TECHNICAL FIELD

The present invention relates to a new process in obtaining representative hydrocarbons PVT (Pressure-Volume-Temperature) samples at production line pressure and temperature i.e., a homogenous PVT sample. The present invention also relates to a new process flow of analysing a homogenous PVT sample taken from production line. The present invention offers a new technical solution to solve current issues related to representative PVT hydrocarbons sampled at production platforms. The present invention also offers a new approach to analyse PVT samples taken from production line.

BACKGROUND ART

Capturing representative PVT samples is the first important step towards obtaining good quality results in PVT study. The objective of the PVT study is to determine the fluid behaviour of the hydrocarbons against pressure and temperature. During exploration phase, two types of PVT samples can be taken:

1. Single phase PVT sample - The PVT sample is taken close to the reservoir in a single-phase condition using a downhole sampler or it is taken at the wellhead if the sample is still in single phase condition. and/or

2. Separator PVT samples - Surface separator is used to separate water, condensate/oil and gas on surface during a well flow testing. The PVT samples (condensate/oil and gas) are taken separately at the same time at the separator. Condensate/oil is sampled from the oil line whereby gas is sampled from the gas line at the separator. Liquid gas ratio provided from the flow rates data or stock tank measurement will be used to recombine these samples in PVT laboratory to represent a single-phase sample of the well. If stock tank liquid ratio is used, shrinkage factor of liquid hydrocarbon is needed to recalculate back to production line or separator conditions. The present invention focuses on PVT samples taken during production phase i.e., to capture surface PVT samples. However, the invention can also be applied during exploration phase. During production phase, generally hydrocarbons which flow to the surface is already in two phases due to changes in reservoir pressure and temperature conditions over time. And the PVT data obtained from the samples taken during exploration might already be invalid and the PVT data requires to be updated periodically.

Production platforms use onsite meters, namely Multiphase meter, Venturi meter, Clamp on meter etc., for continuous metering purposes. These meter readings need to be verified and most of them require input of new PVT data to provide better accuracy of the flow rates readings. The updated PVT data is used to optimize current production processes as well.

The flow rates testing and PVT sampling at production platforms are currently done via 3 ways, a surface well testing package, an iso-split wellhead sampling method and a tracer dilution method.

The well testing package consists of large footprint and heavy equipment. Most platforms have an issue with space and some satellite platforms do not have cranes with sufficient lifting capacity to lift these large and heavy equipment. Thus, most production platforms will have a hurdle to utilize standard surface well testing equipment to perform their well flowrates verification and PVT sampling activities.

Another method available is iso-split wellhead sampling which consists of small and accommodating footprint and light equipment. This method involves isokinetic sampling via a sampling probe installed after a mixing manifold. A fraction of hydrocarbon sample in production line is directed into a mini separation system isoki netica lly via a probe after flowing through a static mixer. Then, the flow is processed in stage separators usually under reduced conditions i.e., temperature is reduced. The hydrocarbon flow is separated into liquid and gas while the flowrates are measured accordingly. This method can provide liquid ratio data and can capture PVT samples. However, there are possible uncertainties associated with this method which are related to data input, equipment, and flow e.g., potential insufficient mixing, potential inaccurate isokinetic rate calculated, potential use of wrong probe area, etc. Tracer dilution method is another technique to measure flowrates of hydrocarbons usually performed at production platforms. The principle of this method is injecting a selected tracer at a specific injection rate into the production stream. A sample of the produced liquid is subsequently taken at a certain distance downstream of the injection point. Analysis of the samples to determine the tracer dilution ratio allows the direct phase production rate to be calculated. However, this method provides only a few numbers of flowrates data, typically only 5 data within 1 hour due to expensive tracer used.

The tracer dilution technique measures flow at atmospheric conditions. This technique utilizes a mini separator to separate gas and liquids for sampling. The mini separator must be set to conditions of the production line. Incorrect conditions of mini separator used will lead to incorrect shrinkage factor and flow rates at test conditions. Shrinkage measurement is required to calculate the flow at test conditions.

Potential flash gas from liquid in sampling tubing or mixed gas from condensate evaporation will lead to inaccurate dilution factor for gas measurement in tracer dilution technique. If this occurred, the measured gas flowrates will be inaccurate. Gas tracer technique is focusing on Helium measurement in both reference sample and collected samples. If the Helium concentration is altered by additional amount of flash gas to both reference and collected sample, this will lead to wrong gas flowrate reading.

Potential retrograde condensation of gas or liquid evaporation will lead to inaccurate dilution factor for liquid measurement in tracer dilution technique. If this occurred, the measured liquid flowrates will be inaccurate. Liquid tracer technique is focusing on fluorescent concentration in both reference and collected samples. If the fluorescent concentration in both samples is altered by either gas condensation or liquid evaporation, this will lead to inaccurate liquid flowrates and consequently inaccurate liquid ratio will be provided to the lab.

