugal pump with a delivery capacity of 100 gal/min. The screen end was closed with a bull plug that allowed the displacement fluid to pass through screen into tubing/casing annulus. The fluid was circulated through five ports.

The clear outer casing allowed flow conditions to be viewed and videotaped. The differential pressure across the screen was recorded by two transducers and transmitted into a laptop computer. The 6-gauge prepack screen was used to simulate filtercake deposition on the formation face.

Flow experiments

The flow model was saturated with base ester and the initial differential pressure across the screen was recorded for steady state conditions at a flow rate of 84 gal/min (annular velocity of 134 ft/min). Thereafter, the base ester was displaced by 11.0 ppg Petrofree mud containing 7% Rev Dust (simulated drill solids). The mud remained in the model for 16 hours at 150°F and 100 psi to deposit a competent filter cake on the screen.

The Petrofree mud was displaced with base ester. The base ester, after 5 minutes contact time, was displaced with Gold Flush SA and allowed to soak for 5 minutes. The base ester was pumped again to displace Gold Flush SA and to record the final differential pressure across the screen. The flow rate was maintained at 84 gal/min during pumping of fluids. The test was repeated with the Petrofree containing 13% Rev Dust (simulated drill solids).

The differential pressure data was recorded at 30 second intervals. The maximum differential pressure across the screen for each spacer, and the pressure versus time were plotted.

The displacement of the Petrofree containing drill solids with lighter base ester resulted in some dilution of the mud. During this process, in a laminar flow conditions, only about 50% of the Petrofree was removed from the screen/model due to gravity segregation. The subsequent displacement of base ester with Gold Flush SA reduced the interfacial tension and facilitated the complete removal of the residual mud from the model.

The displacement of Gold Flush SA with the final spacer of base ester resulted in 100% cleanup, in the case of the Petrofree mud containing 7% drill solids. The final spacer of base oil was used to simulate the production of hydrocarbon.

In the case of the Petrofree containing 13% drill solids, the slightly lower removal efficiency was due partly to flow restriction through the screen, caused by higher solids content.

Conclusions

Based on the flow model data, the following conclusions can be drawn:

The initial spacer of base ester diluted the Petrofree mud and only partially removed the mud from the screen/model due to gravity segregation.
The reduction of interfacial tension between the Petrofree and the metal surface by the Gold Flush SA played a critical role in effectively removing the residual mud from the screen.
It appears that maximum limit of drill solids in the Petrofree is around 7% for efficient cleanup.

In wellbores completed with slotted or perforated liners, the Petro free cleanup should be done by displacing mud out with base ester, followed by Gold Flush SA, and then bring the well on production.

Acknowledgment

The authors would like to thank the management of Chevron U.S.A. Production for permission to publish this article.

Authors

Syed Ali is Technical Advisor for Chevron USA Production in New Orleans. He specializes in sandstone acidizing, formatin damage control, rock-fluid interaction, mineralogy, and oilfield chemistry. He holds an MS from Ohio State University and a PhD from Rensselaer Polytechnic Institute.
Byron Sketchler is a Project Drilling Superintendent for Chevron in the US Gulf of Mexico. He has over 18 years of experience in drilling and completion engineering and applications. He is a member of Chevron's Best Practices Sharing Team and holds a BSME from Louisiana State University.
David Hines is Vice President of Technical Services with Deep south Chemical in New Orleans. He has 32 years of oilfield experience, including 25 with Dowell Schlumberger and seven in chemical treatment (last four focused on wellbore displacement and cleanup of drilling fluids.
S. K. Baijal is a Technical Specialist with Deep South Chemical and has more than 30 years experience in formation damage, product development, log analysis, and reservoir engineering. Most recently, he was Director of Research & Development for Ambar.
Warren Ray is President and CEO of Deep South Chemical in Lafayette, Louisiana. He has been technical manager for several service companies and has over 25 years of chemical and wellbore experience. He holds a BS degree from the University of Southwestern Louisiana.