Interference tests verify reservoir continuity, offshore Africa

Neil Humphreys, Larry Myers
Mobil Equatorial Guinea Inc.
Malabo, Equatorial Guinea

Arlene G. Pollin
Mobil Exploration & Producing Services Inc.
Dallas

Steve Hill
Expro North Sea Ltd.
London

Ian Treherne
Plus Design Ltd.
London

Interference testing during start-up of the Zafiro field, offshore Equatorial Guinea, determined the degree of reservoir continuity within the field. This testing involved producing a single well at a fixed rate while observing the reservoir pressure response in four offset wells.

The test included both analytical and numerical techniques with a clearly defined plan for sequencing well production to maximize reservoir data while minimizing well shut-in times.

The interference test showed that all wells completed in the main Zafiro sands were in pressure communication.

The investment in installation of bottom hole pressure gauges and interference testing during field start-up rapidly acquired data critical for future reservoir management. Vital concerns over reservoir continuity were answered immediately, allowing the operators to implement fast-track plans for pressure maintenance to optimize oil recovery.

Zafiro field

Mobil Equatorial Guinea Inc. (75%) and partner United Meridian Corp. (25%) discovered the Zafiro oil field, License Area B., in March 1995. The field (Fig. 1 [95,483 bytes]) lies about 22.5 miles southeast of Mobil's Edop field in Nigeria, and 42 miles west-northwest of the island of Bioko. Water depths range from 450 to 1,900 ft.

The field is being developed with subsea wells tied back to a floating production, storage, and offtake (FPSO) vessel, the Zafiro Producer. Field development was on a fast-track schedule. First production started on Aug. 25, 1996, only 18 months from discovery.

The discovery well, Zafiro 1, penetrated a gross 190 ft (119 ft net) of highly permeable oil-bearing sands in the Pliocene age Intra Qua Iboe formation. The well flowed 10,505 st-tk b/d on a drill stem test. The Zafiro producing formation is a sequence of major sands separated by sections of laminated sands and shales. Wire line formation tester data showed that all sands penetrated were in vertical pressure equilibrium, indicating that individual sands might be connected vertically at some distance away from the well.

Subsequent appraisal wells encountered similar sands and sand/shale sequences that were stratigraphically equivalent to those penetrated in the discovery well. The wells also discovered oil in deeper and shallower sections. Correlation of sands from well to well was difficult, and sand pay thickness varied considerably across the field.

Although wire line formation tester and DST data indicated that all oil sands in the discovery well and appraisal wells were in pressure equilibrium in the main Zafiro sands, PVT (pressure-volume-temperature) data from drill stem tests, although ambiguous, indicated several different fluid types were present.

Consequently, significant uncertainty existed about:

• Amount of oil initially in place
• Overall continuity of the Zafiro reservoir
• Drive mechanism under which the field would produce
• Recoverable reserves in the field.

The Zafiro reservoir is believed to represent a deep marine slump/debris flow depositional environment. ¹ In this area, seismic amplitude data correlate extremely well with the presence of hydrocarbon-bearing sands in the Zafiro section, and have provided a primary method of mapping the Zafiro reservoir and delimiting individual sand bodies within it. This process, described in detail by Stephens, et al., ² shows an extremely complex reservoir comprising many intersecting and nonintersecting sand bodies.

At the seismic scale, data resolution is insufficient to determine definitively which sand bodies are or are not in lateral and vertical communication.

Because of reservoir complexity and uncertainty in sand continuity, the decision was taken to obtain production data from the field prior to committing funds for pressure maintenance. Production data could have been obtained either by an extended well test with two or more of the discovery/appraisal wells, or by bringing a portion of the field on production and analyzing early field performance to determine an optimum development plan.

Economic and risk analysis showed compelling reasons for developing the field rather than conducting an extended well test. These analyses further showed that a fast-track field development approach would maximize project returns.³

In October 1995, the decision was made to develop the field with a floating production facility. This was only 7 months after its discovery.

At that time, only four wells had been drilled, and only 2D seismic data were available. The entire design, construction, and installation process for this 40,000 st-tk b/d facility was completed within 12 months from the decision to proceed with a fast-track field development.

Permanent gauges

Critical questions remaining unresolved at the time the decision was made for producing the Zafiro field were:

• How much oil is in place in the Zafiro reservoir?
• What is the reservoir connectivity between wells?

The only way to begin answering these questions was to observe the reservoir response to production.

