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| A lack of reservoir interconnectivity necessitated formation testing at Omoueyi field, onshore Gabon. |
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Formation pressure testing with logging-while-drilling (LWD) technology was studied in a series of wells in Gabon’s complex Omoueyi field. The trials were conducted to integrate the pressure-testing tool with the BHA, determine operational procedures, and assess data quality and efficiency relative to wireline operations.
The Gabon experiments showed that the LWD data quality was equal to the ongoing wireline program, and that significant gains in rig efficiency were possible. Following the studies, operator Maurel & Prom changed their Omoueyi field, formation evaluation program to an entirely while-drilling operation, using the LWD pressure tool in concert with a triple-combo suite. Plans call for drilling more than 30 wells in 2014.
Following the Gabon tests, this LWD pressure technology has had similar success in Colombia, Ecuador, the U.S., the North Sea, and, most recently, the Middle East.
COMPLEX RESERVOIR CHALLENGE
Geologic complexity necessitates formation pressure testing in Gabon’s Omoueyi field. The primary producing formation, Kissenda, consists of heavily interbedded sands, with complex fluid flow paths and varying production profiles.
The sand bodies can range in thicknesses from less than 1 m to several meters, across hundreds of meters of wellbore interval, which dramatically affects production. Accurate reservoir connectivity mapping is critical to production allocation and reserves management.
Produced since 1989, some parts of the Kissenda formation have experienced 900–1,000-psi depletion, while other regions have remained at almost virgin reservoir pressure, Fig. 1. A primary reason for formation testing in the field is this lack of reservoir interconnectivity. Pore pressures measured by the formation tester reflect the impact of production activity away from the near wellbore environment, and identify areas of sub-optimal production.
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| Fig. 1. An example of reservoir pressure connectivity, or lack thereof: gamma ray and ROP in Track 1; formation pressures in Track 2; resistivities in Track 3; porosities in Track 4; and fluid mobility in Track 5. |
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The difficulty of acquiring accurate formation pressures throughout the Kissenda formation is compounded by a strong permeability variation, ranging from a few hundred millidarcies to less than 1 millidarcy, often within the same well.
LWD TECHNOLOGY
The Gabon trials examined Weatherford’s PressureWave LWD pressure testing tool and service. The device provides real-time data while drilling, via mud pulse telemetry and high-resolution 32-bit data in memory. It measures formation pore pressure, wellbore annular pressure and temperature while drilling, and directly measures formation pressures, when required. As a byproduct, fluid mobility is also calculated.
LWD formation pressure testing has several unique environmental considerations. These tools must operate at downhole conditions, after surviving the temperature, pressure, shock and vibration encountered during the drilling process. Generally speaking, formation pressure testing tools are delicate, in that they rely on quartz crystal pressure gauges, and moving mechanical parts and seals, to provide the measurement.
In some conditions, mud pulse telemetry can induce noise on the pressure signal, and cloud the true formation pressure. Formation pressure data and key diagnostic information are recorded to onboard memory at much higher resolution and frequency than real time, and processed later at the surface.
In addition, as with any LWD tool, pressure-testing tools must operate autonomously, because bandwidth restrictions preclude any possibility of dynamic control of testing operations. Typically, bandwidth for any LWD sensor is in the range of a few bits per second.
GABON EXPERIENCE
The performance and measurement quality of the LWD formation pressure-testing tool were benchmarked against previous wireline operations. The well tests confirmed that the measurement quality of the triple-combo (gamma ray, resistivity, formation bulk density and neutron porosity) and formation pore pressure measurements were identical to what had been acquired previously with wireline. The choice of systems then became an economic one.
The initial deployment experienced operational issues attributed mostly to the crew’s lack of familiarity with the new system. Steady improvement was observed as experience grew. Because the LWD formation tester is part of the drilling BHA, there were some underlying concerns about how this would affect drilling complexity, and relative ease of directional steering.
Over the course of the introductory period, operator geologists and Weatherford experts fine-tuned the acquisition processes, with a primary goal of minimizing the overall formation pressure acquisition time, acquiring an average of 30 pressure stations per well. This included experimenting with ways to maximize the capabilities of the LWD formation tester. During these initial 10 runs, the PressureWave tool achieved a success rate greater than 95% in obtaining a mechanical seal with the formation. Of nearly 400 stations attempted during this initial period, approximately 60% of the stations were able to obtain valid pore pressure readings. The remainder did not meet the established pressure quality control criteria.
Omoueyi wells are commonly drilled until the basement rock is identified from information provided by the triple-combo logs. A typical triple-combo log with pressure stations is seen in Fig. 2. After TD, the formation pressure-testing program commences. Pressure station depths are chosen by Maurel & Prom’s geologists from the real-time porosity and resistivity data provided.
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| Fig. 2. A typical Omoueyi triple-combo with pressure stations. |
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The data density can vary from one point per meter, to several points per centimeter, depending on ROP. It is common practice to reduce the penetration rate through the zones of interest, to maximize real-time data quality. Fortunately, all data are recorded into onboard memory, which is available after the LWD tools reach surface.
