This paper probes the usefulness of establishing the traditional
time-variant, absolute-open-flow potential (AOFP) on a given well. Our
contention is that a well’s AOFP is not a measure of its future potential in a
volumetric system owing to ever-declining reservoir pressure. To circumvent
this reality, we suggest a two-step approach. First, conduct a multirate test
to establish reservoir parameters, such as permeability, mechanical and
non-Darcy skin, and average pressure. Second, with these known parameters, use
an analytic tool to describe the deliverability potential for a well or a group
of wells, including reservoir uncertainty and/or operational constraints.
This paper presents a simple methodology for establishing reservoir
parameters and predicting a well’s future deliverability potential. Field
examples show that computing reservoir parameters from buildup and drawdown
data and establishing the deliverability relation instills confidence in
analysis. We also show that the traditional log-log graphing of the
backpressure equation is no longer required because we avoid the notion of a
stabilized deliverability concept.
An analytic reservoir simulator was developed to handle well location in
various drainage shapes using the pressure-transient analog for rate
computation. Material-balance calculations form the backbone when depletion
sets in. This simulator is also capable of handling the uncertainty of various
input variables and performs full-factorial design calculations for a
three-level design (that is, P10, P50, and P90). This feature facilitates
capturing uncertainty of drainage area and/or any other variables while
predicting future rates.
Gas-well deliverability testing traces its origin to the work of Rawlins and
Schellhardt (1936). This landmark study presented the well known empirical
backpressure equation for analyzing conventional flow-after-flow (FAF) test
data. Further work showed that this equation also could be used to analyze
isochronal (Cullender 1955) and modified isochronal (Katz et al. 1959) data. In
addition, Forchheimer’s quadratic equation is thought to be a more reliable
tool for estimating a well’s AOFP. Attempts were also made to correlate the
coefficients of the two deliverability equations. The Alberta Energy Resources
Conservation Board (Theory and Practice 1975) provides a comprehensive
treatment of the well-established test and interpretation methods.
In contrast to multirate testing, the analysis methods of Meunier et al.
(1987) and Horne and Kuchuk (1988) showed that a single transient, such as a
buildup test, yields AOFP in addition to reservoir parameters. However, this
single-transient method requires downhole flow measurement with pressure.
Meunier et al. (1987) also proposed a transient flow-after-flow (TFAF) test
method. By eliminating the intervening shut in periods of the popular modified
isochronal test, the total test duration can be reduced by one-half. In most
cases, the stabilized AOFP can be computed from reasonable inputs of drainage
shape and size, thus avoiding (Brar and Aziz 1978) the need for conducting the
stabilized segment of the test.
Brar and Aziz (1978) were the first to point out that the stabilized
deliverability segment of the test is expendable for establishing a well’s
AOFP. This finding constituted a major advancement in deliverability testing.
That is because discerning the onset of the pseudosteady-state (PSS) flow
period is very difficult, if not impossible, in practice. Maintaining a
constant wellhead rate is demanding due to time-variant fluid temperature
(Hasan et al. 2005) along the well length. This problem is exacerbated with
increasing reservoir temperature and increasing kh formations.
Two major issues are addressed in this paper. First, we show that multirate
drawdown tests, followed or preceded by a buildup test, allow one to estimate
the necessary parameters (k, s, D, and p) for future deliverability
predictions. In this context, we skirt the notion of stabilized deliverability
potential and simplify deliverability-test interpretation. Second, an analytic
simulator is used to predict future well performance with full-factorial design
calculations by incorporating uncertainty.
© 2006. Society of Petroleum Engineers
View full textPDF
- Original manuscript received:
31 May 2004
- Revised manuscript received:
25 July 2005
- Manuscript approved:
3 January 2006
- Version of record:
20 April 2006