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Utility of Multiphase Production Logs in Optimizing Performance of Horizontal Wells Exhibiting Crossflow Faleh Al-Shammeri, Brett Fischbuch, SPE, Saudi Aramco; Mahmoud Abd El-Fattah, Mustafa Bawazir, and Hussain Al-Shabibi, SPE, Schlumberger

Copyright 2013, International Petroleum Technology Conference This paper was prepared for presentation at the International Petroleum Technology Conference held in Beijing, China, 26–28 March 2013. 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

Horizontal wells are increasingly being completed with inflow control devices (ICDs) in order to equalize the flow profiles, avoid water coning, enhance oil production, and minimize or eliminate downhole crossflow. Evaluating ICD completions is important to assess well performance, water entry intervals, completion efficiency, and potential remedial actions. Advanced multiphase production logging tools can be employed to evaluate the effectiveness of ICD completions.

This paper examines case studies of two horizontal wells drilled along well trajectories with large heel-to-toe pressure differentials. These wells were drilled in a large carbonate reservoir with moderate fracturing in areas of high structural curvature, which added to the heterogeneity. Crossflow can occur under static and flowing conditions with sufficient contrast in reservoir pressure along the wellbore. Crossflow is undesirable especially when water enters the wellbore in one region and flows into the formation at another region, thereby reducing oil relative permeability in the latter region. This can adversely affect well performance and ultimate recovery. Advanced multiphase production logs and wellbore simulation are useful in the determining minimum well production rate required to avoid downhole crossflow.

Multiphase production logging profiles were obtained for the two ICD-equipped horizontal wells in this study. These logs demonstrate the efficiency of ICD completions in minimizing crossflow when wells are produced above critical flow rates. However, the problem of crossflow remains when such wells are shut-in or produced at rates below their respective critical rates.

These results show that comprehensive evaluation of wells exhibiting crossflow is necessary to minimize or mitigate crossflow and optimize well performance. Additional ICD design enhancements are recommended to control crossflow below the critical flow rate and to minimize undesirable gas/water production. Introduction

Horizontal wells are commonly used in the oil industry to accelerate production and lower unit development cost. Horizontal well performance is affected by many factors, including reservoir heterogeneity, well placement and completion design. In heterogeneous reservoirs, the displacing fluid (water or gas) tends to move faster in zones with higher permeabilities, which will cause early breakthrough of unwanted fluids with eventual bypass of some undisplaced oil (Ouyang 2009). This can affect the pressure distribution and hence can cause crossflow between layers.

In addition, a pressure differential will also exist between injectors and producers. Wells drilled along streamlines instead of along isobars can encounter high heel-to-toe reservoir pressure differentials. High reservoir pressure differential along the wellbore can induce crossflow between wellbore regions, under static and in some cases flowing conditions. Crossflow is undesirable especially when water enters the wellbore in one region and flows into the formation at another region, thereby reducing oil relative permeability in the latter region. This can adversely affect well performance and ultimate recovery. The

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pressure profile along the wellbore can have a significant impact on well performance and therefore, must be accounted for in the well completion design.

Crossflow can be overcome by reducing bottomhole flowing pressure (BHFP) to the point where all intervals produce into the wellbore or by isolating the low pressure reservoir interval from the wellbore. Inflow control device (ICD) technology can help in minimizing or eliminating downhole crossflow during production by lowering the critical flow rate (Al Marzooqi et al. 2010). An explanation of ICD technology follows. ICD Technology

ICD technology was developed to overcome the production challenges inherent to horizontal wells. A typical ICD completion consist of ICD joints with packers that segregate the openhole horizontal section into compartments (Fig. 1) (Leung et al. 2010). The main drivers for ICD completions are to balance inflow along the wellbore, delay water or gas breakthrough, control water cut and heel-to-toe effect, promote initial well cleanup, reduce or eliminate crossflow, and control sand production. The use of ICDs provides a cost-effective solution to many of these challenges.

Generally, ICD completions are designed and optimized based on LWD data (gamma ray, porosity, density, resistivity, saturation) and reservoir parameters (derived permeability, relative permeability, and pressure/volume/temperature data). Any uncertainties in the data can result in flaws in the completion design. The degree of uncertainty will determine whether a uniform or a variable nozzle setting is applicable (Gottumukkala 2011). In general, a uniform nozzle setting with short compartment length is efficient when uncertainty about reservoir data is high.

The optimal ICD completion design results in minimal heel-to-toe effect and balanced inflow across the horizontal section in heterogeneous reservoirs. This can be achieved by compartmentalizing the horizontal section with packers based on permeability, pressure, and viscosity variations and by managing the pressure drops between the reservoir and the annulus and the annulus and the tubing across the reservoir section. Care is necessary to avoid emphasis on production from sections with very low permeability because this could mean higher pressure drops in prolific sections.

