Reservoir

A Full-Field Interwell Tracer Program on a Giant Carbonate Oil Field

This paper reviews the design and implementation of a full-field interwell tracer program for a giant onshore oil field in Abu Dhabi.

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This paper reviews the design and implementation of a full-field interwell tracer program for a giant onshore oil field in Abu Dhabi. The field is under peripheral waterflooding in order to maintain reservoir pressure and provide a mechanism to sweep the oil. However, the existence of strike-slip fault planes juxtaposed across the reservoirs added a variable to the complexity of waterflood management. To improve the understanding of reservoir heterogeneity and reduce the uncertainties associated with major faults, a full-field water-tracer program has been designed.

Introduction

On the basis of the streamline model, unique chemical tracers have been injected in 21 water injectors covering all the peripheral areas; dozens of associated offset producers across the field were selected to monitor the tracer movement. The design of the program is explained in the complete paper in terms of tracer selection and capability tests, ­chemical-volume calculation, ­injection-well and monitoring-well selection (made on the basis of the streamline model), and design of the comprehensive monitoring plan. Ultimately, the results of this tracer program will be used in combination with other reservoir-­surveillance tools to facilitate better management of the reservoir.

Background

The subject field is located approximately 110 km to the southeast of Abu Dhabi Island. The field was discovered in 1965, with first production in 1973 and first water injection in 1976. The main oil-bearing zones are X, Y, and Z. These zones are in communication through faults. Each zone is bounded at the top and base by a dense argillaceous limestone and further subdivided by the occurrence of stylolite-bearing dense intervals. Of these, Zone Y, the subject of this study, contains more than two-thirds of the field’s original oil in place.

Zone Y is an undersaturated, heterogeneous, and highly stratified reservoir with an average porosity of 30%. It is divided into five subzones. The Upper Y contains the higher-permeability zones in excess of 700 md, while the Lower Y has a lower permeability of approximately 10 md. The current development scheme, with selective injection and production under peripheral water injection, was implemented in 1982.

The existence of major faults in Zone Y resulted in splitting the reservoirs into 12 main sectors. Among the major monitoring tools and techniques, tracer injection is one of the most important for monitoring the effectiveness of a waterflood project because of the lack of direct information on flow from individual injection wells. Tracer injection can be used to determine early water breakthrough in different areas, crossflow between zones, and the degree of water slumping caused by reverse coning.

Design

Objective. A clearly defined objective is the first step toward achieving a successful tracer study. Overall, the full-field ­water-tracer program is aimed at achieving the following objectives:

  • Improve understanding of reservoir heterogeneity and reduce the uncertainties associated with major faults.
  • Determine the fluid pathways and verify channels for (or barriers to) flow.
  • Monitor water breakthrough of selected patterns in Zone Y.
  • Determine directional permeability trends.
  • Improve tuning of reservoir-simulation models for future prediction.

Tracer-Type Selection. There are primarily three types of tracers used widely in the petroleum industry: radioactive, chemical, and partitioning. Radioactive tracers are chemicals containing radioactive isotopes that can be identified by their unique type and the energy of their emitted radiation. Chemical tracers are also referred to as non­radioactive tracers and can be identified with common analytical methods. Partitioning tracers are soluble in both water and oil and thus are useful in estimating remaining oil saturation in the swept zone. For sensitivity, safety, and stability reasons, chemical tracers were selected ultimately over radioactive tracers.

Volume Calculation. An important step in designing an interwell tracer program is to inject sufficient tracer material into the reservoir to ensure a detectable tracer concentration to be presented in the well scheduled to be sampled under a wide range of flow scenarios. Therefore, there must be an understanding of the parameters that affect the produced-tracer concentration. The quantity of tracer material required for ­injection is dependent on the following reservoir parameters:

  • Distance between injectors and producers
  • Total reservoir thickness open to flow
  • Current water saturation in the reservoir
  • Analytical detection limits of the tracers injected

