IPTC 17579
Improvements of Sampling and Pressure Measurements with a New Wireline Formation Tester Module in Carbonate Reservoirs Koksal Cig, Schlumberger, Halil Ibrahim Osunluk, Schlumberger, Radwan Naial, ADCO, Tarek Mohamed Ihab, ADCO, Ahmed Yahya Al Baloushi, ADCO
Copyright 2014, International Petroleum Technology Conference This paper was prepared for presentation at the International Petroleum Technology Conference held in Doha, Qatar, 20–22 January 2014. This paper was selected for presentation by an IPTC Programme Committee following review of information contained in an abstract submitted by the author(s). Contents of the paper, as presented, have not been reviewed by the International Petroleum Technology Conference and are subject to correction by the author(s). The material, as presented, does not necessarily reflect any position of the International Petroleum Technology Conference, its officers, or members. Papers presented at IPTC are subject to publication review by Sponsor Society Committees of IPTC. Electronic reproduction, distribution, or storage of any part of this paper for commercial purposes without the written consent of the International Petroleum Technology Conference is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of where and by whom the paper was presented. Write Librarian, IPTC, P.O. Box 833836, Richardson, TX 75083-3836, U.S.A., fax +1-972-952-9435
Abstract A multilayer hydrocarbon reservoir in Abu Dhabi land is in an appraisal stage before experiencing an extensive field
production operation. The hydrocarbon reservoir, having medium to low permeabilities, consists of a number of carbonate
layers with their associated oil-water contacts. One of the challenges is to sample hydrocarbons in tighter layers as well as to
measure valid reservoir pressures to determine oil-water contacts. While the goal is to accomplish the objectives with
wireline formation testers (WFT) in openhole conditions, stationary times during logging are limited due to wellbore
conditions. The time limit has been a longstanding challenge in the layers having especially lower permeabilities (<1md).
A typical sampling operation involves advanced modules of WFT including a Dual-Packer and an Insitu Fluid Analyzer to
identify fluid types and provide downhole compositions with densities. Reservoir pressures are measured generally with
Single-Probe modules. The new WFT inlet module is introduced first time in Abu Dhabi across the carbonate formations to
accelerate the stationary operations. The new inlet module showed an improvement over a Dual-Packer and a Single-Probe
modules in several aspects: (1) Stationary times during sampling are reduced due to very low interval volumes in comparison
to a Dual-Packer module and up to 60% faster oil breakthrough times are achieved. (2) Tight zone pressures are measured as
fast as a Single-Probe module with lower supercharging effects. (3) Set and retract times are shortened so that a new
sampling method of a set-retract-reset is developed without exceeding stationary times.
This paper summarizes the recent achievements by reducing risks associated with long stationary times. The field benefits
are demonstrated in two separate WFT operations by comparing data qualities and job efficiencies in the same reservoir
layers. Results show faster sampling, more accurate pressure and permeability measurements in the carbonate reservoir.
Introduction The Abu Dhabi land field is a newly developed field located approximately 260 km south of Abu Dhabi city. It has an oil
production target of 30000 BPD and will start producing from 2013 as planned in phase I. The field has several carbonate
sequences each having its own oil-water contact (OWC), fluid and rock properties. In order to identify OWCs and obtain
fluid compositions, extensive reservoir characterization has been underway. The reservoir consists of a layered carbonate
system with medium to low permeabilities with heterogeneities. There have been difficulties identifying hydrocarbon in
certain layers due to low resistivity signature in the rock. The challenges also extend to establishing pressure gradients
although virgin formation pressures are expected in the field. This is solely related to formation tightness. Due to carbonate
related challenges, special openhole logs as well as wireline formation testers (WFTs) are run in the field to delineate such
formations.
The data gathered with WFTs deliver pressures with zonal mobilities, examine layer barriers, collect samples and provide
real-time detailed compositions. Interval Pressure Transient Test (IPTT) provides vertical and horizontal permeabilities.
Geomechanical properties such as minimum horizontal stress, closure and breakdown pressures are obtained from Stress
Testing analyses (Ayan 2001, Ayan 2012, Zimmerman 1990, Pop 1993, Kuchuk 1998, Mullins 2008, Mullins 2010).