When designing ICD completions for expected crossflow, all well parameters should be considered (Krinis et al. 2009). In general, openhole compartments should be short, ICD flow areas should be small, backpressure across the completion against relatively low-pressure reservoir layers should be avoided, and the production rate should be above the critical flow rate. Horizontal Production Logging

Evaluation of horizontal well ICD completions is important to assess well performance and completion efficiency. Use of advanced multiphase production logging tools to determine accurate diagnostics is common (Mubarak et al. 2007); flow regimes are generally stratified and different from those in vertical wells. In small completions, mixture of bubbly flow at higher velocities and segregated flow at lower velocities is usual. On one side of the logging tool’s retractable arm are four miniature spinners designed to measure the phase velocities of oil, water, and gas. On the other side are arrays of five electrical and five optical probes for measuring localized water and gas holdups, respectively. Additionally, a fifth miniature spinner and a sixth pair of electrical and optical probes mounted on the tool body measure flow properties on the low side of the well. All sensor measurements are made at the same depth simultaneously. Fig. 2 shows an illustration of such a tool.

The multiphase production log has become the benchmark for most Saudi Aramco downhole flow diagnostics. Fluid flow rates and phase fractions are based on spinner, holdup, and temperature data and a series of calibration passes in a well section with known geometry. An advantage is that the total rate of production log can be calibrated against surface measurements such as those from separators or surface multiphase flowmeters during stabilized flow periods. Study Workflow

A steady-state wellbore hydraulics simulator is used to build the ICD completion model and manage the interaction between the near-wellbore region and the completion (Al-Khelaiwi et al. 2007). The completion design depends heavily on openhole logs, especially the calculated permeability profile. However, assessing permeability in carbonate formations is very challenging, and it is important to understand the fracture trends, conductivity, and uncertainty parameters within the area of the well being drilled; well logs assist to a certain degree around the near-wellbore. Calculations of permeability do not consider the effect of minor fractures, which are detected by image logs for fracture conductivity qualitative analysis. Therefore, the production

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logging profile is needed for model calibration and will achieve a better effective permeability profile, which includes the effect of minor fractures and water-entry intervals.

Fig. 3 describes the study workflow, which begins with collection of the available well and reservoir data and the current completion schematic. A model is then built and matched to the production logging results (downhole fluid flow rate profiles, shut-in and flowing tubing pressures). If model results do not match the log results, then one or more reservoir properties with high uncertainties are modified to achieve the match: permeability profile, fluid saturations, or reservoir pressure.

Once there is a match between model and log results, the model is considered calibrated and representative for the target well. Additional sensitivity runs are carried out to calculate critical flow rate for openhole and ICD completions to show the reduction in the critical flow rate. Field Background

The horizontal wells in this case study were drilled in a complex carbonate system consisting of a coupled micro/macro porosity matrix (Clerke et al. 2008), targeting the upper zones. The reservoir has localized diffuse bed-bound fractures along with periodic fracture corridors following a dominant orientation with greater concentration in the crestal, high-curvature regions. The reservoir consists of four main Zones capped by a continuous evaporate layer. The depositional environments that created this reservoir consist of a lower slope marine (low energy) for Zones 3 and 4, shoal and upper slope marine (high energy) for Zone 2, and tidal flat and lagoon (low to medium energy) for Zone 1, as depicted in Fig. 4 (adapted from Lindsay, 2006). The high energy environment has produced limestone facies dominated by oolites and grainstones, whereas, the lower energy environment has produced varying combinations of wackestones, packstones, and mudstones (Lindsay et al. 2006). Zone 1 facies consist of dolomitized grainstones and wackestones, which can occur as single or dual porosity lobes and can be laterally discontinuous. Nodular anhydrite micro-stringers are frequently found between porosity lobes and often between Zones 1 and 2.

As a result, both wells encountered heterogeneous permeability distributions along their respective wellbores. Field Examples

Well A

Well History. Well A was drilled as a long-radius single lateral oil producer with a horizontal section of more than 5,000 ft. The well was completed with a cased-hole assembly consisting of 17 ICDs and six mechanical packers, as shown in Fig. 5.

Multiphase Production Logging Results. The logging tool was conveyed using 2 3/8-in. coiled tubing with 98% coverage of the completed interval. The well was logged at flowing (two choke settings) and shut-in conditions.

The 100% flow rate was obtained at the high choke setting with a water cut of 9%. The main oil and water contribution interval is from the bottom equalizer, X1535–X2165 ft (26% total oil and 69% total water). No crossflow was observed during the flow at high rate. While the flow rate recorded at the low choke was 2,950 res bbl/d oil and 200 res bbl/d water, the water cut was 8%. The main oil contribution interval is the bottom equalizer, X1535–X2165 ft (34% total oil). However, water is contributed equally from the bottom equalizers (X0820–X1515 ft and X1535–X2165 ft).

