Determining Reservoir Heterogeneity and Flow Geometry from a Successful Interwell Tracer Test (IWTT)

By: Wei Tian and Alex Darnley

Waterflooding and gas/CO2 flooding are often utilized to enhance oil recovery (IOR/EOR) by displacing hydrocarbons that were left unrecoverable during primary recovery. The injected water or gas will sweep mobile oil to offset producers and significantly increase hydrocarbon recovery. In such processes, understanding sweep efficiency between well pairs is critical for engineering teams to evaluate the success of a flooding operation and optimize field development.
Reservoir heterogeneity and flow geometry are two critical parameters that need to be defined before attempting to optimize a flooding operation. A successful flooding operation should yield a uniform fluid distribution (vertically and aerially) by evenly sweeping the reservoir surrounding the injector. Matrix bypass events (MBE) such as fractures, thief zones, high-perm streaks etc. should also be avoided as these heterogeneities will greatly reduce sweep efficiency and cause unwanted cycling of injected fluid.

How it works

An interwell tracer test (IWTT) provides a simple and effective method for defining the heterogeneity of each well pair by tagging each injector with a unique chemical signature. The operations involved in executing a tracer test are simple: inject unique chemical tracers at each injection well and monitor tracer breakthrough at each offset producer. The data is captured by taking produced water/gas samples and sending the samples to the laboratory.

The tracer production profile at each producer is reported as ‘tracer concentration vs. time’. Reservoir properties can be calculated from a successful tracer test by integrating tracer data with injection/production data and then using residence time distribution analysis (RTDA). The following reservoir parameters can be calculated:

1. Well Connectivity
   Offset producers that communicate with each traced injector can be mapped easily by monitoring tracer production at each producer. The operator can then create a map of interconnected well pairs, allowing the operator to confirm each injector’s influence on offset producers and identify any directional permeability trends in the field.
2. Allocation Factor
   Based on the principle of mass balance, the total recovered mass of tracer produced from an offset well can be calculated as long as the tracer production profile and production rate are known. Then, dividing the total recovered tracer mass by its total injected mass gives the allocation factor of the well pair (injector – producer). Allocation factor measures the connectivity between the injector and producer. A higher allocation factor means a larger portion of the injected fluid is flowing towards the producer. A successful flooding operation should produce similar allocation factors for each well pair within a particular pattern.
3. Reservoir Heterogeneity

   For each well pair, the diagram of flow capacity (F) versus storage capacity (φ) can also be plotted from the tracer production profile. Both flow and storage capacity are normalized, ranging from 0 to 1. The area between the plotted tracer curve and unit slope line in the F- φ diagram gives the value of the Lorenz coefficient, which is a measure of reservoir heterogeneity. A highly heterogeneous reservoir has a Lorenz coefficient approximate to 1.

4. Swept Pore Volume
   Combining the tracer production profile with production data and applying residence time distribution analysis (RTDA) will yield the volume swept by the tracer between the well pair. This is the reservoir volume between injector and producer that has been swept by the injected fluid. This important reservoir parameter indicates the potential amount of recoverable oil after a flooding operation.

ResMetrics Tracer Selection & Field Operations

An interwell tracer test requires the use of conservative tracers, that is, chemical compounds that are exclusively soluble in one phase only. Depending on the type of flooding operation (i.e. waterflooding or gasflooding), different chemical tracers are used to tag the injected fluid. A water-based chemical compound is required for waterflooding, whereas a gas-based chemical compound is required for gas/CO2 flooding. Resmetrics provides unparalleled tracer screening to ensure performance of each tracer in the target formation.

During tracer injection, chemical tracers are pumped simultaneously with the injected fluid as a tracer slug, resulting in a high tracer concentration when the tracer breaks though at the producer. This method requires significantly less tracer, thereby reducing costs to the operator, and results in the best signal in the laboratory. ResMetrics also utilizes a high-pressure, high-rate chemical injection pump that can inject tracers downhole in less than an hour. This allows the operator to trace multiple injectors simultaneously while minimizing operational footprint in the field.

ResMetrics provides white-glove service and consulting for the design, execution, analysis, and interpretation of an IWTT. We will work with the operator’s engineering teams to design a cost-effective tracer test that maximizes data gleaned from the results. All tracer data is delivered to our cloud platform (www.petroxy.com) which includes built in modules for calculating the reservoir properties mentioned above.

Themes: Interwell tracer (IWTT), waterflooding, gasflooding, CO2 flooding, sweep efficiency, reservoir conformance, chemical tracer, sweep efficiency, reservoir heterogeneity