Lessons From 10 Years of Monitoring With Chemical Inflow Tracers
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Initial development of inflow tracers was designed to provide qualitative information about the location of water breakthrough in production wells. The proof of concept and application for water detection initiated the development of oil tracers for oil-inflow monitoring. The evolution of inflow-tracer-signal interpretation also has provided valuable insight to inflow characterization. A model-based approach to match the measured signals with proprietary models also has been developed.
Unique Chemical Tracers for Reservoir Surveillance
During the past decade, many unique chemical tracers have been designed. The aim of this development has been to obtain many unique signatures with properties as similar as possible. The strategy involved finding families of chemical molecules that had the potential to provide a large number of unique tracers with a small change in the chemical structure. Today, more than 160 unique signatures exist. More than 80 are within one family of oil markers.
The chemical tracers and the polymer matrix need to be stable and inert at a wide range of well conditions. No applicable accelerated test methods exist for polymer materials, so long-term functionality must be tested in the laboratory with an environment as realistic as possible to estimate inflow-tracer longevities.
Another important feature of these tracers is the concept of the detection limit. To be able to mark production over decades with a limited amount of material placed in the well, the detection limit must be extremely low. The amount of material is limited because of space, cost, or environmental concern. In 2005, the detection limits were parts per billion, while the detection limit today is below parts per trillion and is expected to move toward parts per quadrillion.
Years of laboratory work and field experience have proved that designing systems from experience is crucial. The effect of nontarget fluids and treatment chemicals on different materials is tested over specific time periods according to time of exposure under actual conditions. The qualification tests include treatment chemicals, including solvents and strong acids or bases that may affect the tracer or polymer matrix in which the tracers are embedded. Temperature is one of the major parameters affecting release, and stability and functionality tests for the design will be performed at various temperatures. The use of permanent installed tracers to optimize oil production and reduce water production will reduce the need for water treatment and interventions, thereby reducing the overall environmental impact.
Production-Monitoring Applications Using Inflow Tracers
Since the first constant-release inflow-tracer pilot installation was trialed more than 10 years ago, the main monitoring objective was a qualitative indication of the area from which water is produced. Today, several hundred well installations worldwide feature thousands of tracer systems or chemical-monitoring locations in the ground.
Most installations have been in openhole completions in both sandstone and naturally fractured carbonate reservoirs on land or offshore in both platform and deepwater environments producing through long tiebacks to floating production storage and offloading vessels. The typical well deviations can be vertical, deviated, horizontal, or undulating multilateral wells. More recently, inflow tracers have been increasingly used to monitor multistage fractured completions, providing years of valuable monitoring information. In most cases, the sensors can provide permanent downhole production-surveillance information from cleanup and restarts and during steady-state production. A summary of inflow tracer case studies, discussed in detail in the complete paper, explains the value of inflow-tracer-signal interpretation performed since the inception of the method.
Qualitative Inflow-Tracer Interpretation
One of the simplest monitoring objectives is to identify which well has broken water, when it occurred, and at which location along the producing zone. One operator piloted three passive-inflow-control-device (PICD) wells in the Pyrenees field offshore Western Australia. Each well was installed with four unique water tracers that were activated at different times to reveal how water-breakthrough development occurred. The surveillance information obtained was useful in assisting the early history matching of the reservoir-simulation model to constrain modeling outcomes. The Ormen Lange, the second-largest gas field in Norway, contains a complex big-bore subsea development with a 120-km tieback to shore. Two unique water tracers were installed above the gravel-pack packer in a pup joint in conjunction with a multiphase meter (MPM) to identify which well was producing water. Installation of two unique tracers was a contingency to verify that there was no doubt that the well was producing water. This was advisable because of the known limitations of MPMs in detecting low water cut. Inflow tracers also have been installed in numerous horizontal slotted liners in the Val d’ Agri field, which is an onshore naturally fractured carbonate reservoir in southern Italy.
Quantitative Inflow-Tracer Interpretation
To obtain quantitative results, a transient must be created to build up tracer concentration. When the well is started, the tracer shot will be transported by flow toward the sampling point. If the tracer concentration in the fluids at the sampling point is measured as a function of time or produced volume, then the high-concentration fluids passing the sampling point will present a concentration peak vs. time or cumulative production volume. This can be referred to as the arrival time, or arrival volume, of the given tracer. Knowing both the geometry of the lower completion to the sampling point and the average production rate, the produced volumes between different arrival times of tracer peaks can be estimated. By manipulating relative flow contributions in a simulator, one can attempt to match the measured and simulated arrival volumes.
The arrival model can be applied only when sampling is performed at the wellhead because a time separation will be observed. It is not suitable, however, for quantification in long tiebacks. Therefore, a new patented tracer-flowback-interpretation method named the Flush Out model was developed to allow zonal contributions to be estimated from the transient flush-out signals of tracer shots. This model overcomes the sampling-point limitations necessary for the arrival model to be applicable.
The basis for the Flush Out model is the rate of change of the released tracer material, which is dependent on the flow rate past the tracer carrier. From a high-permeability production interval, a high-tracer-concentration response is expected to decline rapidly toward the steady-state level. In a producing zone with a lower permeability, a slower rate of decline is observed. These declines are exponential in nature and have been observed in hundreds of flowbacks.
Flush-Out Model Verification
Observation of hundreds of tracer signals after a transient was created, usually from a shut in, revealed that the peak of the signal to constant steady-state levels indicated an exponential function match consistently. This initial observation formed the basis of the patented model through history matching to obtain quantification from inflow-tracer responses. To validate the observations, a full-scale flow loop was constructed with the flexibility to control base pipe flow and annulus flow and configure any completion joint to perform controlled tracer experiments. The theoretical model did indeed show an exponential function under various flow conditions, with base pipe flow ranging from 630 to 12,580 B/D, thus verifying the physics observed from realistic production rates. Fig. 1 above shows a close-up of the PICD rig; the top view of the entire rig is shown in Fig. 2.
The original monitoring objective of inflow tracers was to identify water breakthrough along a horizontal well, but they had limited longevity. Subsequently, oil-tracer systems have been developed to extend longevity to up to 10 years while using less polymer and fewer tracers compared with the first water-tracer system. Monitoring sensors have been developed and proved in wells to withstand harsh reservoir and fluid conditions. Furthermore, model-based approaches have been developed to enable the quantification of flow. The models have been verified in a full-scale loop using actual completion components and have also been compared with the incumbent production-logging tool. Several hundred wells have been installed with inflow tracers worldwide in many different types of openhole and cased and perforated completions. The authors write that they believe that inflow-tracer technology will be adopted as an industry standard, being installed in most wells to provide risk-free monitoring.
Lessons From 10 Years of Monitoring With Chemical Inflow Tracers
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