At the recently held SPE/IADC Drilling Conference in Amsterdam, Netherlands, Schlumberger unveiled its new family of measurement-while-drilling (MWD) and logging-while-drilling (LWD) tools called Scope. Although not specifically designed for deepwater, the implications are obvious. Risk reduction is one of the chief beneficiaries of the Scope tools' design, and that has long been a critical factor in deepwater drilling and development decisions.

Scoping it out

To understand how the new family of tools impacts risk, a description of the initial tool systems is in order. At the heart of the system is TeleScope, the new high-speed telemetry tool. According to the company, the tool delivers four times the data rate of the industry standard mud-pulse telemetry devices because it uses the Orion telemetry protocol. In user-terms this means that when drilling at 100 fph, the company can transmit 25 log curves with data updates every 6 in. to surface in real time compared to six curves previously delivered. Like other telemetry subs, the tool can record raw data from all sensors in non-volatile memory for downloading after the tools reach the surface. The tool is compatible with earlier generation LWD tools and rotary steerable systems, so all current LWD services can be run.

The new telemetry is able to sustain two-way communication at all times, so drilling and logging can continue uninterrupted while necessary commands are downlinked to the tool string. This can eliminate as much as 10% non-productive time. Besides providing high-speed telemetry for all data transmission to surface, the tool provides drilling dynamics measurements such as downhole weight and torque on bit, triaxial vibration and torsional shock. It also provides azimuth, inclination and toolface measurements as well as a natural gamma ray detector.

Integrating measurements

Several years ago, when the company's engineers introduced the wireline Platform Express system, they experienced a 7X reliability improvement, simply because they integrated the sensors on a single mandrel that eliminated most tool joints. The EcoScope petrophysical LWD system does the same. In a single, 26-ft (7.93-m) collar, the company has integrated the so-called Triple Combo tools including multifrequency resistivity, azimuthal lithodensity, compensated neutron, caliper, annular pressure-while-drilling and azimuthal natural gamma ray. Moreover, they have added the industry's first elemental capture spectroscopy and sigma measurements while drilling. Full formation evaluation can be obtained at drilling speeds up to 450 fph.

Risk is attacked several ways. In addition to the reduction in tool joints, the system contains built-in diagnostic chips that continuously monitor and record tool operations, reducing lost time and increasing confidence in the measurements. The tool's short length reduces the amount of rat hole necessary and cuts rig-up/rig-down time. A major risk factor is the elimination of a chemical radioactive neutron source. A new Pulsed Neutron Generator (PNG) developed by Schlumberger in collaboration with Japan Oil, Gas, and Metals National Corporation (JOGMEC) enables all radioactive measurements except Lithodensity to be provided without a chemical source. In extremely high-risk situations, a density measurement can be obtained from the neutron formation interactions so even the standard Cs-137 density source can be eliminated.

The ability to fine tune the drilling trajectory in real time is highly important in deepwater drilling. With fully integrated, comprehensive, formation evaluation sensors located close to the bit and high real-time data rates, the EcoScope is designed to take a huge bite out of drilling risk.

Formation pressure

The third family member is StethoScope, a multifunction formation pressure-while-drilling (FPWD) service that experienced extensive field testing in Norway. Unique in its approach, the tool does not rely on gravity to help it achieve a seal. Rather, it has a field-replaceable, hydraulically actuated piston that presses the packer against the borehole wall so tests can be taken regardless of the tool's or the borehole's orientation.

Software-controlled pretests confirm the seal eliminating time wasted on dry tests or seal failures. According to Statoil, who tested the system in a variety of scenarios including temperatures to 300°F (150°C) and pressures to13,053 psi (900 Bar), about 20% of the tests were incomplete with half of that being seal failures.

Presently available in 6.75-in. diameter, the tool has both a strain gauge and an advanced crystal quartz gauge. Pretest volume is adjustable up to 25 cc, and the drawdown rate can be varied from 0.1 cc/sec to 2.0 cc/sec. The automated pretest sequence can be selected from a set of four pretest options or customized using client-defined settings. Test time in almost every case is less than that required to make a drillpipe connection.

A major advantage of the system is its ability to perform tests with the rig's mud pumps on or off to improve measurement quality. By providing direct pore pressure and mobility data for fluid typing and mud weight optimization, both reserves prediction and drilling risk are reduced, according to Schlumberger.

Vital to Statoil

In an exclusive interview, Harald Laastad, Statoil's advisor for Geo-Operations explained how important accurate FPWD measurements are to his company. "For some time, Statoil has focused strongly on the development of FPWD technology because we are drilling a lot of satellite fields from installations we already have. Many of the wells are high-cost extended reach step-outs. To understand the reservoir so we can create an effective development strategy we must log each well thoroughly. In addition, many of the wells are challenging from a drilling point of view," he said.

Laastad went on to point out that for many years the only standard measurement they were unable to take during drilling was pressure - they had to resort to risky and expensive drillpipe-conveyed formation testers. Yet the information was so vital, they had to attempt it. "We believe that pressure information is most valuable when acquired during drilling, because it can improve drilling efficiency," he said. "In fact, we pushed all LWD vendors to develop the ability to measure FPWD, and have tested several designs."

"In production drilling it is very important to know where you are for early information on depleted zones and drilling efficiency optimization." Laastad said. "We believe we can raise recovery factors (increased oil recovery) on fixed platform fields to as much as 70% and on subsea fields to 55%. Some of these goals may be realized by using 'formation pressure steering' to put the wells in the best spot in the reservoir." Aside from production enhancement, Statoil uses FPWD as a drilling optimization tool, and has even used it on subsea fields.

"We are now at the point where we are exploring the operating envelope of the FPWD measurement, especially in low permeability formations. We have also started to get experience with how accurate the measurement is - not only for measuring depletion, but also for fluid typing," Laastad explained. "As you know, if you are off by even a fraction of a bar it can affect the gradient, so we were quite intrigued by the tool's ability to test with pumps on or off. This may reduce supercharging effects. We are still learning how to measure accurate fluid gradients with FPWD due to LWD's inherent depth-control problems, but Statoil and Schlumberger are addressing this issue in an ongoing research and development project. Using the StethoScope tool, we have been able to take tests over a wide mobility range from several Darcies to a few millidarcies. This will help us acquire very high quality information as well as perform good quality control, which is very important to us, especially in the long-reach wells," he said. "Over the last few years, we have qualified all the major vendors of FPWD services and we have had a strong technology focus to solve our business needs," he concluded.

Scoping the future

Latest in the family of Scope is PeriScope 15, the industry's only deep-reading directional electromagnetic measurement. The tool can detect bed boundaries and fluid contacts from as far as 15 ft (4.5 m) and indicate their proximity and apparent dip. This information can be used to geosteer a well to the best reservoir zones without any exits, yielding more productive wells. Because the tool can detect resistive anomalies not yet penetrated by the bit, it can geosteer without dependence on stratigraphic markers. The advance information enables drillers to steer away from trouble, keeping the borehole in the reservoir.

Recently, in developing a Middle East carbonate reservoir, the operator challenged Schlumberger to place the well within the top 10 ft (3 m) of the reservoir. The objective was to stay within the high quality portion of the reservoir while minimizing the oil left behind in the "attic" zone, and while avoiding drilling up into the unstable shale caprock. The tool was able to show the distance to the caprock in real time and maintained the desired trajectory for 2,300 ft (700 m) of the horizontal drainhole within 5.9 ft (1.8 m). More than seven of these wells have been drilled to date using PeriScope 15.