Oil Losses Problem in Oil and Gas Industries

Oil losses is a problem that often arises in oil and gas industries either in onshore or offshore area. There is a loss discrepancy between total quantities from shippers and measurement in the storage tanks; the total sending volume is lower than the measured volume in the mixing tank in a gathering station; this is known as oil losses. When this occurs, an agreement to determine a fair share of the losses must be made. There are two categories of oil losses, they are individual and group losses. Individual loss occurs when oil from one shipper has not been mixed yet with other oils. This includes emulsion and evaporative losses. Group loss occurs during mixing oils in the same storage tank or pipeline. Furthermore, by knowing the causes of oil losses, a way to minimize oil losses can be determined.


Introduction
Oil losses problem often arises in oil and gas industries either in onshore or offshore area.There is a loss discrepancy between total quantities from shippers and measurement in the storage tanks; the total sending volume is lower than the measured volume in the mixing tank; this is known as oil losses.When this occurs, an agreement to determine a fair share of the losses must be made.Hermawan et al. [1] have classified oil losses into two categories, they are (1) individual and (2) group losses.

Individual loss
Individual loss occurs when oil from one shipper has not been mixed yet with other oils.This includes emulsion and evaporative losses.In order to determine emulsion loss, basic sediment and water (BS and W) of oil should be measured.The net standard volume (NSV) excludes sediment, water, and free water.Evaporative loss occurs when light components are released from oil in the storage tank.This happens when the oil temperature in tank is higher than its bubble point.

Group loss
Figure 1 shows typical pipeline system and storage tank for oil gathering activity.The use of the same pipeline to transport the crude oil to a storage tank and oil mixing process either in the same temporary or final storage tank could come up the problem of oil losses.Group loss occurs during mixing oils in the same storage tank or pipeline.Typical oil mixing phenomena in the gathering station is illustrated in Figure 2.
The specific characteristic which has a great effect on group loss is the specific gravity (SG) or API gravity.Petroleum can be classified based on its characteristics, for example density i.e., SG or API, (which indicates heavy oil or light oil), normal boiling point (which indicates the ease with which the oil evaporates), and viscosity (which indicates the ease with which the oil flows).There are five categories of the petroleum fluid, they are dry gas, wet gas, gas condensate, volatile oil, and black oil [2,3].The properties of petroleum fluids will change when they mix together in the same tank.In this case, the oil volume shrinkage occurs when two or more oils are mixed in the same storage tank.As shown in Figure 2, Shipper A and B undergo the mixing process 3 times, i.e., mixing in the Station-1, Station-2, and Station-3.This means that Shipper A and B will experience 3 times the volume depreciation.Shipper C and D experience the mixing phenomena twice and once, respectively.When compared to other Shippers, the volume of shrinkage for Shipper D will be less because it only experiences one mixing phenomena.
The group loss can also occur in the use of the same 3-phase-separator to separate the well stream into three phases of oil, gas, and water, as shown in Figure 3.The 3-phase-separator is often used in both onshore and offshore areas.In separators, gas is flashed from the liquids and free water is separated from the oil.These steps remove enough light hydrocarbons to produce a stable crude oil [4].On the other  hand, setting and controlling of the interface level in 3-phase-separator must be seriously done in order to avoid oil losses due to the offset phenomena.In this case, offset means that water can overflow the weir and follow with oil to the oil storage tank, and vice versa, underflowing oil with the water stream [5].