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However challenges of the carbonate reservoir still not resolved entirely. Formation pressures do not provide fluid gradients
due to tightness of the rock. Fluid sampling exercise may exceed desired station times which are advised by sticking risk
analysis. Openhole logs do not clearly identify OWC levels as well as fluid type in the low resistivity zones. The decisions on
transitional zone saturations and their producibility may often be challenging (Dios 2012, Al Otaibi 2012).
Wireline Formation Tester WFT consists of several modules which can be used interchangeably depending on the job objectives. Pressure
measurements are conducted with a single probe and sampling applications will require a pumpout as well as a fluid analyzer
module for fluid identification. There are several bottle types which can be chosen depending on the data requirements; such
as single-phase pressure compensated bottles or standard PVT quality pressurized bottles. With the advance of the
technology, the latest fluid analyzers are capable of providing accurate downhole hydrocarbon compositions covering C1,
C2, C3-C5, C6+ and CO2 components. Gas/Oil ratio (GOR), Condensate/Gas ratio (CGR), florescence, water resistivity,
fluid density and viscosity values are also measured for real-time fluid identification. The fluid analyzer technology is
extending its capabilities toward the detailed analyses as a downhole laboratory. A fluid density measurement can be
converted into a pressure gradient line extending from a single valid pressure point. This is particularly important for
tight/thin zones where pressure gradients cannot be obtained due to lack of sufficient valid pressure points.
New 3D Radial Probe for Wireline Formation Tester Although WFT is commonly used for fluid identifications and pressures, increasing stationary logging time is the main
challenge in difficult carbonate reservoirs. There have been recent developments in WFT technology such as; straddle
packers with higher differential pressure ratings of up to 5000 psia, increasing probe sizes from 0.15 in. up to 6 in, and faster
and larger volume pretesting tool combinable with openhole logging tools. Despite the different methodologies as well as
various sizes of probes and dual packers, challenges in tight carbonate formations still remain especially in reducing station
times and sticking risks.
Successful WFT fluid sampling and downhole fluid analyses (DFA) require accessing uncontaminated formation fluids in
reasonably short station times. The fluid withdrawal is conducted with an inlet such as probe that sets on the wellbore with
telescopic backup pistons and flowline. The pistons extend the probe and surrounding packer against the borehole wall to
provide a sealed fluid path from the reservoir to the flowline. The flow from the formation into the probe is governed by
Darcy’s law, in which flow rate (q) is a function of permeability (k), drawdown pressure (dp), surface area open to flow (A),
fluid viscosity (μ) and the length (L) over which drawdown is applied.
Figure 1 - Darcy's formulation for absolute permeability
The challenge for sampling is to pump faster with a lower pressure drawdown in a shorter operational time. This, as seen
from above equation, requires a larger flow area with an efficient operation. However having larger flow areas offered by
dual packers come with disadvantages of trapped volume between the packers as well as operational sticking risk. A new 3D
radial probe is developed to overcome these challenges with a radical design that utilizes four elliptical self-sealing probes
with a total flow area of 79.44 sq.in. The probes are radially perpendicular and set simultaneously at the formation (Figure 2).
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Figure 2 - 3D Radial Probe
Figure 3 - Comparison of 3D Radial Probe with straddle (dual) packers and extra-large diameter (XLD) probe
The 3D radial probe enables circumferential flow from the sandface of formation around the borehole, which allows cleanup
process and hydrocarbon arrival quicker without displacing any trapped volume, significantly reducing the time needed to
obtain representative formation fluids. 3D radial probe has 40 times increased flow area over an extra-large diameter probe
with no trapped volume in the sealed interval. Therefore, It is suitable for sampling, DFA, IPTT, and pressure testing in
tighter carbonate environments. Additionally it provides better sealing statistics in rugose and oval boreholes (Al Otaibi 2012,
Dios 2012). The Figure 4 depicts the flow area of different probe types.
Figure 4 - Flow area comparison of various probes.
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Carbonate Field Examples
The following section explains the performance of the 3D radial probe in comparison with XLD probe and dual packers.
WFT logging operations were conducted in two nearby wells in the same carbonate field. The pressures and mobilities are
similar in the formation units due to undepleted nature of the new field. WFT operations conducted in the two wells in the
same field gave the opportunity to compare the pressure and sampling data as well as to evaluate the efficiency of the new
probe.