As oil is produced, reservoir pressure declines. Initial oil in place can be estimated from an analysis of pressure decline rate as a function of production. Similarly, interference testing, the response of individual well flowing and shut-in pressures to offset well production, can be used to evaluate the subsurface reservoir connectivity.

Both observations require accurate downhole pressure measurements in all wells in the field.

The subsea development allowed two options for obtaining these data:

1. Frequent well entries to measure subsurface pressure with retrievable pressure gauges
2. Installation of permanent downhole pressure-monitoring devices in all wells.

Because of high rig costs, the operator selected the permanent downhole pressure gauge option. The option also had potential for gathering additional critical reservoir data at a low cost while producing the field. It was also a simple and elegant approach.

Monitoring system

Besides pressure measurement, the bottom hole pressure-monitoring system had to meet several criteria dictated by the fast-track schedule in a remote location. These included the need to:

• Use existing, field-proven technology and be available in a short time frame.
• Be flexible to accommodate changes made during the fast-track development.
• Allow for data collection from multiple gauges simultaneously.
• Be expandable in the future for additional wells.
• Provide a method for transmitting pressure data to the U.S.

As initially conceived, the design would incorporate permanently installed bottom hole pressure and temperature gauges in all wells. These gauges would be tied back to a computer on the FPSO using dedicated electrical lines in the subsea well control umbilicals. Raw gauge data would then be converted to pressure and temperature and time-stamped at the FPSO by processor cards dedicated to each gauge.

Pressure and temperature data from all gauges would be stored on a single laptop computer, which for reliability would have an uninterruptible power supply. For redundancy in data storage and to allow data export for analysis, the pressure and temperature data would be recorded simultaneously on the laptop computer hard drive, and on a removable Pcmcia hard disc drive. Pcmcia disc drives would be interchanged at regular intervals and shipped to the U.S. for interpretation.

As the fast-track project proceeded, it became apparent that the original design required modification. Transient pressure response analysis of the discovery and appraisal wells, along with design calculations for planned reservoir analyses when the field was on production, gave additional insight into the gauge accuracy, resolution, and sampling frequency that would be required to adequately analyze reservoir performance.

Based on these studies, additional specifications for the pressure-monitoring system were developed. These included:

• Gauge accuracy of 0.02% of full-scale deflection or better
• Gauge resolution of 0.1 psi or greater
• Gauge sampling frequency of 1/sec.

These additional specifications caused a major redesign of the surface data acquisition system. Although the planned system could meet the gauge accuracy and resolution standards, the requirement for rapid sampling had significant design implications for the surface computer system on the FPSO.

Revised data acquisition rates would generate 40-70 megabytes/week of bottom hole pressure and temperature data. This change caused the number of computers on the FPSO to be doubled. This increase allowed additional redundancy because the laptop operating systems were loaded to near capacity. Increased Pcmcia hard drive memory specifications would keep disc interchanges at a reasonable level. The changes also required a satellite telemetry link to Mobil's worldwide computer network to allow on-line monitoring and data export.

Gauge installation

Fig. 2 [80,052 bytes] shows a typical permanent gauge installation. The system has three main parts: the downhole gauge assembly, the cable connecting the downhole gauge through the subsea tree to the well umbilical, and the surface computer hardware and software for processing and storing bottom hole pressure data.

An external slotted gauge carrier conveyed on the completion string houses and protects the downhole pressure gauge. The carrier is ported to provide a hydraulic link between the gauge and the completion string bore. The well bore and annulus are kept totally isolated by two independent metal-to-metal seals between the gauge carrier sealing face and the gauge itself.

The downhole quartz gauge (Table 1 [45,174 bytes]) contains three crystals. These are the reference frequency signal (7.2 mhz), and two measurement frequencies formed by mixing pressure and temperature sensor crystals with the reference. These frequencies (5-60 khz) are measured with a gated reference technique. Gate time is just under 1 sec.

The two independent sets of gauge electronics provide a fully redundant measurement system. These two completely independent sets allow the downhole gauge to be manually switched from primary to backup mode at surface, via the laptop PC, in case of downhole failure to the primary set.

A single cable connects the gauge to the surface. The cable has a central (line) conductor and outer (armor) return path. The line polarity selects between the primary (negative) and backup (positive) systems.

A negative line voltage (-55 v at the surface) powers the primary measurement system. This transmits measured data in a digital format at 600 baud using a frequency-shift-keyed (FSK) transmission system. The message formats include header and checksum information to guarantee uncorrupted data. A pair of readings (pressure, temperature) is transmitted every second.

