Balancing increased risk with savings
In addition to achieving penetration rates with the Security FM2943 bit, the feat allowed the 13-3/8-in. casing string to be eliminated from the primary well design of the Donnatello well. The well is being drilled by the semisubmersible Maersk Jutlander. The vertical exploration well, with a planned total depth of about 4,500 meters, was drilled to the 9-5/8-in. casing point.
While the best offset comparison wells enjoyed the efficiencies of long-term development drilling programs, the Donnatello record 12-1/4-in. run in January was achieved in a hostile weather area while catching and hauling oil-based mud cuttings.
In addition, customer requirements for real-time Gamma Ray information limited BHA selection to a packed, rotary assembly, which was control-drilled through most of the lower 500 meters of hole.
Given these constraints, a drilling analysis system was employed to devise bit preservation techniques that enabled the single 12-1/4-in. FM2943 nine-bladed PDC bit to drill the record interval, which typically would have required a total of four to ten bits in 17-1/2-in. and 12-1/4-in. hole sizes.
In addition, in an innovative move for an exploration well, an oil-based mud was used, which required the rig to be upgraded to capture and contain oil-based cuttings.
Risk versus savingsAfter a review of the potential hole problems in both 17-1/2-in. and 12-1/4-in. sections of offset wells, the mud weight, pore pressure, and kick tolerance requirements for the target exploration well were established. This allowed identification of a "narrow drilling window" in which both sections could be risk-managed by drilling a single, extended 12-1/4-in. interval.
The narrow drilling margin created by a well design with one long 12-1/4-in. hole interval, and the problematic wellbore conditions anticipated, increased risk for operations in the Donnatello well. This increased risk had to be balanced against expected savings of more than $1.5 million dollars, resulting from improved drilling efficiencies and elimination of the 13-3/8-in. casing string.
The site of several large discoveries and field developments, the mid-Norway Haltenbanken is often drilled in water depths of more than 300 meters. The record Donnatello exploration well is in 325 meters of water. Exploration wells in the area typically are drilled primarily using water-based mud, while development wells are drilled with both water and oil-based muds.
A typical well on the Haltenbanken requires 20-in. casing at about 1,000 meters true vertical depth (TVD), with a 17-1/2-in. hole drilled through overpressured, soft Tertiary claystone/shales to about 2,000 meters TVD. On average, this interval requires one or two bits to complete.
The 12-1/4-in. section is then drilled through the shales and sands in the Cretaceous. The section is completed in overpressured shale in the top Jurassic. Problems with lost circulation in the sands, and extremely slow drilling in the Cromer Knoll interval of the lower Cretaceous, are characteristic of this complex interval, which typically requires a minimum of three bits to complete, and can take as many as eight.
The record-setting FM2943 bit was used through both of these intervals, drilling 2,167 meters from the 20-in. casing shoe to produce one long 12-1/4-in. interval, eliminating the 17-1/2-in. section entirely.
The FM2943 achieved an average rate of penetration (ROP) of 19.35 meters/hr. However, due to customer requirements on coring criteria, the lower Cretaceous interval containing the notoriously slow-drilling Cromer Knoll, was control-drilled at an average 10 meters/hr. By comparison, the best-in-class offset well drilled 1,767 meters over a similar interval at an average ROP of 13.0 meters/hr.
The two largest contributors to the successful bit run were use of oil-based mud and development of bit preservation techniques that enabled a single bit to drill the extended interval through difficult formations.
Oil-based fluidA review of offset wells revealed that the 17-1/2-in. interval was drilled almost exclusively with water-based mud, and experienced hole stability problems and lost drilling time due to the overpressured Tertiary claystones/shales. The 12-1/4-in. offset intervals experienced even worse hole stability problems when drilled with water-based mud.
In the offset well with the lowest mud weight, use of oil-based mud allowed stabilization of the overpressured shales while drilling the interval. In the record well, achieving the necessary balance between a mud weight high enough to hold back the overpressured shales while still preventing lost circulation, required a mud weight 0.75 ppg lower than any offset well using either water or oil-based mud.
Rig upgrades were also required in the Donnatello well to enable oil-based cuttings to be caught directly into closed container skips while achieving instantaneous penetration rates of 100 meters/hr, and drilling rates of more than 50 meters/hr.
Bit preservationIn order to extend bit life through the extended 12-1/4-in. section planned, the GeoMechanics(tm) computerized drilling analysis program from Security DBS was used to develop specific bit preservation techniques.
GeoMechanics provides rock compressive strength and bit mechanical efficiency analyses to predict and optimize bit performance and improve bit selection. In addition, a penetration rate model predicts expected performance at given operating parameters.
Based on recommendations, specific operating parameters were employed at the rig to limit PDC cutter damage due to sand, limestone and dolomite stringers. A significant number of laminated intervals were drilled containing stringers up to two meters in thickness, and with compressive strengths of 20 kpsi. In these sections, bit penetration rates would drop to 1-3 meters per hour.
Using the GeoMechanics drilling analysis system, a review of offset log data identified potential problem intervals before drilling began. Output from GeoMechanics analysis, available up to poster-size, may include stratigraphic column, formation logs, and a bit wear analysis, in addition to predictions of bit performance and recommended operating parameters for the given application. Certain parameters were posted on the rig floor so that the drillers could easily identify where the stringers were to be expected. In addition, continuous correlation with area offsets was done to identify and modify interval tops and thickness.
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