Case study of a successful wax diagnosis and intervention

Collection, evaluation critical to diagnosis

Kosta J. Leontaritis
AsphWax/Kosta
Oil Field Technologies

Crude oil wax deposition envelope (WDE) typical of US Gulf of Mexico oils. [41,761 bytes]
An EOS oil characterization is a description of the fluid in terms of its components and their molecular weights, critical properties, and acentric factors. [33,864 bytes]
The data and steps necessary to tune a wax model to the wax data. [22,580 bytes]
The steps recommended in a wax deposition study. [26,778 bytes]
PART II: The following article is Part II of a three-part series on asphaltene and wax deposition. Part III of the series will focus on asphaltene problems.

Frequently, oil company personnel are confronted with a seemingly unconquerable organic deposition problem. Many times, the problem is severe and upper management pressures for solving it are very high. In this paper, a methodology is described which allows for prediction or correct diagnosis of an asphaltene or paraffin deposition problem.

The recommended approach for investigating an existing or potential problem is prediction (where oil is to be produced - new discovery), diagnosis (where oil is being produced), prevention, and mitigation. Is the problem asphaltenes or paraffins? One must know the nature of the problem in order to decide how to study it. Before any study can be started in the laboratory, representative oil samples must be obtained.

This paper prescribes a preliminary, economical suite of diagnostic tests to determine the organic deposition problem. Many times, these initial tests are all one needs to complete the diagnosis and provide solutions.

There are three general categories of samples that should be obtained in a wax/asphaltene project: bottom hole samples, pressurized anaerobic surface samples, and bulk oil samples.

Bottom hole samples

Asphaltene flocculation is sometimes irreversible. Once asphaltenes flocculate from the oil, a portion will not deflocculate (return to original state) by simply reinstating the original thermodynamic conditions.

This results in a lowering in the onset pressure of asphaltene flocculation of the oil. Hence, measurements of asphaltene onset pressures in oils that have previously entered their asphaltene deposition envelope (ADE) are questionable. Also, asphaltene-induced formation damage from oils that undergo irreversible asphaltene flocculation is more severe.

That is why a representative virgin bottom hole sample of oil must be obtained to check whether asphaltene flocculation occurs and whether the process is reversible. In many cases, this is easier said than done. However, if a correct methodology is followed and the recommended technology is used, the chances of obtaining and interpreting the data correctly will increase. Bottom hole samples that are destined for asphaltene flocculation testing must be maintained at reservoir pressure and temperature at all times.

Currently, the author knows of no bottom hole sampling tool (BHS) on the market that maintains both pressure and temperature. There are tools that maintain the oil pressure equal to or above the reservoir pressure. The volume of the BHS oil required for a complete asphaltene or paraffin study is about 2400 cc/zone, but no less than 1800 cc/zone (three 600-cc cylinders).

Anaerobic surface samples

About 80% of the time, asphaltene flocculation is reversible. If flocculation is reversible, properly sampled surface samples may be used for just about all types of tests. However, surface oil samples may be at flocculated conditions at the point of sampling.

This occurs in cases where the reservoir fluid flocculates asphaltenes by a pressure reduction inside the formation or in the tubing. This reduces dramatically the likelihood of obtaining representative samples.

The same can be said if wax crystallization is taking place. Special isokinetic sampling procedures should be followed to improve the chances of obtaining representative samples. Two 500-cc pressurized oil cylinders should be obtained at the well head (before the choke) and two 500-cc pressurized oil cylinders of the separator liquid should be obtained. Two 1-liter cylinders of the separator gas for recombination with the separator liquid are also recommended.

Bulk oil samples

Bulk oil samples are samples obtained in buckets, drums, etc., at atmospheric pressure. These samples can be used in many circumstances for a number of tests. In general, these samples should be obtained in containers not larger than five gallons to facilitate restoration of the oil (one-gallon cans are preferred).

Because many oils deteriorate when in contact with oxygen (especially those that contain large amount of asphaltenes and resins), the storage containers should be purged with nitrogen prior to and after sampling. One 5-gallon and five 1-gallon bulk oil isokinetic samples from the flow line are recommended.

Diagnostic study

For preliminary economical diagnosis of a asphaltene/wax problem, many basic tests on high quality stock tank oil from every producing formation are recommended. The recommended tests are:

Oil restoration
Asphaltene content
Wax content
Onset of asphaltene flocculation
Onset of wax crystallization (cloud point)
Pour point
Solubility test (usually at lowest encountered field temperature).

The results from the above tests help guide the analysis and interpretation of the available reservoir and production data. They also allow for a more accurate diagnosis of any expected organic deposition problems. More importantly, the tests provide enough information to decide whether a complete laboratory testing and simulation study is warranted.

Furthermore, the data can be used in conjunction with a PVT study and PARA oil analysis to develop an EOS type fluid characterization. The data will permit simulating the reservoir fluid asphaltene and/or wax deposition envelope (ADE or WDE).

The above-recommended program does not always provide conclusive information. Occasionally, additional work with live reservoir oils may be necessary to complete the diagnosis and carry out the simulations. But generally, the above preliminary diagnostic tests allow for determining expected problems.