The volumetric method, also referred as the total-dilution method, is used to calculate the required amount of tracer material. The calculated tracer amounts for each of the injectors are listed in Table 1 of the complete paper.
Well Selection. Streamline modeling was used to obtain a list of potential injectors and corresponding producers for the subject tracer study to improve the understanding of reservoir heterogeneity and reduce the uncertainties associated with major faults. In addition to the streamline-simulation model, model water saturations, well allocations, and well-production tests were also used to update the list of wells. The main criterion for well selection is assurance of adequate areal coverage of the reservoir for the injection wells, also considering wells on either side of major fault corridors. As for the production wells, the selection criterion was wells that have existing water cut, giving a higher probability of detecting the tracers, which are associated with the water. Twenty-three injectors in Zone Y are identified for water-tracer injection and, correspondingly, more than 50 strings for the observation of tracers. Fig. 1 shows the streamline modeling, while the tracer-injection and sampling-location map can be seen in Fig. 2.

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Fig. 1: Streamline modeling.

 

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Fig. 2: Tracer-injection and sampling-location map.

 

Monitoring Plan. Following the injection of the chemical tracers in the reservoir, samples of the produced fluids from specific producers selected from the streamline modeling will be taken at predetermined intervals and analyzed for the presence of tracer. Usually, to capture tracer-arrival time precisely and to define the entire tracer wave accurately, a high frequency of sampling is scheduled and thus a large amount of samples will be taken; however, not every sample has to be analyzed. The plan was to collect regular monthly samples and to analyze every third sample for detection of the tracer. Once tracer has been identified in a sample, previous samples will be analyzed to determine the exact time of tracer arrival; every sample will be analyzed thereafter to determine the tracer curve. After the tracer has been in the reservoir for a considerable amount of time, the tracer band will have been prolonged, so sampling can be less frequent. Nevertheless, the monitoring plan should be updated continuously to capture any change that may occur within the reservoir.

Execution

Preinjection Preparation. Before the commencement of any tracer injection, a scout trip is always necessary to investigate the operational and logistical aspects of the tracer study, such as risk assessment, wellhead integrity, and injection-point determination. Another important procedure is to collect initial preinjection samples for compatibility tests and baseline laboratory analyses, so that a background reading of existing chemicals in the reservoir is obtained.

Tracer Injection. The tracers injected are chemicals in water solutions. A specially designed pump will be used for injection. The injection point should be picked carefully so that the tracer can be introduced into the individual injection lines with no possibility of contamination. The injection should be carried out after injection wells reach stable injection. Also, because of the possible existence of some natural fractures or high-permeability streaks, it is recommended to perform the injection of the tracer material into each well over a longer period of time to facilitate acquisition of a better tracer curve from the sampled producers. All relevant information (e.g., well number, injected-tracer name, volume of tracer injected, and displacing time) should be recorded carefully.

Sampling. A few points to be considered during the taking of water samples from the wellhead are listed as follows:

  • Before taking the water samples, vent the sampling point and let the fluid flow to the atmosphere for a sufficient amount of time to ensure that the subsequently obtained sample is representative of the flow stream.
  • Make sure that the same hose is not used for collecting the wellhead samples to avoid cross-contamination.
  • Usually, a mixture of produced oil and water will be collected. The volume to be collected depends on the water cut of the sampled well: The lower the water cut, the higher the volume that has to be collected, so that a sufficient amount of water can be obtained after separation for the tracer testing.

Summary

  • Twenty-three water injectors were identified on the basis of streamline modeling for tracer injection and dozens of producers were selected for monitoring.
  • Injected tracers were selected on the basis of uniqueness, stability, detection limit, and environmental friendliness, and compatibility tests were performed in advance between tracers and reservoir fluids.
  • The total-dilution method was used to calculate the quantity of tracers injected in each of the wells.
  • A comprehensive monitoring program was scheduled to capture the arrival of the tracer and to depict the entire tracer wave.
  • Sampling frequency should be properly designed to capture the breakthrough (high-frequency sampling is required in the beginning).

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 177527, “The Design and Implementation of a Full-Field Interwell Tracer Program on a Giant UAE Carbonate Oil Field,” by D. Wang, A.B. Al-Katheeri, S.M. Al-Nuimi, and A. Dey, Abu Dhabi Company for Onshore Petroleum Operations, prepared for the 2015 Abu Dhabi International Petroleum Exhibition and Conference, Abu Dhabi, 9–12 November. The paper has not been peer reviewed.