During the flowing survey at low choke, an upward crossflow of 250 res bbl/d was observed toward the upper equalizer (X8188–X8855 ft). At shut-in, there was an upward oil crossflow of 2,270 res bbl/d from the last two equalizers (X0820–X1515 ft and X1535–X2165 ft) toward the upper three equalizers (X8188–X8855 ft, X8874–X9502 ft, and X9522–X0151 ft) as shown in Fig. 5.

ICD Completion Design and Crossflow Sensitivity Analysis. The contrast in reservoir pressure is about 158 psi between heel (low reservoir pressure of 3,507 psi) and toe (high reservoir pressure 3,665 psi). The average permeability across the horizontal section was considered to be about 300 md with minor contrast. The ICD completion design was performed at a well flow rate of 5,000 res bbl/d. A uniform ICD nozzle setting was used with short compartment lengths at heel to minimize backpressure, eliminate downhole crossflow, and balance fluid influx from this low-pressure reservoir section. The well completion model predicted downhole crossflow in an openhole completion would occur at a well flow rate of 5,000 res bbl/d, and predicted that with an ICD completion the crossflow would be eliminated at the same rate.

After the multiphase production logging run, the well completion model was calibrated using log downhole rates and BHFP.

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Reservoir permeability, fluid saturation, and pressure were changed in the well completion model to match the logging results for both high and low choke rates and pressure; reservoir pressure contrast was increased to 163 psi (reservoir pressure was 3.508 psi at the heel and 3,671 psi at the toe). The initial reservoir pressure had been 3,507 psi at the heel and 3,665 psi at the toe. The permeability profile was increased at the toe section to 800 to 1,500 md as shown in Fig. 6. Water saturation was added to match the actual water production and transmissibility profile. The well completion calibrated model results perfectly match the low and high choke logging results (downhole rate and BHFP). See Fig. 7.

Different sensitivity scenarios run with the calibrated model show that an ICD completion in open hole reduces the well critical flow rate from 18,000 res bbl/d to 4,300 res bbl/d; see Figs. 8 and 9.

Well B

Well History. Well B, as illustrated in Fig. 10, is a horizontal producer drilled in the Arab reservoir. The well was completed with a cased-hole assembly consisting of 20 ICDs and five mechanical packers, as shown in Fig. 10.

Multiphase Production Logging Results. The logging tool was conveyed using a tractor with 99% coverage of the completed interval. Production logging was run at two flowing choke settings and shut-in conditions.

The recorded total flow rate at high choke was 5,200 res bbl/d with 780 res bbl/d water, yielding 17% water cut. While the total downhole oil rate was 2,270 res bbl/d, the water rate was 360 res bbl/d. A 17% water cut was detected at the low choke flowing condition. No crossflow was observed at either flowing condition. However, crossflow was detected (500 res bbl/d of oil) during shut-in from X9547–X0035 ft measured depth (MD) to X1406–X1690 ft MD as shown in Fig. 10.

ICD Completion Design and Crossflow Sensitivity Analysis. Well B has a lower reservoir pressure at the toe than at the heel; this makes the well influx balance and downhole crossflow more challenging. The production logging data show no downhole crossflow for low or high choke rates. The permeability profile was decreased across the openhole completion, as shown in Fig. 11. The log and model match was achieved as shown in Fig. 12. The reservoir pressure contrast decreased to 27 psi (3,030 psi at the heel and 3,003 psi at the toe); the initial reservoir pressure had been 3,195 psi at heel and 3,142 psi at toe. The calibrated model shows the ICD completion reduced the well’s critical flow rate from 5,400 res bbl/d for an openhole completion to 1,450 res bbl/d. Fig. 13 and Fig. 14 show drawdown profiles and critical flow rate sensitivity for ICD and openhole completions, respectively. Conclusions

Multiphase production logging is a powerful diagnostic tool for completion evaluation, reservoir dynamics (crossflow) and for potential remedial actions, especially after breakthrough.

The logging data enable a comprehensive understanding of the reservoir parameters under dynamic conditions, which play a significant role in calibrating the static model.

Many factors control the volume of crossflow: pressure and permeability contrast between different reservoirs or within the same reservoir, and flowing or shut-in conditions that can create pressure contrasts between the different reservoirs.

With ICD completion technology, downhole crossflow can be minimized or eliminated only in the flowing phase. Downhole crossflow will continue to occur during shut-in conditions.

The methodology presented here aims to produce each well without crossflow by exceeding the critical flow rate and curtailing water production.

