Abstract

This paper presents a solution to the problem of heat transfer between a gas inside a wellbore and the surrounding formation during a pressure buildup test. Our results show different curve shapes caused by thermal effects, many of which have been observed in the field. None of the conventional type curves and well test analysis methods can be used to analyze and predict these kinds of pressure behavior. These wellbore thermal effects mask the true reservoir behavior and can be easily misinterpreted as reservoir behavior. The method presented in this paper is based on a semi-analytical wellbore/reservoir model constructed from mass, momentum, and energy balance equations.1 The results of this paper will help engineers identify thermal effects in gas well pressure buildup tests and find the correct transient period to analyze the tests, which is especially important in high-temperature gas wells. This will ensure accurate results from the interpretation. With the help of a simulator developed from our new model, engineers can effectively design gas well pressure buildup tests by running the simulator to determine the approximate test duration required to obtain data not distorted by wellbore effects. Examples are presented to illustrate how to use the results from this paper to analyze and simulate gas well pressure buildup tests.

Introduction

Field observations2-3 indicate that wellbore effects often cause difficulties when analyzing transient tests from gas wells, especially buildup tests with surface shut-in. These effects can be mistakenly interpreted as reservoir responses. These wellbore effects mainly include mass (wellbore storage) effects, momentum effects, and thermal or heat loss effects. The momentum effect, which is usually caused by high flow velocity in the wellbore, has a very strong impact on the transient pressure data at very early time, but it dies out very quickly. Wellbore storage effects, including variable wellbore storage, have been studied and modeled by many.4-12 Heat loss effects have been observed in many gas well pressure buildup tests. These behaviors have not been generalized or modeled with physical justifications, but they can continuously distort the pressure transient response even after wellbore storage effects end. Failure to recognize these effects in the pressure data will result in errors and misleading well test interpretation. Further, the extended duration of the heat loss affects the design of gas well pressure buildup tests.

The various aspects of heat transfer between a wellbore fluid and the formation have been studied over several decades.13-21 Most studies use steady-state models, which assume that fluid properties and flow rates are not functions of time in the wellbore. A few wellbore models, mainly for geothermal wells with two-phase transient flow inside the wellbore, exist in the literature.22-26 Generally, these models do not work for gas wells without modifications. Hasan27 and Kabir28 presented a transient wellbore/reservoir model for estimating bottomhole pressure from measured wellhead pressure in high-temperature gas reservoirs.

To improve design and analysis of gas well pressure buildup tests, we have developed a wellbore model that can describe wellbore effects during a gas well pressure buildup test.1 The model can be used in the forward mode to predict pressure transient behavior. This paper summarizes the major results generated with a simulator developed from this model and illustrates the use of the simulator in field applications.

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