Wax phase behavior

Once the preliminary study has been completed and a wax deposition problem is predicted or taking place, a complete phase behavior study is recommended to determine the WDE of the reservoir oil.

An example of a crude oil WDE is shown in an accompany figure. This WDE shape is typical of US Gulf of Mexico oils. The wax deposition envelope is defined by all points to the left of the onset pressure. No wax deposition occurs at conditions outside the WDE.

If the temperature and pressure profile of the produced fluid is superimposed on top of the WDE and the two lines cross each other, then wax deposition is to be expected in the facilities that will be operating at conditions inside the WDE. The only thing that is not provided by the WDE diagram is a measure of the severity of the problem.

Sometimes, the wax crystal formation rate inside the WDE is so small and the flow dynamics so unfavorable that wax deposition is very small. In such cases, properly designed periodic pigging is an appropriate remedy.

One of the uses of the wax solubility test is to calculate the average rate of wax crystal formation inside the WDE. By comparing it with the WDE of other oils, a semi-quantitative idea of the expected wax deposition rate can be generated.

Obtaining the WDE

The phase behavior diagram (accomapnying figure) is an overlap of the vapor-liquid envelope (V-L-E) and of the WDE. Hence, one would have to calculate both PVT and wax phase behavior simultaneously to obtain the WDE. In those cases where an equation of state (EOS) is used to calculate the PVT phase behavior, an EOS oil characterization is required as input to the EOS.

An EOS oil characterization is a description of the fluid in terms of its components and their molecular weights, critical properties, and acentric factors. An accompanying figure shows the sequence of events required for obtaining an EOS description of the oil. When used as input to an EOS (Peng-Robinson EOS, or P-R EOS, or Soave-Redlich-Kwong EOS S-R-K EOS), it is capable of simulating the V-L-E of the fluid.

Since the EOS oil characterization will be used to calculate the WDE of the oil, it should show the waxes as distinct separate pseudo-components. This type of oil characterization is called PARA (Paraffin-Aromatic-Resin-Asphaltene).

After a bottom hole sample is obtained, it should be restored to reservoir conditions, if not maintained at those conditions at all times. The preferred restoration time is usually 5-7 days at reservoir conditions. Then, the restored oil is used to perform a standard PVT study and a PARA analysis.

Then, the PARA analysis data are used to generate an initial PARA EOS oil characterization. An EOS is used then to tune the initial PARA EOS oil characterization to the PVT data. A tuned EOS - P-R EOS or S-R-K EOS - is then used to calculate the V-L-E.

Tuning the data

Up to this point, a PARA EOS oil characterization has been developed that is tuned to the PVT data. To ensure better accuracy, the PARA EOS oil characterization should be tuned to some wax data before it is used to calculate the WDE.

The main product at the bottom of an accompanying figure is the WDE of the subject oil. The next logical step is to simulate and plot the pressure and temperature profile (P&T profile) of the produced fluid and super-impose it on the WDE.

The software used to calculate the P&T profile of the produced fluid at this time does not have to have wax deposition simulation capability. However, if the P&T profile and the WDE cross each other, wax deposition does occur and alters the P&T profile. In this situation, a wax deposition study is necessary to determine the severity of wax deposition and specify remedies.

Wax deposition study

The objective of a wax deposition study is to determine the severity of wax deposition in a system which is operating at conditions inside the WDE of the produced fluid. The wax deposition propensity of the produced fluid is determined in the laboratory and the data are then used to tune a wax deposition simulator.

The laboratory test conditions are determined by scaling down the expected field conditions. The two most important scaling criteria in such a system are the wall shear rate and the Reynolds number. The wall shear rate scales down or up the hydrodynamic drag that is applied on the wax crystals depositing on the pipe wall and the Reynolds number scales down or up the flow regime.

In the laboratory, one cannot scale down or up the wall shear and Reynold's number simultaneously. Only one of the two may be set in the laboratory test equal to the field conditions. The author prefers maintaining shear rate as it appears to have the highest impact on wax deposition.

An accompanying figure shows the steps recommended in a wax deposition study. Before one can simulate wax deposition, one must simulate accurately the oil phase behavior. That is the purpose of the steps in the previous figures.

A wax deposition simulation provides extremely useful information. Wax deposition severity, pigging frequency required, heating and insulation requirements, and many other useful system performance parameters are obtained through a carefully done study. The wax deposition model used should be capable of calculating hydraulics, heat transfer, phase behavior, and wax deposition simultaneously.

References

Leontaritis, K., "Offshore Asphaltene and Wax Deposition: Problems/Solutions," World Oil, May 1996.

Leontaritis, K., "The Wax Deposition Envelope of Gas Condensates," OTC Paper No. 8776, 1998 Offshore Technology Conference, Houston, Texas, 4-7 May, 1998.

Leontaritis, K., "PARA-Based (Paraffin-Aromatic-Resin-Asphaltene) Reservoir Oil Characterization," SPE Paper No. 37252, SPE International Symposium on Oilfield Chemistry, February 18-21, 1997, Houston, Texas.