Cable and wellhead penetration

The downhole cable supplies power to the downhole tool and transmits data back to surface. A mechanical cablehead connects it to the gauge. The cable comprises a solid copper conductor surrounded by an insulator and a layer of filler, encased in a Incoloy 825, 0.25-in. metal sheath. This is then protected by thermoplastic encapsulation that resists accidental damage, crush, or abrasion during the installation.

A cable protector is installed at every tubing connection throughout the completion. The protectors are designed individually for the tubing type and support and protect the downhole cable. Additionally, special protectors are installed at the subsurface safety valve and gas lift mandrels.

A proprietary male-female wet connector system provides electrical continuity and penetration through the tubing hanger and subsea tree. System integrity is assured by monitoring gauge operation throughout the running and installation procedure.

Surface system

Surface hardware comprises a rack of data interface cards (flowcards) feeding pressure and temperature readings from each gauge to two computers via a processor.

The input rack provides slots for up to 10 flowcards. Each flowcard supplies power and data interfaces for a single downhole gauge set and calculates the pressure and temperature in engineering units from the gauge signal with the quartz calibration files stored in nonvolatile memory on the card. The calculated data are transferred to a processor with a serial rack protocol at 2,400 baud.

The processor card collects data from up to 10 flowcards and issues a single data stream at 9,600 baud. This data stream is connected to the data logger. All card configurations in the rack can also be changed with a terminal process on the processor unit. This allows for entering new calibration data, or for changing flowcard modes from primary to backup mode.

The data-logging system is based on PC hardware. The extremely short time scales and limited rack space at the Zafiro field led to the use of notebook PCs for the initial installation. Two systems were run in parallel. Both shared the same input data because of the data volume. A proprietary software package was used to log and display the input data from the processor unit.

The logging system software was modified from an existing proprietary package to permit simultaneous 1 sec logging of 10 wells. A new data base structure was added to enable fixed 1 sec logging rates on the hardware available. The anticipated data acquisition rate was about 9 megabytes/day for the raw data, and this required additional data management techniques for transferring data to shore for analysis.

Objectives, plans

Interference testing was carried out among wells in the main Zafiro reservoir (Wells Zafiro 1, 2, 3, and 4) during production start-up from Aug. 25 to Sept. 5, 1996. The test objectives were to:

• Determine whether the wells in the field produced from isolated sand bodies or were in pressure communication.
• If in pressure communication, determine which wells in the field communicated.
• Derive information on the inter-well reservoir properties.

Fig. 3 [55,348 bytes] shows the well layout in the initial development area overlain on a sketch showing individual sand body outlines mapped from seismic amplitudes. The complex subsurface reservoir is apparent even in two dimensions. Because of this complexity and the high reservoir transmissibility (kh/µ 100,000 md-ft/cp) and storage (fc_tµ 50 x 10^-5 ft/psi), it was decided to maximize signal-to-noise ratio in responding wells by having a single active well. All other wells remained shut in until a response was observed.

The nomenclature is as follows:

c_t = total compressibility, psi^-1

k = effective permeability, md

h = formation thickness, ft

Dp = pressure change, psiDt = elapsed time, hrf = porosity, fraction

µ = viscosity, cp.

Consequently, the test used Zafiro 1, the most central well in the field, as the active well. This minimized the distances between the active and observation wells.

The magnitude and timing of the expected pressure response in the observation wells that resulted from Zafiro 1 production was modeled analytically with both a simple Ei-function approach,⁴ and more-sophisticated computer well test modeling. Both results agreed closely. This showed that the simpler Ei-function approach is adequate for designing interference tests in similar situations.

These studies indicated that the absolute magnitude of pressure response to production from Zafiro 1 would be small, and that Zafiro 1 should be produced at a high rate to maximize the pressure signal being generated in the reservoir.

Initial modeling predicted that if Zafiro 1 were produced at a withdrawal rate of 10,000 reservoir b/d, a drop in static pressure in Zafiro 2 would be observed in about 22 hr. The pressure-drop would be 0.5 psi in 25 hr.

Similar responses would be expected in Zafiro 3 and 4 after about 40 hr. These would have a magnitude of 0.5 psi after about 50 hr.

Significant uncertainty existed prior to the test on both the timing and magnitude of observation well response, if any. Also a wide margin of uncertainty existed in estimates of transmissibility, storage, and the true flow path length between wells.

