This paper proposes a variation of ‘slug test’ for sucker-rod pumping oil wells. Application of slug testing simply expedites the buildup process. A slug test is conducted by admitting a slug of reservoir liquid such that the hydrostatic head of the wellbore liquid column balances the reservoir pressure; that is, the well kills itself. Bottom-hole pressures (BHP) monitored in such a test have been routinely analyzed by the hydrologists and petroleum engineers alike over the past 35 years.
Although a slug test is typically associated with a new well, the same concept can be extended to pumping wells producing for years. A variation of slug test can be conducted on a pumping well by leaving the casing-head valve open and recording pressures either down-hole or by using the acoustic method. The data so gathered lends itself to slug test interpretation.
An analysis of wellbore storage phenomenon arising from flow of a two-phase gas/liquid mixture is presented. The analysis shows that a slug test would allow the pressure buildup process to occur much faster than the conventional buildup method wherein a surface shut-in is required. Two tests conducted on the same well by the two methods, using down-hole recorders, demonstrate the value of the proposed method. Reservoir parameters computed from the two tests are in very good agreement; thus, lending credence to the method proposed.
Testing sucker-rod pumping wells in North America is a demanding task. The long-duration storage arising from low reservoir transmissivity and/or energy necessitates along test duration. Often tests conducted over live to seven days are required to furnish interpretable data. If formation characteristics dictate further increase in shut-in time, low-rate wells may prove to be economically unattractive for additional test duration.
Methods have been proposed to enhance test interpretation, which has often led to the reduction of test duration. By using sand-face rates together with pressure, convolution, rate normalization and deconvolution methods have been used to interpret pumping well data1–3. In this work we propose to reduce the test duration by performing a variation of slug test and analyzing the data using the slug test interpretation technique.
Test data can be gathered either by direct gauge measurement, or by using the indirect Acoustic Well Sounding (AWS) method. Given the advancement made in the AWS technique, both in terms of measurement4 and understanding of two-phase wellbore flow during buildup5,6, this method is recommended. Besides, the AWS method is economically attractive. Note that unlike a conventional buildup test wherein the gas rate is computed from material balance considerations5, the rate needs to be measured at surface so that proper liquid gradient and hence, the BHP can be computed.
During a buildup test, the fluid influx can be equated to the accumulation term in an open pumping-well annulus (Fig. 1). Thus,
Influx = Accumulation
Equation (1) (Available In Full Paper)
The underlying assumption of Eq. 1 is that the free gas, if any, is continuously vented.