The objective of well conditioning is to replace the non-representative reservoir fluid located around the wellbore by displacing it into the well with original reservoir fluid from the more distant parts of the reservoir. Simply shutting-in the well to restore the pressure around the well bore will not necessarily bring the fluid in the affected area to its original condition or composition. It is necessary to flow the well at a low flow rate to allow the altered oil to be displaced by representative reservoir oil.
Conditioning the well before sampling is almost always necessary and is especially important when the reservoir fluid is saturated at prevailing reservoir pressure. This is because the reduction in pressure around well bore, which results from producing the wells, can alter the fluid composition before it reaches the well bore and well string.
• Conditioning of flowing oil wells:
To collect representative reservoir fluid from a selected oil well it is necessary that the well should be new with minimum gas saturation and should not be producing free water.
Before collecting the sample it is necessary to see that the well is producing with the flowing bottom hole pressures much above the saturation pressure so that there is no change in the state or composition of in- place oil. A flowing well must be subjected to reduction in flow rates by a multi-bean test (by systematic reduction of bean sizes). A stabilized oil & gas rates, water-cut and bottom sediments should be recorded with each bean.
When the gas-oil ratio remains constant after the first reduction in flow rate bean size), flow of an undersaturated oil into well bore is indicated. This means that there is no change in phase state and composition of in-place reservoir oil entering into tubing. In this event the well can be considered to be conditioned.
When gas oil ratio decreases after rate reduction, the presence of the gas saturation in the formation around wellbore is indicated. The gas saturation can results from a) coning of gas cap gas into oil bearing formation around the well bore; b) flowing bottom hole pressure being less that saturation pressure. In this condition the well is conditioned by reducing the producing rate by stages. The stage-wise reductions in flowing rate is continued until minimum stabilized GOR is reached and when further reduction in rate do not affect the gas oil ratio this indicates that the non-representative oil around the wellbore has been replaced by representative in-place oil flowing in from a greater distance in the reservoir and the well can be assumed as conditioned for sampling.
When the gas-oil ratio increases after rate reduction, the simultaneous production of a gas from a gas bearing zone and oil from an oil-bearing zone is indicated. The increased gas-oil ratio could be caused by subsidence of an oil zone. Although a representative sample of the reservoir oil can often be obtained, it is better to use a well which does not indicate oil coning, because it is difficult to determine when the well is adequately conditioned.
• Conditioning Gas-Condensate wells:
The procedure for conditioning a gas-condensate well prior to sampling is based upon interpreting the changes in the gas-condensate ratio that result from reducing the producing rate in a series of steps. When the pressure on a gas – condensate type fluid is reduced below its dew point pressure, a liquid phase is formed. As a result the vapour
phase, which is the fluid produced, will have a lower concentration of condensable hydrocarbon. This loss of condensable hydrocarbon results initially in an increase in the producing gas-oil ratio. Since the largest part of the pressure drop occurs in the area close to the wellbore, retrograde liquid saturation in that area can build-up enough to allow the liquid to become mobile. This mobile liquid can cause unpredictable but significant short-term changes in the gas-condensate ratio to accompany changes in the producing rate.
The well is conditioned by placing it on a producing schedule consisting of a series of successively lower rates. After each rate reduction, flow is continued until the gas – condensate becomes stabilized. The trend of the stabilized gas-condensate will generally be found to decrease as the rate is decreased. The well is considered to be conditioned when the stabilized gas-oil ratio does not change when the producing rate changes.
• Conditioning wells producing a near critical fluid:
The reservoir which contains a near critical reservoir fluid presents specially difficult problems in well conditioning. When the pressure on this type of fluid drops below saturation pressure, usually both of the phases which form are mobile and therefore, flow the well. The rates of production of the two phases, however, are usually in the production that results in a well effluent which is not the same as the reservoir fluid composition. The well effluent can contain either too much or too little gas in combination with the liquid hydrocarbon phase.
Conditioning the well is accomplished by flowing at a succession of slower rates for the purpose of removing the non-representative hydrocarbon phases. The problem lies in determining when the non-representative fluids have been produced. Production from near-critical reservoir, however, often will exhibit a relatively small change in gas oil ratio even though the well effluent has undergone significant changes in composition. When early production information indicate a near-critical reservoir field, sampling should be conducted as soon as possible after the well has been completed. Samples taken after the reservoir pressure has declined only a small amount below original saturation pressure are in many cases, virtually useless for determining the original reservoir fluid
properties and cannot be used in laboratory tests designed to predict fluid properties at later stages of reservoir depletion.