What Is the Real Measure of Gas-Well Deliverability Potential?
- C. Shah Kabir (Chevron Energy Technology Co.)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Evaluation & Engineering
- Publication Date
- April 2006
- Document Type
- Journal Paper
- 126 - 134
- 2006. Society of Petroleum Engineers
- 5.5.8 History Matching, 4.2.3 Materials and Corrosion, 5.6.4 Drillstem/Well Testing, 4.1.2 Separation and Treating, 3.3.3 Downhole and Wellsite Flow Metering, 5.5 Reservoir Simulation, 4.6 Natural Gas, 3.3.1 Production Logging, 4.1.5 Processing Equipment
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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.
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