Communication Testing

Thu, 9 Jun 2005

Communication Testing relies on changes in pressure at one location producing measurable pressures changes at the second location. The most common communication tests used in the production of oil and gas are the Interference Test and the Pulse Test.

Interference Testing

Interference Tests are used to determine if two wells are producing from the same reservoir. When two wells are known to be in the same reservoir, the Interference Test can be used to measure the reservoir permeability between the wells. The Interference Test is conducted by producing from, or injecting into, the "active" well while monitoring the pressure in an "observation" well. The magnitude of the pressure response and the time required to observe the response are functions of the reservoir rock, the reservoir fluid properties and the sensitivity of the pressure recording device. The SPIDR has a maximum resolution of 0.01 psi.

When designing an Interference Test, the first step is to estimate the time required to detect a measurable pressure response at the observation well. The reservoir and fluid properties must be known or estimated. The table below lists the information required along with sample values for a gas reservoir:

The test time may also be shortened by increasing the size of the rate change in the active well. A rate change of 200 MCF/D rather than 100 MCF/D would reduce the test time for a 1 psi resolution gauge from 97 hours to 37 hours.

The SPIDR offers the same resolution as the best downhole electronic gauges with the advantage that on single phase observation wells, pressures may be monitored from the surface at far less cost and risk than with downhole gauges. Additionally, pressure data acquired by the SPIDR is unaffected by temperature change.

The procedure to determine a specific pressure response at an observation well from a rate change at an active well requires use of the Exponential-integral curve by Earlougher (1). The three dimensionless terms from this curve that must be evaluated are:

Using the data from Table 1, Pd is calculated to be 708 times the gauge resolution or .708 for a gauge with 1 psi resolution. From Figure 4 (not shown), the term is read as 1.6. Equations 2 and 3 are combined and time t is calculated to be 97 hours.

To calculate reservoir properties from an interference test, pressures are recorded at the "observation" well beginning with the moment of rate change at the "active" well. It is assumed that the reservoir has been in steady state prior to the test. A log/log plot of delta-time versus delta pressure is constructed. Using type curve matching procedures, the dimensionless terms of equation 2 and 3 may be evaluated for permeability and compressibility. Fractured reservoirs can effect observation well response.

Pulse Testing

Pulse testing has the same objectives as interference testing; i.e., to detect communication and determine reservoir properties. The pulse test utilizes a controlled sequence of withdrawals or injections at the "active" well while monitoring an "observation" well. The pattern of production pulses should be reflected in the observation well if communication exists. The pulse test helps separate "noise" and the baseline trend in reservoir pressure changes from the response to the change in flow at the producing well. The active well in a pulse test should be free of skin damage and well bore storage should be minimal. Selectively testing wells around the active well will determine reservoir flow patterns and reservoir anisotropy.

The pulse test does not require as much time to run as the interference test, however, it requires gauges with better resolution and is more complicated to run and analyze. A typical Pulse Test is shown below:



The duration of all flow periods and shut-in periods must be equal. The flow duration, however, may be different than the shut-in duration. Figure 2 shows the time lag, t1, between the end of the first pulse and the pressure peak at the observation well. The size or amplitude of the pulse, is the vertical distance between the peak and the two adjacent valleys. The pulses may be analyzed to determine reservoir properties.

Data from multiple Pulse Tests show that the first and second pulses are not only different from each other but also different from all subsequent pulses. After the first two pulses, all odd numbered pulses are similar and all even numbered pulses are similar. This behavior pattern has led to series of eight type curves developed by Kamal and Brigham (2). There are two curves for the first odd pulse. The same is true for the even pulses. Each pair of curves plots the ratio of time lag (t,) to cycle length (tc) against the pressure response of that cycle and the time lag for that cycle. for the first odd pulse. Space limitations do not allow reproducing all eight type curves which may be found in the cited references. The term F' is the ratio of the pulse time to the total cycle time.

(4) = pulse time/cycle time.

Three dimensionless variables must be evaluated for the analysis of pulse tests:


 

Dimensionless time lag =
Dimensionless distance between wells =
Dimensionless pressure response =


After measuring the time lag and amplitude of a specific pulse, the values of dimensionless time and pressure can be read from the appropriate graphs. By substitution in the above equations, reservoir properties may be calculated.
1. Earlougher, R.C. Jr.: Advances in well Test Analysis, Monograph Series, Vol 5, SPE, Dallas (1967).
   
   
2. Kamal, M. and Brigham, W.E.: Pulse-Testing Response for unequal Pulse and Shut-in Periods, "Sec. Pet. Eng. J. (Oct. 1975) 399-410: Trans., AIME, 259.
   
   
3. Lee, John. : Well Testing, Sec. Pet. Eng. Textbook Series Vol 1. 89-97: Dallas 1982.
   
   
4. Slider, H.C.: Worldwide Practical Petroleum Reservoir Engineering Methods, PennWell Books, Tulsa 1983.