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Production histories of gas and oil wells can be analyzed to estimate reserves and production rates and to validate results of complex reservoir studies. Because accurate production data are commonly available on most wells, production data analyses can be widely applied by the production engineer. Two main analysis methods are decline curve analysis and type curve analysis. These methods are simple and economic to apply and can provide valuable data on the quality of the reservoir. Decline curves are generally easy to use, but type curves usually provide better forecasts for complex reservoirs. If large risks or large capital expenditures are needed to conduct a project, well testing and simulation can be used to supplement and/or upgrade the quality of forecasts generated by production histories (see “[[Production testing]]”) and “Reservoir Simulation” in Part 10).

Production histories of gas and oil wells can be analyzed to estimate reserves and production rates and to validate results of complex reservoir studies. Because accurate production data are commonly available on most wells, production data analyses can be widely applied by the production engineer. Two main analysis methods are decline curve analysis and type curve analysis. These methods are simple and economic to apply and can provide valuable data on the quality of the reservoir. Decline curves are generally easy to use, but type curves usually provide better forecasts for complex reservoirs. If large risks or large capital expenditures are needed to conduct a project, well testing and simulation can be used to supplement and/or upgrade the quality of forecasts generated by production histories (see [[Production testing]] and [[Reservoir simulation]]).

==Decline curve analysis==

==Decline curve analysis==

Decline curve analysis relates past performance of gas and oil wells to future performance, but it does not anticipate changes in performance due to operating conditions or changes in reservoir behavior (for more on decline curves, see the chapter on “[[Reserves estimation]]” in Part 10.)

Decline curve analysis relates past performance of gas and oil wells to future performance, but it does not anticipate changes in performance due to operating conditions or changes in reservoir behavior (for more on decline curves, see [[Reserves estimation]].)

[[file:production-histories_fig1.png|left|thumb|{{figure number|1}}Semi-log decline history.]]

[[file:production-histories_fig1.png|left|thumb|{{figure number|1}}Semi-log decline history.]]

===Flow systems===

===Flow systems===

Either linear flow or radial flow is commonly used to analyze production data and to forecast reserves. In ''linear flow'' systems, fluid flows in parallel paths to a fracture. In ''radial flow'' systems, fluid flows in a path that converges to the wellbore, causing high pressure drops near the wellbore because of high fluid velocity (see “Fundamentals of Fluid Flow”).

Either linear flow or radial flow is commonly used to analyze production data and to forecast reserves. In ''linear flow'' systems, fluid flows in parallel paths to a fracture. In ''radial flow'' systems, fluid flows in a path that converges to the wellbore, causing high pressure drops near the wellbore because of high fluid velocity (see [[Fundamentals of fluid flow]]).

Linear flow is observed as a straight line ([[:file:production-histories_fig2.png|Figure 2]]) on a log Δ''p'' or log 1/''q'' versus log Δ''t'' graph of field data. Linear flow is common in low permeability reservoirs where pressure changes very slowly and fluids move slowly through the reservoir. Radial flow is expected in wells with high permeability or in old wells where the drainage area is large.

Linear flow is observed as a straight line ([[:file:production-histories_fig2.png|Figure 2]]) on a log Δ''p'' or log 1/''q'' versus log Δ''t'' graph of field data. Linear flow is common in low permeability reservoirs where pressure changes very slowly and fluids move slowly through the reservoir. Radial flow is expected in wells with high permeability or in old wells where the drainage area is large.