Pressure Measurements in Low Mobility
Figure 5 - Well-A 3D radial probe pressure measurement and its pressure derivatives
Figure 6 - Well-A Large-diameter probe pressure measurement and its pressure derivatives
Figure 7 - Well-B dual packer pressure measurement and its derivatives
Formation pressure from Spherical
P*=2804.33 psia. Mobility 0.22 is md/cP.
Formation pressure from Spherical P*=2806.9. Mobility is 0.25 md/cP.
Buildup pressure is below 2800 psia and still building up after 1 hr. Mobility is 0.27 md/cP.
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Figures 5 to 7 show the pressure measurements with three different inlets in two wells in the same zones. 3D radial probe
completes the pressure measurement in roughly 30 mins, about the same time taken by the large-diameter probe whereas dual
packer pressure measurement took 60 mins, yet still not achieved stable pressure. The advantage of the 3D radial probe over
the large-diameter probe is in the fact that the final pressure has reduced supercharging effect which is often significant in
pressure measurements done with single probes in tight formations.
Pressure Transient Analyses
Figure 8 - Well-A IPTT example 1 for horizontal and vertical permeability calculation
Well-A IPTT testing was conducted with the 3D radial probe after DFA operation providing horizontal and vertical
permeabilities. Entire station operation was only one hour that highlights the new probe efficiency. Blue pressure curves
represent the 3D radial probe and the green is the observer probe. 3D radial probe detects effectively very early spherical
flow followed by radial flow regime in the reservoir section of 17 ft thickness. Horizontal mobility is around 160 md/cp with
kv/kh ratio of 0.2.
Figure 9 - Well-A IPTT example 2 for horizontal and vertical permeability calculation
Well-A IPTT testing was conducted with the 3D radial probe after DFA operation providing horizontal and vertical
permeabilities. Entire station operation took two hours. Blue pressure curves represent the 3D radial probe and the green is
the observer probe. 3D radial probe detects early spherical flow followed by radial flow regime in the reservoir section of 17
ft thickness. Horizontal mobility is around 75 md/cp with kv/kh ratio of 0.17.
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Sampling in Low Mobility
Figure 10 - Well-A 3D radial probe sampling station. The station time was 11.7 hours including pretest and pressure build-up.
Figure 11 - Well-B dual packer sampling station. The station time was 13.2 hours without a pretest and a pressure build-up.
Table 1 - Comparison of Well A and Well B sampling performance of 3D radial probe and dual packers
Table 1 shows that 3D radial probe achieves a better efficiency in overall sampling operation from the same tight carbonate
reservoir even though the measured mobility is over 3 times lower. The time of hydrocarbon breakthrough is significantly
reduced which proves that the mud filtrate cleanup process is faster as well. Set and retract times are shorter with the 3D
radial probe due to its improved design that adds to the overall efficiency.
Inflate time Pretest TimePumpout
Time
PBU/Mini-
DST/VIT TimeDeflate Time
Total
Station TimeFirst HC Show
Pressure
Drawdown
Drawdown
Mobility
(hrs) (hrs) (hrs) (hrs) (hrs) (hrs) (hrs) (Psia) (mD/cP)
A 3DRP 0.2 0.3 10.4 0.7 0.1 11.7 6.2 2,266 0.22
B Dual Packers 0.3 0 12.7 0 0.3 13.3 11.5 1,712 0.70
Well Inlet Type
Sampling oil with PVT bottles
Sampling oil with PVT bottles
IPTC 17579 7
Tool String and IPTT Setup
Conclusions
3D radial probe enables sampling in tighter formations with a quicker hydrocarbon breakthrough and a shorter station time,
which reduces sticking risk. The pressure buildup analysis has a lesser wellbore storage effect due to no trapped volume in
the 3D radial probe. The 3D-Radial Probe has shown good ability to seal in rugose holes in comparison with dual packers.
Overall operational efficiency during setting and retracting is at least 50% better compared to that of dual packers. This
allows a new technique referred as retract-move-reset. In this technique, WFT string is stationary for a maximum allowed
period of time imposed by operator’s requirement. When the time limit is reached, the string is retracted and moved to
confirm that there is no sticking. Once this is established, the string is positioned to the original depth for resetting the 3D
radial probe and resuming the cleanup. This technique is not feasible with dual packers due to time requirements of longer
inflation and trapped volume displacement.
Acknowledgements The authors wish to thank the management of ADCO and Schlumberger for granting the permission to publish the content of
this paper.
Figure 12 - WFT tool string run in Well A
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