Pulse Testing

Introduction

In most well tests, a measurement is made of the pressure response at the formation face of a well to a stimulus within that same well.  In the case of interference testing, the pressure response to a stimulus at one well is measured at another well.  The second well is relatively remote from the active well (the well where the stimulus is applied).

 

A pulse test is a modification of standard interference testing that is designed to assist in identification of the signal produced by the stimulus.  It can be considered as a special form of multi-well interference testing.

 

A pulse test uses shorter pulses (with smaller observed pressure changes) than an interference test.  The test is carried out between two wells, one of which is an observation well and the other well is subjected to a series of injection or production “pulses” followed by shut-in.  The process generates an identifiable pressure pattern, which can be detected by the observation well and can be isolated from the general field pressure trends.  The advantages, in comparison to interference testing can include:

 

1.     Only two wells are involved in the test at a time, and the other wells in the field can continue injection/production.

2.     The test time for determining formation properties is much shorter than for conventional interference testing.

3.     There is a basis (as in conventional interference testing) for determining storage (fc_th), diffusivity (k/fmc_t) and interval transmissibility (kh/m).

4.     Since an identifiable pressure pattern is used, there is more certainty for discriminating noise.

5.     Wells other than the observation well do not need to be monitored.

 

Basic Theory (A Single Pulse)

Consider single pulse testing in an infinite reservoir.  Since the rate in the observation well is zero and injection/production in the active well is maintained for a time Dt before it is shut in:

 

 

The subscript “L” indicates the time lag from the end of injection/production at the active well to the pressure peak at the observation well.  For the duration of the injection/production period, Dt, expressed in minutes:

 

 

In fact, if the standard assumptions of an infinite, homogeneous and isotropic reservoir are made, storage and transmissibility can be determined relatively easily from a single pulse measurement. 

 

Basic Theory (Multiple Pulses)

Consider the situation shown in Figure 1.  This shows three square pulses applied at the active well.  Be careful.  These are production pulses – injection is handled in the same way but it is related to negative rates.  The red line shows the response at the observation well and the green line shows an arbitrary general pressure trend in the reservoir.  The background general pressure changes are eliminated by only considering the pressure data between the orange dashed lines.

 

Figure 1.        Multiple pulse test rate and pressure profile.

 

General solutions exist.  They all depend on the time lag and the amplitude of the pulses.  These can be progressively simplified by approximations.  For example, if it is assumed that Dp is the same for all pulses and that the shut-in and production/injection durations are the same.  The slope of the tangent lines can be calculated and this can lead to a solution for the diffusivity.

 

A good reference on the specifics for the calculations is Sabet, 1991.  In fact Sabet suggests that simulation might be the preferable method for interpreting pulse data.  Earlougher, 1977, also provides a good discussion.

 

Skin and Storage

1.     If there is only skin at the active well, this does not change the observed pressure drop at the observation well (whether the skin is positive or negative). 

2.     The presence of infinitesimal skin at the observation well would merely decrease r_w and this does not impact the calculations.

3.     The presence of substantial skin at the observation well delays the arrival of the pressure wave (for positive skin and vice versa for negative skin).  This is most significant when the wells are close together.

4.     There is no way in practice to account for wellbore storage without assuming that the reservoir is homogeneous and isotropic.  Precautionary measures can include packers in both the active and observation wells and the use of late time data in reservoir simulation matching of the observed data.

 

Designing a Pulse Test

1.    You must first make an estimate of:

 

·         Permeability

·         Porosity

·         Thickness

·         Total Compressibility

·         Viscosity

·         Formation Volume Factor

 

2.     Estimate the background noise in the field.  It is necessary to monitor BHP in one of the wells for several weeks.  For example, if it is found that there is a ±2 psi fluctuatuion within a two hour period, it is inappropriate to design a pulse test that only fluctuates by 2 psi.  Noise (other than tides) may be less of a problem if all of the wells are shut-in.

3.     From the noise, estimate the required signal amplitude that can be discriminated.

4.     Estimate the required duration of the pulse, Dt.  Assume a value for Dt.  Calculate a value for t_DL:

 

 

5.     By trial and error determine a reasonable value for the absolute value of Dp:

 

 

 

References

1.                 Daltaban, T.S., and Wall, C.G.: Fundamental and Applied Pressure Analysis, Imperial College Press, London (1998).

2.                 Earlougher, R.C. Jr.: Advances in Well Test Analysis, Henry L. Doherty Memorial Fund of AIME, SPE, Dallas (1977).

3.                 Matthews, C.S. and Russell, D.G.: Pressure Buildup and Flow Tests in Wells, Henry L. Doherty Memorial Fund of AIME, SPE, Dallas (1967).

4.                 Sabet, M.A.: Well Test Analysis, Gulf Publishing Company, Houston (1991).