Simultaneously measured bottomhole pressure and sandface flow rate data have been used to study the transient behaviour of oil wells. Recent studies show that improved estimates of reservoir permeability - thickness product (kh) and skin (s) result when two independent measurements - pressure and rate - are available. Additionally, identification of the reservoir flow model (pD − tD function) is possible by deconvolving the early-time after-flow-dominated data.

Clearly, convolution of rate-pressure-time data provides more reliable results than conventional methods using the pressure-time data alone. This study extends the use of the technique to gas well testing.

This paper introduces a new pressure function, and addresses two aspects of gas well-test analysis: (1) convolution of simultaneously measured pressure and sandface rate data, and (2) prediction of stabilized deliverability from a flow-after-flow test.

The new pressure function results from a normalization of the pseudopressure and has the dimension of a pressure. It offers several advantages, for instance, gas flow in a porous medium can be described by an equivalent ‘oil equation’ which retains the units of pressure, i.e. psi or kPa. Examples of variable rate tests demonstrate the application of the proposed pressure function. Results thus obtained compare favourably with those calculated by using the Al-Hussainy-Ramey pseudo-pressure function.

A new time function, resulting from the normalization of Agarwal's pseudotime is also proposed.

Transient flow-after-flow tests are analyzed to obtain the stabilized deliverability. Estimation of stabilized deliverability is possible by evaluating the constants of Forchheimer's equation from the test data, with an assumed reservoir radius. We show that the absolute open flow potential (AOFP) is not sensitive to the reservoir radius. This observation implies that the duration of deliverability test could be significantly reduced. Theoretical validity of transient flow-after-flow test is also established in this work.

Keywords:
gas well, compressibility, time function, drillstem/well testing, assumption, pseudotime, sandface rate, flow equation, mass rate, pressure function
Subjects:
Well & Reservoir Surveillance and Monitoring, Fluid Characterization, Reservoir Fluid Dynamics, Formation Evaluation & Management, Phase behavior and PVT measurements, Fluid modeling, equations of state, Flow in porous media, Pressure transient analysis, Drillstem/well testing, Well performance, inflow performance
Copyright 1984, Society of Petroleum Engineers of AIME
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