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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