Published: 02/28/2011
Published: 02/28/2011
Well Information |
|
| Location | Hungary |
| Spud | May 5, 2009 |
| Interval drilled | 8 ⅜ in. hole for 442 m (1,450 ft) and 5 ⅞ in. hole for 408 m (1,339 ft) |
| Maximum Bottomhole Temperature | 168° C (335° F) |
| Completion type | emented Liner |
| Maximum Mud weight | 2.28 s.g. (19.0 lb/gal) |
| Total well depth | 3,750 m (12,303 ft) |
A major operator wanted to drill an exploration well with the target a high-temperature high-pressure (HTHP) tight gas reservoir. Other challenges that had to be considered were acid gas contamination (CO2 and H2S) and environmental limitations that would put constraints on cuttings disposal and OBM usage.
Based on previous experiences in the area and extended lab testing, M-I SWACO recommended the newly engineered ENVIROTHERM NT HTHP water-based drilling fluid. The polymer-based drilling fluid not only resists gelling at high temperatures, but its chrome-free water-base fluid formulation is designed to meet or exceed most environmental regulations. The system has been shown to resist temperatures as high as 400° F (204° C).
The well was drilled safely to total depth (TD) with the drilling fluid proving to be very stable for this harsh environment. The HTHP fluid loss was controlled easily while the rheology was actually lower than that shown in the lab tests. The reduced rheology helped minimize the equivalent circulating density (ECD) for the extremely high mud weight.
After the 9 5/8-in casing was run and cemented, the drilling fluid used in the previous interval (KCl/K2CO3/GLYDRIL*) was treated for HTHP conditions. The first step was cleaning with centrifuges and flocculation to lower drilled solids content. Afterwards, the standard rheology and fluid loss polymers were replaced with HTHP-resistant additives. To provide solids suspension, hole cleaning and HTHP fluid loss control, high-temperature polymers, including CALOVIS FL, were used. In addition, ASPHASOL SUPREME* and BLACK FURY* were added to improve filter cake quality and help stabilize HTHP filtration. With increasing density to control pore pressure, the addition of high-temperature stabilizers were required (CALOTHIN, CALOSPERSE Zr and K-17). Same strategy was used during both 8 3/8 in and 5 7/8 in sections, HTHP product concentrations being gradually increased towards well TD.wn in the lab tests. The reduced rheology helped minimize the equivalent circulating density (ECD) for the extremely high mud weight.
The level of reactive and drilled solids was minimized to prevent high viscosity. The solids control equipment included two MONGOOSE* PT shakers dressed with 230-mesh screens; two high- speed centrifuges in the barite recovery mode and a flocculation unit. This configuration proved to be very effective. The pH ranged between 10.5 and 11.5 and total hardness was maintained at 300-400 mg/l by continuous addition of caustic soda, lime and gypsum to prevent CO2 contamination. The VIRTUAL HYDRAULICS* engineering software proved invaluable in predicting ECD for drilling and surge / swab pressures while tripping.
“When drilling hot wells with very high density mud, an oil-base mud (OBM) is usually the only option. The new ENVIROTHERM* NT water-base drilling fluid provided stability and high performance in a very challenging environment.”Cristian Martinescu – Technical Services Manager (Fluids) - Europe
If you’d like to know more about ENVIROTHERM NT and how it’s performing for our other customers, please call the M-I SWACO office nearest you.
Challenge: Barla field is located in Arges County, Center-South Romania, and together with the Bacea and Surdulesti fields, is known for partial-to-total losses in a relatively shallow and loose Meotian shaly sand formation. Many wells drilled in these fields encountered significant losses; blind drilling had to be employed in some of the most severe instances. A new drilling campaign was initiated in 2007 to drill several production wells in the depleted fractured limestone Inferior Cretaceous formation that lies below the loose Meotian. The operator requested a solution that would first prevent or minimize lost circulation but still sustain directional drilling. Also, the Meotian formation was not as thick as expected; medium-reactive clays had to be drilled at the same time as the thief zone, demanding a system with superior inhibitive characteristics. In addition, the field is in an environmentally sensitive area requiring minimum waste.
Solution: The EMS-2700 system was chosen because of its well-documented abilities to control losses and cuttings-carrying capacity even at reduced circulation flow rates. These characteristics (combined with elevated low-shear-rate viscosities) to reduce erosion of the mechanically weak Meotian formation were considered essential to enable fast drilling, very good hole cleaning in the directional section, and most important, to reduce/eliminate lost circulation that could result in lengthy remedial operations. The EMI-795 inhibitor facilitated trouble-free drilling of the clay interval while supporting high efficiency of the solids-control equipment and reduced generated waste.
Results: 1. The EMS-2700 system allowed drilling the interval without mud losses and reduced drilling time and costs. 2. The interval reached TD ahead of time with a 36.4% reduction versus planned days. 3. The new system, in addition to preventing losses, enabled faster drilling, improved hole stability and reduced interval costs. 4. Using the EMS-2700 system, the total interval cost, including chemicals, engineering, solids-control equipment and waste management, was 37.1% less than planned, enabling significant savings to the operator. 5. The system’s inhibitive nature resulted in hole stability and improved solids-control equipment efficiency (80% versus 75% planned). 6. The actual disposal factor was 1.77 m3/m3 hole volume compared to 2.12 m3/m3 planned hole volume, resulting in 155 MT less generated waste and reduced disposal costs.
