Definition

WFP – Well Performance Analysis is a specialized analysis of correlation between surface flowrate and wellbore pressure as function of formation pressure and associated production optimization procedures.

Application

Setting up the optimal production targets
Create WFP – Well Flow Performance as input for 3D simulations

Technology

WFP is performed on stabilised flow and does not cover transient behavior which is normally covered in Well Testing domain.

The flow is called stabilised if the well productivity index is not changing over time.

It's important to remember the difference between constant rate flow and stabilised flow.

The stabilised flow may go through a gradually changing flow rate due to formation pressure change, while the productivity index stays constant.

On the other hand, the constant rate flow may not represent a stabilised flow as the bottom-hole pressure and productivity index maybe still in transition after the last rate change.

The WFP methods are not applicable if the well flow is not stabilised even if the flow rate is maintained constant.

There are two special reservoir flow regimes which are both stabilised and maintain constant flow rate: steady state regime (SS) and pseudo-steady state regime (PSS).

The steady state regime (SS) regime is reached when the flow is stabilised with the full pressure support at the external boundary.

The pseudo-steady state (PSS) regime is reached when the flow is stabilised with no pressure support at the external boundary.

In both above cases, the drawdown and flow rate will stay constant upon productivity stabilisation.

As for formation and bottom-hole pressure in PSS they will be synchronously varying while in SS they will be staying constant.

The table below is summarizing the major differences between SS and PSS regimes.

	Steady state regime (SS)	Pseudo-steady state (PSS)
Boundary	Full pressure support	No pressure support
Productivity index	constant	constant
Flow rate	constant	constant
Drawdown	constant	constant
Botom-hole pressure	constant	varying
Formation pressure	constant	varying

It's again important to avoid confusion between the termines stationary conditions (which mean that refered properties are not chaning in time) and stabilised flow conditions which may admit pressure and rate vraition.

In practice, the productivity index is usually not known at all times as there is not routine procedure to assess it.

It is usually accepted that a given formation takes the same time to stabilise the flow after any change in well flow conditions and the stabilisation time is assessed based on the well tests analysis.

Although, this is not strictly true and the flow stabilisation time depends on well-formation contact and reservoir property variation around a given well.

This is also compromised in multi-layer formations with cross-layer communication.

The conventional WFP – Well Performance Analysis is perfomed as the cross-lot with two physical models:

IPR – Inflow Performance Relation

and

Vertical Lift Performance also called Outflow Performance Relation or Tubing Performance Relation

IPR – Inflow Performance Relationship

IPR – Inflow Performance Relation represnts the relation between the bottom-hole pressure and surface flow rate during the stabilised flow:

p_{wf} = p_{wf}(q)

which will be non-linear in generic case.

The IPR analysis is closely related to well productivity index which is defined as below:

J_s(q_{\rm liq}) = \frac{q_{\rm liq}}{p_R-p_{wf}}

for oil producer with surface liquid production (water and oil)

J_s(q_g) = \frac{q_g}{p_R-p_{wf}}

for gas producer

J_s(q_g) = \frac{q_g}{p_{wf}-p_R}

for gas injector

J_s(q_w) = \frac{q_w}{p_R-p_{wf}}

for water injector

where

	water, oil, gas flow rates at separator
	field-average formation pressure withing the drainage area of a given well

Based on these notions the general WFP – Well Flow Performance can be wirtten in univseral form:

p_{wf} = p_R - \frac{q}{J_s(q)}

providing that has a specific meaning and sign as per the table below:

	for producer
	for injector
	for oil producer
	for gas producer or injector
	for water injector or water-supply producer

Despite of terminologival similarity there is a big difference in the productivity definition and productivity analysis between WFP and Well Testing which is summarized in the table below:

	WFP	Well Testing
Formation pressure	– field-average pressure within the drainage area	– pressure value at boudary of the drainage area
Flow rate	– surface liquid rate	– total flowrate at sandface

Для однопластовой залежи со слабосжимаемым флюидом продуктивность не зависит от дебита и график ИКС представляет собой прямую линию (Рис. 1).

GLOSSARY > Well Flow Performance = WFP > ipr.png

Fig.1. IPR for low-compressible fluid production (water, undersaturated oil)

Это характерно для водозаборных скважин, водонагнетательных скважин и нефтяных скважин выше давления насыщения.

Для газовых скважин, газоконденсатных скважин, скважин с легкой нефтью, а также нефтяных скважин, где давление опустилось ниже давления насыщения , сжимаемость фильрующегося агента являестя высокой, закон фильтрации в прибойной зоне скважины становится нелинейным и кривая ИКС начинает отклоняется вниз от прямой линии (Рис. 2), что означает ухудшение продуктивности скважины с ростом скорости фильтрации в призабойной зоне.