Bottom Hole Pressure (BHP) Survey
BHPs are carried out in wells to obtain pressure data to define local and average reservoir pressures.
Reservoir pressures are required to obtain
knowledge about:
1. reservoir drive mechanism
2. productivity
3. length of hydrocarbon column
Early knowledge of each parameter may influence development and production policy of the reservoir.
The Need for BHP Survey
Knowledge of active drive mechanism of a reservoir is essential to determine the optimum number and siting of ultimate drainage points.
Infill drilling, recompletion of existing wells to obtain a nearer optimal position of drainage points, and/or secondary recovery may be
required.
The taking of accurate pressure (and temperature) measurements, the where, when, and how of such information, are indispensable Productivity information, e.g. drawdown and buildup information, makes it possible to
determine reservoir permeability and presence of skin in-situ. Both these
parameters are required in reservoir studies.
Furthermore, subsurface productivity information is a necessary tool in evaluating productivity impairment which can be instantaneous and due to consolidation technique or workover fluid used, or gradual as a result of gas, water or sand influx.
The pressure determined at a specific point in the undrained hydrocarbon column sometimes yields new information about the approximate length of the gas or oil column present, provided the pressure regime in the underlying aquifer and the gas or oil gradient is known.
Such information is helpful in determining the optimum appraisal/development plan for the reservoir.
Survey Chart
1 True Zero Line.
2 Wellhead Pressure in Lubricator.
3–5 Flowing Gradient Steps.
6 Flowing Bottom Hole Pressure
at Final Survey Depth.
7–13 Readings on BU Slope.
14–16 Static Gradient Steps.
17 Static Bottom Hole Pressure at
Final Survey Depth.
18 Wellhead Pressure in Lubricator.
19 True Zero Line (check-back).
Survey Runs: Important Points
• Drift run to check hole condition and to ensure instrument can safely re-enter tubing.
• Record Zero Point on top flange of Xmas tree.
• Zero Line must be marked when gauge is cold.
• Run gauges (in tandem) to depths D-600, D-400, D-200, D for specified duration.
• CI well with gauge at D for BU duration
• Pull gauge to D-600 and run static survey
• POH to the Lubricator for Final (true) Zero Line.
Surveys Requirements
• Routine surveys on monthly or annual basis.
• After-completion Surveys.
• For government Reporting.
• Communication Tests.
• Through Tubing Stimulation/Sand Consolidation Surveys.
• Project (Water/Gas Injection) Surveys.
• Gaslift Well Tests.
• Surveys for Well Problem Identification.
• Repeat Surveys in Wells with unreliable survey results.
• Other Special Surveys.
Planning Constraints
• Seasons: Seasonally flooded fields and their favourable seasons need to be known.
• Area Logistics: Choice of candidates for routine surveys should take account of area logistics to permit optimal barge and truck movements in the swamp and land resp.
Type of Surveys
FG/BU/SG should be taken after the completion of a well.
In addition to reservoir pressure, information is obtained about PI, permeability to oil and skin.
BU surveys taken at a later stage may not be representative of the initial conditions if movable water and/or gas have penetrated the wells.
FG/SG surveys would suffice for highly permeable reservoirs.
Selection Criteria
Frequency of Survey should be based on the reservoir content (STOIIP) and pressure history.
Pressure history enables prediction of the decline of the static reservoir pressure with production. Strong water drive reservoirs require less frequent surveys only to confirm the established pressure trend.
BHP surveys are also of utmost importance to determine the initial pressures directly after completion.
BHP Surveys
