Figure 1. The feasibility of using electromagnetic transmission with a logging-while-drilling triple combo tool was recently tested successfully. (Images courtesy of Weatherford)
Logging-while-drilling (LWD) tools are past the gee-whiz stage in the oil and gas industry. But despite their wide use and acceptance, there are limitations to where these tools can effectively be used. For LWD tools to work, information has to be transmitted to the surface. One available method is mud-pulse telemetry, which restricts and opens the flow of the drilling fluid, causing pressure waves in the drilling fluid for carrying data information to the surface. Currently, there is a limit to the bottomhole circulating pressures that can be achieved using conventional mud-pulse measurement-while-drilling (MWD) systems due to the limited ability to decode pressure pulses through a compressible, multi-phase drilling fluid.

The other alternative is the use of electromagnetic (EM) telemetry. The electromagnetic signal is transmitted by propagation of electromagnetic waves through the formation with the drill string and casing acting as a wave guide. Underbalanced drilling (UBD) using compressible fluid (i.e. multiphase fluid) eliminates the mud-pulse option when the gas volume fraction of the fluid becomes too high. But EM transmission combined with LWD triple combo logs and annulus pressure sensors had previously not been tried in underbalanced applications.

Saudi Aramco wanted to test these limits. The company had determined that UBD could provide a step change in performance in one of its producing fields by enabling the drilling of power water injection (PWI) wells without excessive formation damage while improving injectivity and eliminating the need for acid stimulation. The water injection would in turn maintain pressure for optimum production of oil from the fractured, oil-bearing carbonate reservoir.

In the past, hydrocarbon recovery in this reservoir had been achieved by drilling overbalanced vertical and deviated well bores. Lost circulation and stuck pipe were common headaches. Additionally, the mud weight required exceeded the reservoir pressure, often resulting in mud filtrate invasion and drilled solids penetration into the carbonate fracture system. Extensive acid stimulation was often required to bring back the injectivity of the PWI wells.

Weatherford engineers were brought in to assess the viability of UBD to reduce well costs and optimize well design. Ultimately the team settled on a well design where the entire lateral length of the well was placed in the lower reservoir pressure. The reservoir sections were drilled as underbalanced open hole using flow drilling techniques, paving the way for optimum UBD operations.

Geosteering
The next step was to evaluate the feasibility of EM transmission in conjunction with LWD triple-combo and annulus pressure sensors in the horizontal section. The LWD measurements are important to keep the well in the zone, and the pressure information is required to monitor the underbalanced conditions. Gas injection is also needed to maintain underbalance, thus precluding the use of mud-pulse telemetry. As an alternative, EM telemetry needed to be proven to allow a continuation in the UBD planning process to include real-time LWD technology.

Weatherford introduced the EM-LWD concept to Saudi Aramco by testing it in a high-pressure PWI well. A successful proof-of-concept would allow Aramco to use two-phase flow down the drillstring to maintain UBD conditions in depleted reservoirs while at the same time acquiring real-time LWD data for geosteering, downhole pressure monitoring and survey applications. Saudi Aramco engineers preferred the drillpipe injection method as opposed to concentric casing injection because it gave them the ability to achieve the target bottomhole circulating pressure, maintain hole cleaning requirements and optimize drilling performance regardless of reservoir pressure and well profile. Another benefit is overall cost savings with a reduced requirement for gas injection and UBD surface equipment.

Case study
To study the EM transmission, the formation resistivity logs, casing configuration, well geometry and drilling fluids data were compiled to determine their effect on the EM signal propagation. Based on this work, it was determined that the EM transmission would be marginal for wells with total measured depth of 11,000 ft (3,355 m). An extended-range setup was run with a lengthy downhole antenna.

The intention of the trial was to prove the feasibility of EM-LWD technology in this environment. The 61¼8-in. horizontal reservoir section was drilled from a measured depth of 8,109 ft (2,470 m) to a total depth of 11,198 ft (3,413 m), a total of 3,080 ft (940 m) in the formation.

The UBD concept used the flow drilling technique with a semi-closed loop freshwater system and bottomhole circulating pressures engineered to be below the reservoir pressure throughout the section drilled.

Figure 2. The EM-LWD triple combo.
After 53 ft (16 m) of the lateral section was drilled, the well was shut in at the UBD manifold choke to establish the static reservoir pressure. This was monitored in real time via the EM-LWD system. This pressure test is required to confirm the UBD model and minimize influx.

Drilling resumed at 100 ft/hr (33 m/hr) to verify the density of real-time LWD data for geosteering purposes. The rate was increased once the log performance was deemed satisfactory. A strong EM signal allowed 100% detection of the LWD data while drilling. Real-time bore and annulus pressures were also transmitted to monitor the static and equivalent circulating densities to ensure adequate hole cleaning.

The entire lateral section was drilled in one bit run and in 35 hours. All directional drilling targets were met. Because the EM tool could transmit data back to surface independent of rig operations, surveys were taken while making up drill pipe connections, saving significant rig time.

The future
Despite the determination that the EM signal might be marginal at depth, the excellent readings obtained during the run indicate that the extended-range antenna might not be necessary. This proof-of-concept test will allow Saudi Aramco to expand the use of UBD to both high-pressure and low-pressure wells where the use of two-phase fluids might be a necessity.

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