Bottom Hole Pressure Survey

A bottom hole pressure (BHP) survey measures the pressure, and often the temperature, at depth inside a well. BHP surveys are carried out to obtain pressure data that define the local and average reservoir pressures. This article explains why surveys are run, how a survey is carried out, and how the types of survey are chosen.

Why Reservoir Pressure Is Needed

Reservoir pressure data give knowledge about:

1. the reservoir drive mechanism
2. productivity
3. the length of the hydrocarbon column

Early knowledge of each of these parameters can influence the development and production policy of the reservoir.

The Need for a BHP Survey

• Drive mechanism. Knowing the active drive mechanism is essential to determine the optimum number and siting of the final drainage points. Infill drilling, recompletion of existing wells, or secondary recovery may be required.
• Productivity. Accurate pressure (and temperature) measurements, and the where, when and how of taking them, are indispensable. Drawdown and buildup information makes it possible to determine reservoir permeability and the presence of skin in situ, both needed in reservoir studies.
• Impairment. Subsurface productivity data are necessary to evaluate productivity impairment, which can be instantaneous (from the consolidation technique or workover fluid used) or gradual (from gas, water or sand influx).
• Hydrocarbon column. The pressure at a specific point in the undrained column sometimes gives new information about the approximate length of the gas or oil column, provided the pressure regime in the underlying aquifer and the gas or oil gradient are known. This helps determine the best appraisal and development plan.

Survey Chart

The numbered points on the chart are:

Survey Runs: Important Points

• Run a drift to check hole condition and ensure the instrument can safely re-enter the tubing.
• Record the zero point on the top flange of the Xmas tree.
• Mark the zero line when the gauge is cold.
• Run the gauges (in tandem) to depths D−600, D−400, D−200 and D (in ft) for the specified duration.
• Close in the well with the gauge at D for the buildup duration.
• Pull the gauge to D−600 and run the static survey.
• Pull out of hole to the lubricator for the final (true) zero line.

When Surveys Are Required

• Routine surveys, on a monthly or annual basis
• After-completion surveys
• Government reporting
• Communication tests
• Through-tubing stimulation or sand consolidation surveys
• Project surveys (water or gas injection)
• Gas lift well tests
• Surveys to identify well problems
• Repeat surveys when earlier results are unreliable
• Other special surveys

Planning Constraints

• Seasons: seasonally flooded fields and their favourable seasons must be known.
• Area logistics: candidates for routine surveys should be chosen with area logistics in mind, so that barge and truck movements in the swamp and on land are optimal.

Types of Surveys

FG, BU and SG surveys should be taken after the completion of a well. In addition to reservoir pressure, they give information about productivity index (PI), permeability to oil, and skin. BU surveys taken at a later stage may not represent the initial conditions if movable water or gas has reached the wells. FG and SG surveys are enough for highly permeable reservoirs. Pressure surveys are often run together with flow tests.

Selection Criteria

The frequency of surveys should be based on the reservoir content (STOIIP) and the pressure history. Pressure history makes it possible to predict the decline of the static reservoir pressure with production. Strong water-drive reservoirs need less frequent surveys, only to confirm the established pressure trend. BHP surveys are also very important for determining the initial pressures directly after completion. The results of each survey support the well performance evaluation of the field.

Frequently Asked Questions

What does a BHP survey measure?

The pressure, and often the temperature, at one or more depths in the well while it is flowing or shut in.

What is the difference between FG, BU and SG?

FG is measured while the well flows, BU while pressure builds after shut-in, and SG with the well shut in and stabilized.

Key Takeaways

• BHP surveys give reservoir pressure, productivity, permeability and skin.
• A standard survey has flowing steps, a buildup and static steps, with zero-line checks before and after.
• Frequency depends on reservoir size (STOIIP) and pressure history.