Formation Damage
Return permeability testing was conducted to compare the performance of the HDNZ system against a conventional calcium bromide/zinc bromide solution of the same density.
Testing was conducted using cores of comparable permeability and minerology. The cores were vacuum saturated with completion brine, then heated to 265°F with a confining pressure of 1500 psi. Formation brine was flowed in the production direction until permeability stabilized, then 10 pore volumes of completion brine were introduced in the injection direction, and shut in for 60 minutes. Formation brine was again introduced in the production direction and allowed to flow until permeability had re-stabilized.
As expected, the HDNZ system outperformed the conventional calcium bromide/zinc bromide solution, offering return permeability of 60%, compared to 41% for the calcium bromide/zinc bromide.
Temperature & Pressure Effects
As with all clear brine fluids, the fluid density of the HDNZ system is a function of temperature and pressure. Due to the unique nature of the constituents of the HDNZ system, the thermal expansion and pressure compression coefficients utilized in API Recommended Practice 13J2 do not apply. Several downhole density models were developed, and the model selected offers a high degree of correlation with both experimental and field data.
Environmental
The HDNZ system was tested to ensure compliance with GOM and North Sea environmental regulations.
The system was tested to ensure that it met the Free Oil and Oil & Grease requirements of Section 6 of the EPA NPDES permit for Gulf of Mexico (GOM) discharges, using EPA Method 1617, Static Sheen, and EPA Method 1664A, Oil and Grease. Testing revealed no static sheen under Method 1617 and less than 5 mg/L oil and grease using Method 1664A, demonstrating compliance with the applicable sections of the NPDES permit, as well as the Oil and Grease requirement of Section 4 (Produced Water) of the NPDES permit. It was also verified not to contain any of the priority pollutants listed in Section 307 of the Clean Water Act and 40 CFR 401.1, thus meeting the Priority Pollutants requirement of Section 6 of the EPA NPDES permit.
7-Day Toxicity testing was conducted on both the HDNZ system and a 14.1 lb/gal calcium bromide solution, using Menidia beryllina and Mysidopsis bahia populations according to EPA-821-R-02-014 methodology. There was no notable toxicity difference between the HDNZ system and the conventional 14.1 lb/gal calcium bromide solution.
The system earned a Gold environmental rating for compliance with North Sea regulations.
Field Performance
The system was evaluated in two Gulf of Mexico wells between February and October 2015. The average water depth was in excess of 7000 ft., with mudline temperatures of approximately 40°F. Bottomhole temperatures (BHT) were approximately 265°F and the formation pressures were in excess of 21,000 psi.
Density
Surface densities were maintained at 14.5 lb/gal with additions of dry calcium bromide and 15.1 lb/gal HDNZ spike fluid. Due to the halide concentration, the system was extremely hygroscopic, and frequent additions of weighting material were required.
The downhole density matched predicted values, yielding an effective bottomhole density of 14.49 lb/gal.
Crystallization Temperature
Crystallization temperatures, both true crystallization temperature (TCT) and pressure crystallization temperature (PCT), remained stable and met expectations, The HDNZ system was extremely stable and maintained PCT values of 30°F at 15,000 psi and TCT values between 6 and 9 °F with very little maintenance.
Compatibility
The compatibility of the HDNZ system met expectations under field conditions. No significant incompatibility with formation fluids was observed. There was minor salt (NaCl) precipitation when the HDNZ system encountered formation water influx, but it was removed with normal filtration operations.
Compatibility of the HDNZ system with drilling fluids was consistent with that of conventional calcium bromide solutions and was minimized by effective displacement system design. The displacement systems used were very similar to those used with conventional systems.
The behavior of the system when exposed to control line fluids was similar to comparable solutions of calcium bromide under similar circumstances.
There were no unforeseen compatibility issues with elastomers. In over 175 days of exposure under wellbore conditions, no incompatibility issues have been observed.
Corrosion
The HDNZ system exhibited no scaling, pitting, or stress cracking corrosion (SCC) issues with the Q125, 13Cr, and 15Cr materials at bottomhole temperatures ranging between 250 and
Thermal Stability
The HDNZ system showed no signs of thermal decomposition at 265°F bottomhole temperatures or at elevated temperatures encountered during perforating operations.
Workability
Workability is defined as applicability of conventional completion fluid additive systems with the HDNZ system, as well as critical system characteristics. During the field trial, several criteria were monitored, including filtration, displacement systems, sweeps, fluid loss pills, viscosity, and friction.
The system viscosity and friction characteristics were extremely stable and met design criteria. The polymer systems utilized in displacement systems, fluid loss control packages and viscous sweeps performed well with respect to yield, yield time, temperature stability, and serviceability.
The HDNZ system was used as an integral component of the displacement systems. In this capacity, it performed well. No incompatibility with other system components was observed, and all field performance criteria were met.
Standard high-flow, high-capacity filtration systems were used with the HDNZ system and flow rates, solids retention, and clarity all met expectations. The filtration process had no impact on the HDZF fluid system characteristics, nor did the addition of oil adsorption materials.
Environmental
The HDNZ system met all HSE design criteria, and no system-specific issues related to GOM environmental regulation compliance were observed.
Lessons Learned
During the field evaluation, several key observations were made:
First, the HDNZ system is extremely stable. The fluid was stored on a workboat for over six weeks with no loss of density or attrition of properties. The fluid was pumped from the workboat through over 20,000 ft. of wellbore and 7,000 ft. of riser, without a non-density-related impact on the crystallization temperature. The system was exposed to a broad range of temperatures and pressures, and experienced no loss of fluid properties.
The system met all crystallization temperature requirements under the full range of operating conditions and was used to test the BOP stack to over 14,000 psi @ <40 °F.
Conclusions
Lab-scale performance testing indicates that the novel high-density, non-zinc, solids-free completion fluid (HDNZ) meets the challenges and requirements of ultra deepwater environments for fluid densities between 14.5 and 15.4 lb/gal, offering significant performance improvements over conventional aqueous halide and formate completion fluid systems.
Testing of the HDNZ system under actual field conditions has confirmed the lab-scale performance testing and revealed that the system is extremely stable and robust, and thus offers a viable alternative to conventional completion fluid systems.
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