There are also attempts to utilize a mini separator for PVT sampling simply because the unit can provide hydrocarbons separation. This shall be acceptable for trace analysis purposes but not for PVT sampling. To capture representative samples, conditions of the mini separator must be the same with the production line conditions during sampling. To keep a mini separator at flow line pressure is usually not be a problem. Temperature control is more difficult because of its small vessel size, making temperature stability hard to achieve. If the temperature used is too low, the liquid volume will increase due to retrograde condensation from the gas phase. The composition of this liquid will be lighter than the correct liquid at flow line conditions. The gas composition will also be too light. If the temperature used is too high, the liquid volume will decrease due to evaporation of liquid and the composition of the resulting remaining liquid will be too heavy. All of the liquid may even evaporate into single phase gas. Due to this, the gas composition will be too rich. In addition to the temperature stability issue mentioned, retention time required for good separation of liquid and gas is not sufficient via mini separator due to small vessel size as well. Liquid carry over and/or gas carry under often occurred during sampling via mini separator, thus resulting in significant differences in opening pressure and saturation pressure of the PVT samples compared to production line pressure, affecting the quality of the PVT samples taken. Consequently, the recombined PVT samples will not be representative due to inaccurate liquid ratio data provided and questionable quality samples taken.

All above methods involve the separation of production hydrocarbons into water, condensate/oil and gas. Current standard process for PVT sampling and analysis for production samples is to capture separate phases of hydrocarbon and recombine them in the PVT laboratory with available liquid ratio data. After the recombination process is done, the recombined samples are then transferred into the PVT cell for further PVT analysis. Or they can be transferred separately into the PVT cell with calculated known volumes from recombination procedure. If wrong liquid ratio data is used for recombination, the recombination samples will not be the representation of the production fluid. Subsequently, data from PVT analysis will not be accurate. Similar results will happen if the PVT samples taken are not of good quality.

SUMMARY OF THE INVENTION

According to the present invention, PVT samples from the production line will be sampled directly from production line conditions via a homogenous PVT sampling procedure instead of performing sampling from the separator or mini separator for separate phases of hydrocarbons. This approach will provide a 'true' representative PVT sample of the production fluids in the line, thus eliminating any uncertainties related to hydrocarbon separation process via methods discussed above. The PVT samples obtained can be transferred into PVT cell directly without the need to perform samples recombination procedure.

In an embodiment of the invention, a static mixer is installed upstream of sampling manifold to produce a homogenous mixture of hydrocarbons inside the production line. Several static mixers are designed to accommodate ranges of flowrates, pressure, and temperature of standard production conditions. The mixer is installed in a sampling manifold that has a bypass line to allow change to a new mixer if needed without having to stop the flow of the well.

In an embodiment of the invention, two or more in-situ density sensors are installed in a radial position after the mixer to provide status of homogenous level of the hydrocarbon mixture. Full homogenous state of the mixture is achieved after the density readings at different radial points are the same or within 5% difference.

In an embodiment of the invention, two or more sampling points are made available at the manifold in a radial position after the mixer to provide uniform PVT sampling.

In an embodiment of the invention, a cylinder equipped with an internal mixer and a heating unit (capture unit) is connected to the sampling head. The cylinder temperature is conditioned to be the same as temperature of the production line to avoid condensation of liquid from gas during sampling. Sampling is done slowly by drawing water/glycol from the cylinder as to avoid pressure drop while the homogenous sample is entering the capture unit. Pressure drop will cause gas to flash from the liquid. Once 600cc of homogenous sample is collected, inlet valve is closed to secure the sample. The objective is to collect minimum of 500cc of the hydrocarbon mixture of gas and condensate/oil with a backup set.

However, water cut is needed to be determined onsite to ensure enough homogenous sample gas and condensate/oil is collected for PVT studies i.e., 500cc.

In an embodiment of the invention, after the 600cc homogenous sample has been secured, the sample from the capture unit is then transferred into stabilization unit at a pressure higher than the production line pressure. The internal mixer is activated for a period of 15 minutes to homogenize the captured samples before it is transferred into the stabilization unit. The stabilization unit is equipped with an internal mixer and a cooler unit containing water or cooling liquid to provide faster water separation. The homogenous sample is then set for stabilization for no less than 30 minutes before water draining procedure is performed to determine water cut percentage. Pressure is kept constant at a pressure higher than the sample pressure to avoid pressure drop during water draining procedure.

If water cut is low e.g., 10%, then water draining procedure may be done in PVT laboratory for subsequent sample. However, if the water cut is high, several sampling and water draining procedures shall be done onsite to collect enough sample of homogenous sample containing gas and condensate/oil for PVT studies.

In an embodiment of the invention, after the water draining procedure is completed, the sample is then transferred to a PVT sample cylinder equipped with an internal mixer at a pressure higher than the sample pressure. The internal mixer is activated for a period of 15 minutes to homogenize the captured samples before it is transferred into the sampling cylinder unit. The sample volume is measured by collecting the drawing water/glycol. The sampling procedure is repeated until 600cc sample volume is collected.

