LWD technology has proven itself as a reliable means to shorten rig time, stay in zone to enhance well productivity, and more efficiently reach target depth. Formation pressure MWD has been viewed as a specialty service, in most cases being limited to use in offshore drilling programs or complex wellbores. A need for optimized LWD systems presented an opportunity for the development of a formation pressure tester as part of a large LWD campaign.

Weatherford developed the new PressureWave LWD formation pressure tester and deployed it in three wells for an operator in Gabon. To better understand the impact of formation tester placement within a bottomhole assembly (BHA) and the concurrent impact of varying operating steps, different BHA designs were used for the individual wells along with different operating procedures.

LWD formation testers

Formation testers create a mechanical seal with the formation through a hydraulic cylinder that extends out from the tool body and seals tight against the wellbore, isolating the contacted area from the wellbore. Once a seal is created, a mechanism lowers the pressure inside the tool and the connected reservoir section (drawdown). Once the drawdown stops, reservoir fluid is then free to flow into the tool, eventually equalizing at formation pressure.

LWD formation testers are specially designed to accommodate use in the BHA. The formation tester must be “ruggedized” to operate in downhole conditions where temperature, pressure, shock, and vibration can be extreme. This proved challenging as formation tester tools rely on delicate crystal quartz gauges and moving mechanical parts to provide the measurement.

LWD formation testers transmit data and receive commands via mud-pulse telemetry. In some instances the telemetry pulses may be visible on the pressure signal and induce noise that can cloud the true formation pressure. Measured pressure fluctuations of up to 5 psi around true formation pressure are not uncommon.

In addition, mud-pulse telemetry does not afford real-time control, meaning that the LWD formation tester must operate autonomously downhole. Onboard memory is available to store formation pressure data while the tool is deployed. These data are downloaded and processed once the tool has returned to surface.

MWD 3 Figure 2

FIGURE 2. A time-depth comparison shows a noticeable increase in efficiency when the correlation and testing activities are combined.

Geology and reservoir considerations in Gabon

The new tool was deployed in a heavily interbedded sandstone formation. The fine laminations and interbedding create complex fluid flow paths and directly impact the production profile within the reservoir. The varying thickness of the sand bodies, ranging from less than 1 m to 3 m (3 ft to 10 ft) along the wellbore, dramatically affects reservoir productivity.

The reservoir also lacks significant interconnectivity, with some producing regions experiencing 900 psi to 1,000 psi depletion while other regions remain at original reservoir pressure. A formation tester can identify the areas within the reservoir that are either well connected or isolated. The measured pore pressures reflect the impact of production activity away from the near-wellbore environment and identify areas of suboptimal production.

Permeabilities ranging from a few hundred millidarcies (mD) to less than 1 mD within the same well further increase the difficulty in acquiring accurate formation pressures in this field. But while the reservoir is geologically complex, the formation itself is at benign conditions, typically less than 1,500 m (4,921 ft) true vertical depth and less than 60°C (140°F).

Field runs in three wells

An initial series of trials was conducted using LWD. Three wells were chosen to investigate the impact that changes to operating pressure and formation tester placement within the BHA assembly would have on the data acquired.

Well 1 – Formation tester at the top of the BHA. For the first well the formation tester probe was placed approximately 25 m (75 ft) above the gamma ray correlation sensor located at the bottom of the resistivity collar. The testing program called for reaching the termination depth and then proceeding with a bottom-up correlation pass to acquire high-resolution formation evaluation data. This correlation pass would confirm the depth of the target sands, thus increasing the confidence in placing the formation tester probe in the correct location.

Once the correlation pass was complete, the pressure station acquisition began, working from the top to the bottom of the reservoir section. At each measurement location the formation tester was lowered past the station depth, followed by reversing the pipe direction and pulling the formation tester upward into the desired station depth.

Well 2 – Formation tester near the bottom of the BHA. In this well the formation tester was placed below the triple-combo logging suite – directly below and within 8 m (26 ft) of the gamma ray correlation sensor. This placed the formation tester closer to the bottom of the hole.

The testing operation was planned such that the correlation pass and the pressure measurement were conducted in the same run, with correlations being performed between pressure stations. This placed additional stress on the field engineers since the operation was continuously shifting between testing and correlation activities.

While this testing procedure would theoretically maximize efficiency, an evaluation of the data provided useful insights. A significant portion of the testing run was allocated to nontesting time – attributed to rig activities such as tripping, making connections, or maintenance. This nontesting time was not apparent in the previous wells. Placing the formation tester directly above the mud motor increased the difficulty in directional steering.

Further directional drilling difficulties arose when large amounts of reactive torque – more than 900° in some instances – were believed to have originated at the formation tester’s integral stabilizer. Since wells in this field are targeting small subsurface locations, maintaining directional control while drilling is imperative.

Well 3 – Formation tester at top, simultaneous correlation and pressure testing. The third well was drilled using the same BHA configuration as the first well but followed the testing program of acquiring pressure stations and performing correlations in the same run. This operating procedure yielded a success rate greater than 80% for pressure testing and coverage of a fairly large section.

Prior to this well, the highest success rate was in the mid-70% range. The high success rate, coupled with a quick operation, increased the operator’s confidence that the optimal testing program had been found.

Comparing results

A comparison of the formation testing operations in these three wells highlights a noticeable increase in efficiency when the correlation and testing activities are combined. In the second and third wells the number of repeat stations was reduced, regardless of the interval being tested. This is demonstrated in Figure 2, where the depth-time plots of the three wells are overlaid.

The number of repeats and issues with probe placement are likely a result of continuous changes in pipe direction. Small depth errors introduced by continuously working the drillpipe up and down during testing may be sufficient to cause a noticeable increase in the number of repeats required. An improvement in the efficiency of the pressure acquisition program – marked by a decrease in the number of repeats and failed tests – was observed when the pipe was moved in a consistent direction.

This study showed that an LWD formation tester is a viable option for acquiring accurate pore pressure information, although the placement of the formation tester in the drilling BHA impacts the ability to steer the well to the desired subsurface target location. The integration of formation tester operations into rig operations creates the maximum measurement benefit while reducing testing time. The best results are obtained by simultaneously combining pressure acquisition with correlation.