Long-Term Production Testing Improves Reservoir Characterization in the Oseberg Field
Authors: Fjerstad Paul A. , Flaate Svein A. , Jens Hagen , Steinar Pollen 
Published: December 2007

Long-term reservoir testing provides an improved understanding of static and dynamic reservoir behavior before full-field development. The long-term test also improves understanding of the geological model and the connected hydrocarbon volumes. The experience gained from long-term testing has strengthened the basis for field development and reservoir management.

Introduction

The Norwegian Oseberg field came on stream in April 1989. The reserves estimate for the field is 1.45×109 STB [231.6×106 stock-tank m3] of oil and 3.25×1012 scf [92×109 std m3] of gas. Long-term production testing was performed as part of the data gathering for reservoir evaluation before field production startup. The testing program comprised two production wells and four reservoirs during a 20-month period. The production test ship Petrojarl I was used to enable comprehensive data gathering. Revenue from the sale of produced oil covered the cost of the operation.

A large portion of the economic risk linked to offshore field developments lies in the forecasted hydrocarbon production profile. Long-term production testing is a major step toward gaining insight into reservoir dynamics and thus reducing reservoir uncertainties. This paper describes the motivation for and process of achieving early information on reservoir features by long-term production testing.

The test performed on the Upper Brent sand in Well 30/9-T1 on the Gamma structure is used as a case history. This long-term production test contained several sequences - pressure-buildup tests, a rate sensitivity test, and a resumed test - designed to evaluate and improve the reservoir geometry and flow description.

On-line measurements of flow and pressure data together with the long duration of the test enabled reservoir engineers to optimize the testing and data gathering according to the observed reservoir behavior.

The experience gained on reservoir and well behavior has improved and strengthened the current basis for field development and reservoir management.

Background

During evaluation of a field development, the reservoir engineer provides an evaluation of the reservoir, including recommended drive mechanisms, required number of well slots, topside processing and injection capacities, and well locations and timing. This evaluation is used to determine the optimum field development concept and as the design basis for planned installations. The corresponding forecasted field hydrocarbon production profile also is provided to enable an assessment of overall field economics, preferably with risks and uncertainties highlighted and evaluated.

Long-Term Production Test

Definition.

A long-term production test is defined as producing the hydrocarbon discovery at realistic field development rates with extensive data monitoring (rates and pressures) for an extended period (usually at least 4 weeks) to learn more about the long-term reservoir performance. It should be possible to alter the test duration and plan as new information becomes available from the test. A long-term production test often is aimed at reducing reservoir pressure in a larger region than can be reached in a short-term test. This may provide information for material-balance calculations and analyses of future or present communication between wells. Obtaining water or gas breakthrough may also be a testing goal for fields with expected multiphase reservoir flow during field production.1-3

Long-term production testing implies producing a reservoir long enough to activate time- or volume-dependent reservoir phenomena. The testing period should be long enough for proper interpretation techniques to be applied on test results. The testing period necessary to achieve representative data to meet test objectives depends on reservoir type and quality and cannot be stated generally.

Bearing in mind the diversity of reservoir properties and flow behavior, we use "long-term" for periods longer than 4 weeks. The term "production testing" is used to indicate that extensive data gathering was performed during production of the well at rates and conditions similar to those of the planned field development.

Definition.

A long-term production test is defined as producing the hydrocarbon discovery at realistic field development rates with extensive data monitoring (rates and pressures) for an extended period (usually at least 4 weeks) to learn more about the long-term reservoir performance. It should be possible to alter the test duration and plan as new information becomes available from the test. A long-term production test often is aimed at reducing reservoir pressure in a larger region than can be reached in a short-term test. This may provide information for material-balance calculations and analyses of future or present communication between wells. Obtaining water or gas breakthrough may also be a testing goal for fields with expected multiphase reservoir flow during field production.1-3

Long-term production testing implies producing a reservoir long enough to activate time- or volume-dependent reservoir phenomena. The testing period should be long enough for proper interpretation techniques to be applied on test results. The testing period necessary to achieve representative data to meet test objectives depends on reservoir type and quality and cannot be stated generally.

Bearing in mind the diversity of reservoir properties and flow behavior, we use "long-term" for periods longer than 4 weeks. The term "production testing" is used to indicate that extensive data gathering was performed during production of the well at rates and conditions similar to those of the planned field development.