In an embodiment of the invention, homogenous PVT sample obtained onsite can be transferred from the sampling cylinder into PVT cell directly after sample conditioning is achieved. The internal mixer in the sampling cylinder is activated during sample conditioning to homogenize the captured samples before transfer.

Separate gas and liquid samples are no longer required to be taken and liquid gas ratio data is no longer needed for the recombination of these samples which is currently practised for production fluid samples. This approach will eliminate the need to perform well testing activities and it will produce true quality PVT samples from production facilities.

In details, the advantages of using this new approach are:

1. Since gas liquid ratio is not required via this new approach, surface production testing which involves high cost, space and crane lifting issues at production facilities might no longer be required. 2. Since surface production testing requirement which mostly involves hydrocarbon burning might greatly reduce, this will provide significant contribution to the net zero emissions target by oil companies.

3. Huge cost saving on PVT sample cylinders rental i.e., PVT gas sample cylinder is no longer required and required amount of PVT piston type sample cylinder may be reduced.

4. Uncertainties related to the quality of surface PVT samples from production facilities can be avoided i.e., significant difference in opening and saturation pressures when compared with separator or production line pressure, liquid carry over and gas carry under, gas condensation and evaporation of liquid, etc.

5. Reduced laboratory time for sample conditioning on each type of samples.

6. True representative fluids can be obtained, thus resulting in good quality PVT analysis data

BRIEF DESCRIPTION OF THE DRAWING

For better understanding on the different approach of PVT sampling of production fluids and current process towards PVT studies, current setup and new approach illustration are provided in this specification.

Figure 1 shows current setup for PVT sampling for gas and condensate/oil via surface separator

Figure 2 shows current process of surface PVT samples which undergo recombination procedure before or during sample charge into PVT cell for further PVT analysis.

Figure 3 shows new approach of homogenous PVT sampling setup at production line

Figure 4 shows new approach of homogenous PVT sample from production line charges directly into

PVT cell without having to undergo samples recombination procedure DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

Referring to Figure 1, this is a current setup to obtain PVT samples from the production line or separator. The process involves proper separation of the hydrocarbon liquid and gas. And the PVT samples are taken separately at about the same time.

Referring to Figure 2, after the surface PVT samples have been obtained, they are sent to PVT laboratory for quality check and for further PVT studies. Once the samples have passed the quality check, the recombination procedure will take place. The gas and liquid samples are recombined to represent well fluid sample with gas liquid ratio data provided from production or well testing. In this case, the recombination is to mimic production fluid sample. Then, the recombination fluid is charged into PVT cell at a constant pressure higher than the separator or production line pressure. The samples can also be charged directly into PVT cell at a known volume of each sample type provided by final recombination calculation.

Referring to Figure 3, the homogenous PVT sampling provides actual and true representative fluid from the production line. Therefore, the recombination procedure is not required. And potential error in recombination fluids prepared can be avoided, thus this new approach will provide more reliable PVT analysis data.

Referring to Figure 4, production fluid flow is diverted to a sampling manifold which consist of homogenous PVT sampling setup (11). The production fluid is flowing through a static mixer (13) where the fluid which consist of a mixture of liquids and gas is turned into homogenous state. 2 or 4 in-situ density meters (15) are installed in radial positions downstream of the static mixer (13) to provide homogenous status of the fluid. If the ins-situ density meters (15) give the same reading at all points, then homogenous state is achieved. The homogenous sample is then sampled from radial sampling points (17) into a capture unit (19) by drawing water/glycol slowly to avoid pressure drop. The radial sampling points (17) provides uniform sampling via four points of sampling to ensure representative production fluid is captured. Once 600cc of homogenous sample is collected, the inlet valve is closed to secure the sample. The true representative of production fluid is now collected. A high-pressure pump (25) is connected to both capture unit (19) and stabilization unit (21). The sample is then transferred to a stabilization unit with a pressure higher than the production line pressure to avoid pressure drop during transfer. The internal mixer (31) is activated for a period of 15 minutes to homogenize the captured sample before transfer. After transfer is done, the sample is allowed to stabilize for a period of 15-30 minutes for water separation to take place. The fluid cooler (29) provides faster temperature stabilization of the sample. Water is then drained from the sample at a constant pressure higher than the sample pressure and the volume is noted to calculate water cut percentage. The remaining sample inside the stabilization unit (21) is then transferred with a pressure higher than the production line pressure to sampling cylinder (23). The internal mixer (31) is activated for a period of 15 minutes to homogenize the captured samples before it is transferred into the sampling cylinder unit. The sample volume is noted. The sampling process is repeated until 600cc of homogenous PVT sample is collected inside sample cylinder (23). The final sample volume, final pressure and ambient temperature are noted.