Completion and drill stem tests had shown strong tidal effects, with amplitudes ranging from 0.1 to 0.4 psi. This magnitude was similar to the expected interference responses in the observation wells.

To ensure that the tidal effects did not obscure any response due to interference, it was decided to record a base line tidal pressure response in all wells prior to commencing the interference test. Subsequently, this could be mathematically removed from the observed well response to isolate the pressure change to production and hence calculate true reservoir parameters.

The schedule for connecting risers and umbilicals to the Zafiro Producer was adjusted to maximize the time available for recording base line tidal response in all wells, while eliminating or minimizing time when the field was not on production. After taking into account safety and mechanical constraints, no time was actually lost during field hookup because of the collection of these base line data.

Reservoir complexity made it unclear which wells, if any, would respond to Zafiro 1 production. Consequently, the planned test sequence brought the wells onstream in an orderly manner while still gathering all reservoir data needed for evaluating reservoir connectivity and properties.

The plan (Fig. 4 [139,896 bytes]) called for Zafiro 1 to be brought onstream at a constant rate of 10,000 reservoir b/d. The pressures in other wells would be monitored for a minimum of 100 hr to see if interference could be observed. If interference was observed in any well, that well would remain shut in until sufficient data had been gathered to interpret the response.

For planning purposes, this additional shut-in time, 50-100 hr, was estimated from the theoretical well pressure response. The criterion was that a semilog plot of Dp-vs.-Dt should clearly define a straight line. Any observation wells responding would be brought onstream following this time, and the pressures in the remaining well or wells were to be monitored for an additional 100 hr.

If no wells responded within 100 hr of commencing Zafiro 1 production, additional wells would be brought onstream singly, using a similar logic for monitoring the remaining observation wells.

Field observations

The permanently installed downhole pressure and temperature gauges were run in the Zafiro wells during the fast-track drilling and completion program. At field start-up, most gauges had been unobserved for several months. Consequently, interim gauge calibration drift was of some concern.

During each well completion, the permanently installed gauges obtained bottom hole pressure data while the well cleaned up through testing facilities on the drilling rig. After completing each well, a limited pressure buildup was conducted which, by extrapolation, estimated initial reservoir pressure. Consequently, as each well was tied back to the monitoring computer on the FPSO, the bottom hole gauge pressure reading was compared with the original well pressure calculated during the well completion.

Additionally, wire line formation tester (WFT) tools had been run in all wells to measure initial reservoir pressures and pressure gradients. These pressures were corrected to gauge depth and compared against permanent gauge pressures as another check on gauge accuracy.

Table 2 [28,395 bytes] shows the remarkably good agreement between all pressure measurements. This testifies to the accuracy and stability of the pressure-measuring devices.

All reservoirs exhibited tidal effects caused by the sun and the moon. These effects act on the air column (barometric tide), sea column (sea tide), and overburden (earth tide) overlaying the reservoir. These tidal effects change the total overburden load acting on the reservoir.

Both air and earth tides are observable, and have been reported in many geothermal and water wells onshore. Sea tide effects are generally larger than barometric and earth tides, and have been observed in many offshore fields.

The change in overburden load, averaging 4 ft of sea water or 1.8 psi at Zafiro, occurs as a complex sinusoidal signal with a period of about 12 hr. These changes cause a sinusoidal variation in reservoir pressure. This reservoir pressure variation is attenuated from the surface tidal variation due to geomechanical effects, but typically exhibits the same periodicity.

Fig. 5a [63,575 bytes]shows the tidal effects measured on the gauges prior to initiating the interference test. As a comparison, the records have been shifted in time to compare directly the tidal effect magnitude between wells. The measurements show that all wells have a tidal period of 24.8 hr for a complete cycle, or 12.4 hr between high tides.

The tidal amplitudes are as follows:

• Zafiro 1-0.42 psi
• Zafiro 2-0.47 psi
• Zafiro 3-0.45 psi
• Zafiro 4-0.17 psi.

All the wells have a similar tidal amplitude with the exception of Zafiro 4, which exhibits an amplitude of about 40% of the other wells. This reduced tidal effect is attributed to tidal signal attenuation by the gas cap overlying Zafiro 4.

Zafiro 1 was brought on stream at about 8,100 st-tk b/d at 5:30 p.m. on Aug. 23. Responses observed in all wells, Zafiro 2, 3, and 4, indicate that the wells are in direct pressure communication with Zafiro 1. Fig. 5b shows the actual field response observed in Zafiros 3 and 4.