Significant effort is expended to ensure that the formations are positively identified through a dedicated correlation pass. Experience has shown that this correlation pass consumes 8% to 15% of the operational time, for a given testing program. Three wells were chosen to illustrate the results of the experimentation done with formation tester placement, within the BHA and the operating procedure.
Well 1. Once drilling was terminated, the planned operating procedure for Well 1 was to conduct a bottom-up correlation pass, to acquire high-resolution formation evaluation data from one or two select downhole sensors. This critical correlation pass confirmed the target depths, by filling in any gaps in the drilling data, and increased the confidence in accurate placement of the formation tester probe.
Acquisition of the pressure stations was then performed from the top to the bottom of the reservoir section. Identification and placement of the formation tester probe was difficult, primarily because the correlation sensor (gamma ray) was approximately 25 m below the formation tester probe. This sensor offset created issues, as the spacing required an increase in the number of connections being made by the rig crew while correlating. In this case, the 25-m length is longer than a typical stand in the rigs being employed currently.
As a standard operating procedure, the station depth is entered from below. This movement requires the formation tester to be lowered past the station depth, and then pulled up to correct depth. With careful planning, connections could be avoided, minimizing the rig time involved.
Because the testing program in this well appeared to be the most efficient, it served as the baseline for assessing the performance of subsequent wells.
Well 2. For Well 2, the formation tester was placed below the triple-combo logging suite, just above the mud motor. This BHA design placed the formation tester closer to the bottom of the hole, and within 8 m of the correlation sensor, effectively putting the formation tester and the correlation sensor in the same drill joint. The hypothesis for this experiment was that reducing the number of connections encountered would lead to an improvement in operational efficiency.
In this well, the pressure testing operation was planned, so that the correlation pass and the pressure measurement were conducted simultaneously. Correlation passes were conducted between the pressure stations, as required. This procedure placed additional demands on the field engineers, as the operation continuously alternated between pressure testing and correlation activities, which were a radical change in procedure from what had been done on previous wells.
While this program, theoretically, maximized efficiency and minimized formation pressure testing time, evaluation of the results showed that total test time was comparable to Well 1, which had a 150-m shorter reservoir section. The conclusion was that the rig activity time had a larger impact on testing operations than initially thought. This rig time was not anything uncommon; some of the activities included tripping, making or breaking connections, and other rig maintenance.
The placement of the formation tester directly above the mud motor in this BHA did have some negative consequences. The configuration placed three almost-full gauge stabilizers within 20 m of the bit, making it difficult to directionally control this well. The formation tester’s integral stabilizer also became a localized source of torque generation, creating over 900° of reactive torque at some points. This reactive torque increased the difficulty of directing the well. Because wells in Omoueyi are drilled from pad locations to reach small targets, they must be accurate.
Well 3. The third well was drilled, using the same BHA configuration as Well 1, which has become the operational standard for Maurel & Prom in Gabon, Fig. 3. This testing program was designed to acquire the pressure stations and perform the correlation at the same time.
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| Fig. 3. Breakdown of a triple-combo BHA with formation tester at top. |
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One notable deviation was made from previous wells. The complementary measurements (resistivity, gamma ray or porosities) made while drilling were incomplete in some regions. To fill in gaps in the real-time data, the reservoir interval was re-logged after the well was drilled. The testing then proceeded to work from top to bottom, correlating between pressure stations.
This operating procedure yielded outstanding results, with a success rate over 80%, compared to the previous best of about 70%. The testing program in this well also covered a fairly large reservoir section of approximately 300 m. The high success rate and relatively quick operation increased confidence that the optimal testing program and procedure had been found.
Compared to the other well procedures, the method of acquiring formation pressures while correlating resulted in a reduction in the amount of repeat testing required. For Well 3, there were only three repeat stations out of a total of 18 stations attempted.
WELL SUMMARY
Results from the three well tests showed a noticeable increase in efficiency, when correlation is combined with the testing program. In Wells 2 and 3, the number of repeat stations was reduced, regardless of the interval being tested.
It was concluded that the number of repeats and issues with probe placement were likely the results of small depth errors, introduced by continuously working the drill pipe up and down during testing and correlating. Potential variations between surface and downhole depth changes created small offsets that could not be accounted for, unless a long correlation pass was made.
An improvement in the efficiency of the pressure acquisition program was observed when the pipe was moved in a consistent direction. Regardless of the direction of testing operations, top-to-bottom or bottom-to-top, the number of repeats and failed tests decreased, once the pipe was moved consistently in one direction. It has become apparent that the depth control operations are an important part of gathering formation pressures in Omoueyi field.
GLOBAL LWD EXPERIENCE
Since the Gabon testing, there have been similar experiences by other operators around the world. In Colombia, the LWD pressure tool provided data for wellbore stability and rock properties. Pore pressure data also validated the dynamic reservoir model. When used with real-time, triple-combo data, the pressure log helped optimize well placement.
In the first offshore deployment, a North Sea correlation log was followed by seven pressure station tests. The LWD suite saved about $1.5 million in rig time, and recovered 100% of logging data with no sensor failures. A testing program in Ecuador, to validate a dedicated wireline program, logged fifteen stations, while hole conditions limited wireline to five. The results successfully identified fluid contacts, and proved the LWD tester’s ability to provide wireline-equivalent data quality. 
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