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Abstract

A drillstem test (DST) can be run prior to completion of a well to determine if a formation is capable of producing oil or gas at commercial flow rates. In the eastern United States, drillstem tests are seldom used because a typical well will not produce at flow rates high enough to measure during a DST, especially in low permeability reservoirs where mud filtrate invasion prior to the drillstem test suppresses the gas flow rates. A closed chamber drillstem test can be used to measure these low flow rates but interpretation is often difficult. This paper examines the effects of mud filtrate invasion upon the results obtained from drillstem tests.

A two-phase, two-dimensional model was used to simulate a drillstem test following a period of mud filtrate invasion. Using the results of these drillstem test simulations, qualitative relationships between absolute permeability, gas relative permeability, filter cake permeability and permeability, filter cake permeability and thickness, and wellbore pressures during the filtrate imbibition period and flow period have been established.

All of the parameters investigated, except for the filter cake thickness, did influence the results from the drillstem test and the ability to forecast flow behavior based upon the drillstem test results. In particular, this study has shown that the parameters that affect the cleanup behavior of a formation can be ranked in order of importance as follows: absolute permeability, gas relative permeability, filter cake permeability, pressure drawdown and, finally, overbalance pressure drawdown and, finally, overbalance pressure during drilling. pressure during drilling

Introduction

Using a two-phase, two-dimensional finite difference reservoir simulator, an analysis has been conducted of drillstem tests (DST) under a variety of reservoir conditions. The objective of this study was to clarify how certain factors would influence this type of short duration well test and, in turn, how to use these results to predict future well performance. Unlike other well tests, drillstem tests are conducted for only a few hours. As a result, the flow rate and fluid production data are highly influenced by factors that enhance mud filtrate invasion or inhibit subsequent cleanup of the imbibed fluids around the borehole.

Ideally, one could correlate actual drillstem test results with the ultimate performance history of a particular set of wells; however, such field data were not readily accessible for this investigation. Instead, a reservoir simulator has been used to model a typical drillstem test and perform parametric sensitivity studies of the most critical parametric sensitivity studies of the most critical variables.

A drillstem test usually refers to open hole operations and, in that sense, is a temporary completion which provides a means of estimating formation and fluid properties before the completion of the well. During a DST, the zone of interest in the open hole is isolated with an inflatable packer which relieves the hydrostatic pressure exerted by the mud column. The zone is pressure exerted by the mud column. The zone is allowed to produce through the drillpipe or drillstem. The basic tool assembly consists of a packer, a test valve and an equalizing valve. The packer, a test valve and an equalizing valve. The packer is used as the isolating element packer is used as the isolating element - ie expands against the hole to segregate the annular sections above and below the element. The test valve controls flow into the drillpipe by preventing mud from entering the drillpipe while preventing mud from entering the drillpipe while running the test assembly iv-to the hole and letting formation fluids enter the drillpipe during the test. The equalizing valve allows pressure equalization across the packer after completion of the flow test so that the packer can be safely released. During the test both pressure and flow rate are measured as a function of time.

For low permeability gas reservoirs, the main purpose of a drillstem test is to determine if the purpose of a drillstem test is to determine if the formation can produce at commercial flow rates; however, due to the low permeability, the DST results can sometimes be misleading.

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