Abstract:
To reach a decision as how best to produce a given reservoir it is essential to know its
deliverability, properties, size and initial gas in place (GIIP). Estimating reservoir properties
has long been a challenge. Traditionally pressure survey or well testing is conducted to
estimate the reservoir properties, which is expensive; also production loss is associated with
pressure survey. The equations used for well test analysis are derived from the constant
terminal rate solution of the radial diffusivity equation.
Well testing data is obtained from a relatively short period of time with a controlled
environment. If properly done, the data quality is good and results obtained from this test can
be reliable. This technique is used to estimate Skin Factor, Formation Permeability,
Reservoir Drainage Area, Average reservoir pressure, distance to faults, Connectivity among
well etc. If well testing is done only occasionally, developing a good understanding of the
reservoir from well testing alone is often difficult.
Conventional Decline Curve Analysis normally used to estimate original gas in place and gas
reserves. The development of modern Decline Curve Analysis began in 1944. This technique
used to analyze and interpret production data and pressure data from wells using Type
Curves. This technique also can estimate skin, permeability and gas in place. But most of the
time it is impossible to maintain controlled condition to collect undisturbed data of decline
curve analysis for a long period of time like short period of time of well testing data.
In this study for a well evaluation real cases was analyzed using both well testing and decline
analysis. First Skin and Permeability are determined with the help of pseudo pressure versus
Horner time. Also non-Darcy flow coefficient is determined from deliverability test equation.
This Skin, Permeability and non-Darcy flow coefficient is a reference point to model a
reservoir. Classical material balance and its output GIIP is also another reference point to
model a reservoir for modern Decline Curve analysis method
The GIIP estimated from classical and flowing material balance methods are 600 BCF and
580 BCF respectively. The same is estimated to be 629 BCF by Fetkovich, 630 BCF by
Blassingame and 473 BCF by Arp’s method. Except for Arp’s the rest of the methods
provided reasonably close results.
The skin factor estimated from well testing analysis is in good agreement with Decline
analysis result. Skin, s from Horner plot is 21.15, from type curve is 20, Fetkovich type curve
is 19, and Blassingame is also 19. Rate dependent skin is also detected during well test
analysis, which was characterized by non-Darcy flow coefficient of 0.456 [MMscf/D]-1. Skin
value is quite high, the major contribution is due to formation damage (sd=13) and the partial
completion is not significant (sp=2.15).
The permeability estimated from well testing is good agreement with past studies.
Permeability, k from Horner plot is 201.6 mD and type curve is 236 mD. k was also
estimated decline analysis method. Both Fetkovich and Blassingame methods provide ‘k’
value close to each other but order of magnitude lower than well testing results.
