Abstract

Measurements associated with drillstem tests (DSTs) comprise surface flowrates and downhole pressure and temperature. Sandface rate and fluid density are important data but are generally not measured for lack of the appropriate instrumentation. These measurements are occasionally available when a production logging survey is conducted during DST operations. This is, however, not often the case because of the risk of spinner clogging when the well is cleaning up or when significant sand production occurs.

This paper presents a DST data set composed of continuous downhole density and mass flowrate measurements in addition to the "standard" pressure and temperature. It was acquired with a prototype fullbore multiphase flowmeter located below the tester valve.

Sandface fluid density provides a means to monitor the clean up period. Sandface rate measurements are used to convolve the reservoir pressure response and to estimate reservoir permeability and skin from flowing transients. Of particular interest in this field example, the additional measurements provide evidence of the formation of a gas cap below the tester valve during the main shut in, allowing interpretation of the apparently incoherent build up response.

Introduction

A drillstem test is generally performed on zones of unknown potential in an exploration well and sometimes on known productive zones in an appraisal or development well. The test consists of a succession of flowing and shut-in periods.

The initial short flowing period is designed to release the high hydrostatic mud pressure. The ensuing shut in is of sufficient duration to allow the measured pressure to approach the initial formation pressure-one or two hours is generally considered sufficient for this purpose. The well is then flowed on various adjustable chokes until completion fluids and debris have been produced and the well has cleaned up. A sequence of shut-ins and stabilized flow periods follows, during which surface flowrates are measured at the separator. The well may finally be opened on a small choke to allow sampling with the smallest possible pressure drop. The test surface flowrate and downhole pressure history are interpreted to yield reservoir parameters such as permeability, skin, heterogeneities and flow barriers.

A review of current DST practices is presented. Interpretation constraints that result from the absence of continuous downhole flow rate and density measurements are outlined. The reader is referred to Ehlig-Economides and Brown for a thorough discussion of these aspects.

Any given drawdown or buildup transient is affected by all previous flowing and shut in periods and cannot be analyzed in isolation. The effects of the prior testing sequence is accounted for through the use of the superposition time function which requires that the entire test flowrate history be known accurately.

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