Electromagnetic measurement-while-drilling and logging-while-drilling (EM MWD/LWD) technologies have been identified as key enabling technologies that could have a significant impact on underbalanced drilling (UBD) operations in Oman. The Nimr field is one of many reservoirs in Oman that is mature and depleting. Technologies such as UBD have served to increase the value of wells drilled in this region during the past years by improving productivity and overcoming conventional drilling challenges.

The current practice of using concentric casing gas injection cannot physically achieve as high a reduction in bottomhole circulating pressure (BCP) as drillstring gas injection. Consequently, some severely depleted reservoirs cannot be drilled underbalanced using concentric casing gas injection because BCP cannot be reduced below pore pressure. Such severely depleted reservoirs could be drilled underbalanced using drillpipe gas injection. Previous to implementation of EM MWD, this technique was considered impractical because mud-pulse MWD/LWD telemetry imposed a significant limitation on the degree of underbalance achievable since the ability to decode mud-pulse telemetry is severely limited by the gas volume fraction in the drillstring during two-phase injection.

EM MWD in the Nimr field

During the Lekhwair UBD campaign in north Oman, Petroleum Development Oman (PDO) discovered that lower-than-expected reservoir pressures stressed the need for lower circulating pressures, and it became necessary to use drillpipe injection to achieve underbalanced conditions. EM tools were sourced and mobilized into the country.

Following the Lekhwair wells, the EM kit was moved to the Amin field in south Oman. Although there was no distinct technical need for the use of EM (Amin's reservoir pressures allowed the use of concentric casing injection of gas), an increase in efficiency in the drilling operations was possible. One could argue that the equipment was in the right place at the right time.

To study the EM transmission in Nimr field, formation resistivity logs, casing configuration, well geometry and drilling fluids data were gathered to determine their effect on EM signal propagation. Based on data received and running of simulation programs, it was concluded that EM transmission was achievable in Nimr field.

To steer the well in the most productive zone of the pay, formation evaluation in real time (gamma ray and resistivity) was required. Annulus pressure also was monitored in real-time to control UBD conditions. Neutron density was a further option not implemented on these wells.

Operations and results

On the first well, the EM signal was detected and data sent to surface through casing in real time while tripping in down to 3,970 ft (1,200 m), 85 ft (26 m) from the 7-in. liner shoe set at 90° inclination (Figure 1). The EM transmitter is usually run above MWD and LWD sensors to get deviation and formation evaluation data closer to the drillbit. However, due to the tight window of vertical depth imposed by geologists on the well, it was critical to obtain EM signal as soon as possible below the casing shoe to steer the well.

Another requirement was to control underbalanced conditions as soon as possible through monitoring annulus pressure while drilling. Therefore, the EM MWD/LWD string was run with the EM transmitter below LWD sensors. This strategy succeeded as EM transmission was detected as predicted when the drill bit was 40 ft (12 m) out of the casing shoe, thereby maintaining the desired underbalanced conditions and steering the well within the target pay zone.

During pipe connections, significant time savings were realized due to the ability of EM MWD to transmit data back to surface independent of rig operations and fluid properties. A survey was taken and transmitted while adding a new stand of drillpipe. Toolface readings were continuously transmitted to surface, allowing the directional driller to resume drilling ahead without waiting for full stabilization of the UBD system as previously required with mud-pulse MWD to decode the signal. Time savings averaged 7 minutes out of 22 minutes per connection performed every two singles. This is equivalent to a 30% time savings per connection using EM versus mud pulse. A total of 8 hours was saved during the 69-hour time period to drill the lateral section from 4,020 ft (1,226 m) to total depth of leg #1 at 7,900 ft (2,408 m).

Also, it was recognized that in this UBD environment, data rates achieved using EM MWD transmission were faster than with mud-pulse MWD.

As with the first well, the EM-MWD successfully transmitted data through the casing when tripping in the hole on the second well. The 7-in. liner shoe was at 4,850 ft (1,478 m). MD and EM transmission was achieved through the casing down to 4,764 ft (1,452 m) measured depth.

The time saved during pipe connections on the first well was not comparable to the second well due to a different connection procedure and the first use of drillpipe injection in this field.

Further results

Based upon the initial results seen in wells 1 and 2, evaluation of the successful introduction of EM technology to Nimr altered the objectives of its use. The increase in drilling efficiency observed during well 1 prompted the team to maintain the concentric casing well design and forego slimming down the well. The results of this are shown graphically in Figure 2, where effective daily rate of penetration increased to almost 2,300 ft (700 m) per day. In this instance, these increases in efficiency have resulted in "cost neutrality" of the underbalanced wells, whereby the total cost of the wells drilled underbalanced equates to those drilled conventionally overbalanced. Achieving cost neutrality in underbalanced wells is significant as the value equation is altered by elimination of the economic factors associated with UBD.

It is worth noting that similar cost savings could be realized if EM telemetry was used on conventional overbalanced wells. The new value equation, however, shows that underbalanced wells may be delivered at the same cost as a conventional well, with the added benefits associated with UBD technology.

Other issues

Observations were made during drilling that the data sampling rate was faster with EM compared with mud-pulse telemetry under the same conditions. Faster data rates helped early directional calculation, especially at the middle of the stand.

Another advantage to successful implementation of EM MWD technology with transmission through the casing was the ability to monitor annulus pressure while tripping out, thus allowing fluid level detection in the annulus (Figure 3). This provided an additional hour of rig time savings, as this operation was required to be performed with an echometer on previous wells where mud-pulse MWD was used.

Conclusions

The EM MWD/LWD system effectively transmitted downhole measured data in underbalanced conditions. PDO identified many advantages to implementing EM MWD technology for both concentric casing injection and drillstring injection techniques. Due to real-time formation evaluation measurement, the well was steered into the most productive zone. This success marks a new step in drilling underbalanced wells in Oman.

The successful introduction of EM technology to the Nimr field opens the door for the use of underbalanced techniques on other, more depleted reservoirs. The ability to use gas injection down the drillstring is key to reducing BCP beyond what is currently achievable with concentric annular injection.

Continual transmission of borehole data while drilling underbalanced had a progressively marked impact on operational efficiency. The use of EM telemetry contributed to underbalanced wells drilled in south Oman, reaching cost neutrality in comparison to their overbalanced neighbors.

Facilitating elimination of concentric casing through use of drillstring gas injection could significantly reduce overall well cost by allowing a slimmed-down well design. PDO estimated cost savings with the slim-well design to be about 15% per well.

Pore pressures in mature fields will continue to decrease over time, and there will be a greater need for enabling technologies such as underbalanced drilling to ensure continued exploitation of these reserves in a cost-effective manner. To effectively capitalize on the advantages of drillstring injection in drilling effectively at lower BCPs, a technology is required that is not subject to the constraints of multiphase flow dynamics. This is the essence of EM MWD technology.

Editor's note

This paper was originally presented as SPE 92617 at the 2005 IADC/SPE Drilling Conference and Exhibition, held Feb. 23-25 in Amsterdam, The Netherlands.

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