Understanding Well Test Analysis

What Is a Well Test?

A well test is a carefully controlled process where a well is either produced or shut in, and pressure and flow rate data are recorded. These measurements help assess the condition of both the well and the surrounding reservoir.

What Does Well Test Analysis Involve?

Well test analysis is the process of interpreting the collected data using analytical models and diagnostic techniques. The goal is to uncover key reservoir properties and evaluate the performance of the well.

What Is Skin Factor?

Skin factor is a dimensionless value used to express how much the area around the wellbore deviates from ideal conditions—whether due to formation damage (positive skin) or stimulation (negative skin).

Common Types of Well Tests

  • Pressure Build-Up (PBU) Test: Conducted after a well has been shut in; monitors how pressure recovers over time.

  • Drawdown Test: The well is flowed at a constant rate, and the resulting pressure drop is recorded.

  • Injectivity Test: Fluids are injected into the formation to measure how easily the reservoir accepts them.

  • Interference Test: Observes pressure changes in a nearby non-producing well in response to production or injection in an active well.

  • Falloff Test: Performed after stopping an injection to monitor the subsequent pressure decline.

Why Conduct Well Test Analysis?

  • To determine reservoir permeability and quantify skin effect

  • To estimate average reservoir pressure

  • To identify formation damage or evaluate stimulation success

  • To define reservoir boundaries and estimate drainage area

  • To detect heterogeneities such as faults, fractures, or flow barriers

  • To improve reservoir models and support reserve calculations

Key Concepts in Well Test Interpretation

  • Radial Flow: A stable flow pattern where fluids move symmetrically toward the wellbore—typically observed during the middle period of a test.

  • Boundary Effects: Detected when pressure waves reach the edges of the reservoir, like sealing faults or recharge zones.

  • Wellbore Storage: The effect of fluid compressibility in the wellbore influencing early-time pressure data more than the reservoir itself.

  • Skin Effect: Reflects additional pressure losses or gains near the wellbore due to mechanical damage or enhancement.

Flow Regimes Explained

  • Transient Flow: Pressure changes dynamically over time, unaffected by reservoir boundaries.

  • Pseudosteady-State Flow: Occurs after the pressure front reaches the boundaries, resulting in a linear pressure decline with time.

Collecting and Interpreting Test Data

What Data Is Needed?

  • Surface and bottom-hole pressure and temperature readings

  • Flow rates throughout the test duration

  • Well completion details and reservoir description

Interpretation Methods

  • Semilog Analysis: Uses straight-line trends on semilog plots to estimate permeability and skin.

  • Log-Log Plotting: Helps identify flow patterns and transitions using diagnostic curve shapes.

  • Type Curve Matching: Involves overlaying field data on theoretical models to interpret reservoir behavior.







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