Recommendations

Appropriate ICD completion design requires knowledge of reservoir and geological challenges that are then used for inputs to the near-wellbore fluid flow model.

Multiphase production logs are major dynamic inputs for well completion design and evaluation; it is highly recommended to perform logging before and after the completion is installed to optimize the performance of horizontal wells.

A new ICD technology that can work as a one-way valve is required to stop downhole crossflow in flowing and shut-in conditions.

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Acknowledgments

The authors thank Saudi Aramco and Schlumberger for the permission to present and publish this paper. Nomenclature

kh Horizontal permeability, md P Pressure, psi References

Clerke, E. A., Mueller III, H. W., Phillips, E. C., et. al. 2008. Application of Thomeer Hyperbolas to Decode the Pore Systems, Facies, and Reservoir Properties of the Upper Jurassic Arab D Limestone. Ghawar Field. Saudi Arabia: A “Rosetta Stone” Approach. GeoArabia, Volume 13, No. 4, p. 113-160.

Al-Khelaiwi, F. T., and Davies, D. R. 2007. Inflow Control Devices: Application and Value Quantification of a Developing Technology. Paper SPE 108700 presented at the International Oil Conference and Exhibition, Veracruz, Mexico, 27–30 June.

Lindsay, R. F., Cantrell, D. L., Hughes, G. W., et al. 2006. Ghawar Arab-D Reservoir: Widespread Porosity in Shoaling-upward Carbonate Cycles, AAPG Memoir 88/SEPM Special Publication, pp. 97-137.

Al Marzooqi, A., Helmy, H., Keshka A., et al. 2010. Wellbore Segmentation Using Inflow Control Devices: Design and Optimisation Process. Paper SPE 137992 presented at the Abu Dhabi International Petroleum Exhibition and Conference, Abu Dhabi, UAE, 1–4 November.

Gottumukkala, V., Abd El-Fattah, M., and Ogunsanwo, O. 2011. Design Methodology for Nozzle Based Inflow Control Devices (ICD). Presentation at Passive Inflow Control Technology Meeting, Houston, 5–6 May.

Krinis, D., Hembling, D., Al-Dawood, N., et al. 2009. Optimizing Horizontal Well Performance in Nonuniform Pressure Environments Using Passive Inflow Control Devices. Paper OTC-20129 presented at the Offshore Technology Conference, Houston, 4–7 May.

Leung, E., Nukhaev, M., Gottumukkala, V., et al. 2010. Horizontal Well Placement and Completion Optimisation in Carbonate Reservoirs. Paper SPE 140048 presented at the Caspian Carbonates Technology Conference, Atyrau, Kazakhstan, 8–10 November.

Mubarak, S., Al-Afaleg, N.I., Pham, T.R., et al. 2007. Integrating Advanced Production Logging and Near Wellbore Modeling in a Maximum Reservoir Contact (MRC) Well. Paper SPE 105700 presented at the SPE Middle East Oil and Gas Show and Conference, Manama, Bahrain, 11–14 March.

Ouyang, L.B. 2009. Practical Consideration of Inflow Control Device Application for Reducing Water Production. Paper SPE 124154 presented at the SPE Annual Technical Conference and Exhibition, New Orleans, Louisiana, 4–7 October.

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Figures

Fig. 1—ICD completion diagram

Fig. 2—Multiphase production logging tool Fig. 3—Study workflow

Fig. 4—Arab-D carbonate geology (adapted from Lindsay, 2006)

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Fig. 5—Flowing and shut-in profiles (Well A)

5 ICDs 4 ICDs 3 ICDs 2 ICDs 1 ICD 2 ICDs

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Fig.6—Initial vs. calibrated permeabilities (Well A)

Fig.7—Production logging tool (PLT) downhole flow rates and BHFP match profiles (Well A)

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Fig. 8a—Drawdown (DD) profiles for ICD completion (Well A)

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Fig. 8b—Critical flow rate sensitivity for ICD completion (Well A)

Fig. 9—Drawdown profiles and critical flow rate sensitivity for openhole completion (Well A)

Critical flow rate 18,000 res bbl/d

Critical flow rate 4,300 res bbl/d

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Fig. 10—Flowing and shut-in profiles (Well B)

4 ICDs 1 ICD 5 ICDs 5 ICDs 5 ICDs

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Fig.11—Initial vs. calibrated permeabilities (Well B)

Fig. 12—Production logging tool (PLT) downhole flow rates and BHFP match profiles (Well B)

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Fig. 13a—Drawdown (DD) profiles for ICD completion (Well B)

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Fig. 13b—Critical flow rate sensitivity for ICD completion (Well B)

Fig. 14—Drawdown profiles and critical flow rate sensitivity for openhole completion (Well B)

Critical flow rate 5,400 res bbl/d

Critical flow rate 1,450 res bbl/d