The figure plots pressure response-vs.-time at each well. Time zero is at 0000 on Aug. 23. Zafiro 1 comes onstream at 65.5 hr. Zafiro 3 shows an initial response at about 105 hr and Zafiro 4 at about 100 hr, or at elapsed times of 40 and 35 hr, respectively.

Although not plotted, a strong response was also seen in Zafiro 2 at 70.5 hr. However, this was not due to interference, but was caused by inadvertent actuation of a subsea tree valve on Zafiro 2. More detailed inspection of the Zafiro 2 response indicates an initial response at about 90 hr, or at an elapsed time of 25 hr from start-up of Zafiro 1.

These response times compare very favorably with those predicted in design of the interference test.

Test interpretation

Zafiro interference tests were interpreted with a computer-assisted well testing design and interpretation program. Results obtained from Zafiro 3 illustrate the methodology.

As has been noted, well response to the pressure signal caused by Zafiro 1 production has the same order of magnitude as the tidal effect observed in the well. Complete removal of this tidal effect requires a Fourier transform analysis. But a simpler approach was taken for the Zafiro interpretation (Fig. 5c).

The observed well response is a combination of both tidal and interference effects. Therefore, it was assumed that this combined response is simply additive, with the form:

Observed response = Tidal effect + Interference response

Consequently, the tidal effect when subtracted from the observed response yields the true response of the well to offset production.

The initial tidal data gathered prior to any interference response generated a tidal curve for each well by selecting a 26-hr subset of these data, and determining its periodicity. For all wells, the observed period is 12.4 hr.

The data were then copied repeatedly, with a time shift of 24.8 hr. These 24-hr segments were then combined to estimate the undisturbed tidal response of the well.

The upper part in Fig. 5c shows the observed well response and the estimated tidal effect. The lower part shows the curve after subtracting the tidal effect.

It is apparent that all of the tidal effect has not been removed during the period where the well is responding to offset production. This is believed to be due to the interaction of tidal and interference effects being more complex than can be described by a simplistic model. Additional work is needed to understand this effect.

The simplistic approach for removing tidal effects was used on data from Zafiros 3 and 4, but not Zafiro 2 because of problems with the data described previously.

Well responses provided estimates of transmissibility and storage with the following three interpretation methods:

1. Type curve matching
2. Semilog analysis
3. Miller-Dyes-Hutchinson (MDH) approach.

Fig. 6a [57,293 bytes] shows the type curve match for Zafiro 3. Pressure data is the lower curve, and the derivative is the upper. The modeled data (curves) and the raw measurements are an extremely good match. Based on this match, the permeability-thickness between Wells Zafiro 1 and Zafiro 3 is 72,000 md-ft and the porosity-thickness is 33 porosity-ft.

The semilog plot (Fig. 6b) shows no clearly defined straight line and consequently, a straight line has been drawn through the last measured data points. Based on this fit, the permeability-thickness between the two wells is 104,000 md-ft. A similar match on an MDH plot yields a permeability-thickness between the wells of 77,500 md-ft, and a porosity-thickness of 20 porosity-ft.

Zafiro 3 interference performance because of Zafiro 1 production was back modeled using these properties. Fig. 6c compares the model with the pressure data measured in the well. Tidal effects have not been superimposed on this modeling that obtained a very good match of the measured data.

Table 3 [40,411 bytes] summarizes the interference tests. A wide range of permeability-thickness and porosity-thickness was calculated from the different methods. This variance is attributed to the difficulty in obtaining a straight line through data on both the semilog and MDH plots.

Data on both plots exhibit residual tidal noise, and also appear to exhibit curvature, with the slope of both plots increasing (decreasing kh and fh) with time. Consequently, if the test duration had been increased, the calculated kh from both methods would be expected to decrease.

References

1. Famakinwa, S.B., Shanmugam, G., Hodgkinson, R.J., Blundell, L.C., "Deep water slump and debris flow dominated reservoirs of the Zafiro field area, offshore Equatorial Guinea," Offshore West Africa '96, Libreville, Gabon, November 1996.
2. Stephens, A.R., Monson, G.D., Reilly, J.R., "The relevance of seismic amplitudes in exploring the Niger Delta," Offshore, October 1996, pp. 54-60.
3. Green, A.J., "Fast track development of the Zafiro field, offshore Bioko Island, Equatorial Guinea," Sub-Saharan Oil and Gas Conference, Capetown, July 1996.
4. Streltsova, T.D., Well testing in heterogeneous formations, Wiley, 1988, pp 59-66.