Procedure of sharing oil losess
The typical block diagram of oil distribution and mixing phenomena as shown in Figure 4 would be used as a case study of oil losses problem in the oil and gas industries.In this case, shippers are defined as the petroleum companies, both government and private companies that are members of the cooperation contract contractor.In general, the criteria for shippers are based on the type of oil produced from the oilfield for examples, heavy oil, light oil, and condensate.
When this study was carried out, the weather conditions are as follows: the air temperature varied from 26.7 to 28.7°C, and humidity varied from 71 to 82%.Climate data (from the Juanda Meteorological Station, Surabaya, Indonesia) shows an annual average rainfall of 1.969 mm/year.The ratio of the dry to wet months is  0.5109 or 51.09%.The climate in the study location is relatively wet because the number of dry months is relatively the same as the number of wet months.
In this case, shipper A and B will have 3 times mixing in TANK-1, TANK-2, and TANK-3, respectively; Shipper C will have twice mixing in TANK-2 and TANK-3; while Shipper D has only once mixing in TANK-3.The tank criteria used in this case is the welded steel tank for storing petroleum at the atmospheric pressure accordance with PTK-013/PTK/II/2007, BPMIGAS, February 12, 2007, Decree of the Head of BPMIGAS: Operation and Maintenance of Petroleum Storage Tanks.Oil tank capacity depends on its production rate.In addition, the tank capacity also depends on its function, whether for temporary or final storage.TANK-1 and TANK-2 are the temporary storage tank with capacity @30,000 barrels, and Tank-3 is the final storage tank with capacity 900,000 barrels.Calculation of sharing oil losses can be determined with the following procedures.

Required data
The data required in the calculation of sharing oil losses are the gross production rate in barrel fluid per day (BFPD), the water cut (WC, %-volume), the tank's conditions (pressure and temperature), the oil specific gravity (SG o ), the formation water specific gravity (SG w ), basic sediment and water (BS&W, %-volume), and hydro carbon composition (%-mole).The required data for calculating of sharing oil losses are listed in Tables 1 and 2.
As shown in Table 1, all shippers produce fluid with different characteristic, water cut and BS&W.Shipper D produces condensate with water cut and BS&W equal to zero.All fluids are stored in the atmospheric storage tank (pressure of about 1 atm and temperature of about 30°C.Water cut is a parameter that shows the water content that is easily separated naturally from oil.While BS&W shows the amount of water and based sediment in the oil which is difficult to separate naturally.In other words, the BS&W separation can only be carried out with the aid of a separator such as a centrifuge. Based on the water cut data, the oil rate of each shipper can be calculated.The oil rates after being separated from free water for shippers A, B, and C are 600, 1800, and 950 BOPD (barrel oil per day), respectively.But these rates are still the gross rate due to basic sediment and water content.The net standard volume (NSV) excludes sediment, water, and free water.The NSV is obtained from the gross volume minus free water and BS&W volume.In order to calculate the total sharing oil losses, the individual losses such as emulsion and evaporative losses must first be calculated.

Calculation of emulsion correction factor
BS&W is required to calculate the emulsion losses.In this case, BS&W in oils shipper A, B, and C are taken the same 0.25%-vol (Table 1).The BS&W in oil of shipper D is zero since this oil is a typical condensate.
For calculation emulsion losses, the empiric emulsion equations for all shippers must be determined.The emulsion parameters (a shown in Table 3.The empiric emulsion equations and emulsion loss can be determined with following procedure: a. First, we make a curve of percentage of the addition of the volume of formation water (in %vol) versus the calculated SG.The first curve produces linear equation: where X 1 is the percentage of the addition of the volume of formation water (in %vol), Y 1 is the calculated SG, a 1 and b 1 are constants.The calculated SG is: where X w is water volume fraction in oil, SG w is specific gravity of formation water, and SG o is specific gravity of oil.The correlation between the calculated SG and the percentage of the addition of the volume of formation water for shippers A, B, and C is shown in Figure 5.  b.Simulation of oil and water emulsion is carried out by mixing of crude oil and its formation water at some levels of water volume, and the BS&W and SG of mixed oil-water are then measured.
c.Then, the changes of BS&W and SG are plotted in a curve.This second curve results linear equation: where X 2 is the measured BS&W, Y 2 is the measured SG, a 2 and b 2 are constants.The correlation between the measured SG and measured BS&W for shippers A, B, and C is shown in Figure 5 d.Then, the Eqs.( 1) and ( 3) are utilized to determine the emulsion loss.The measured BS&W of oil is taken as X 1 in Eq. ( 1) to calculate Y 1 .The calculation result of Y 1 is then used to calculate X 2 using Eq. ( 3).
e. Emulsion correction factor (ECF) in %Vol can be determined with the following equation: The resulted ECF for all shippers are listed in Table 3.
The constants of a 1 , b 1 , a 2 , and b 2 , are taken from Figure 5.

Calculation of evaporative correction factor
The flash calculation is usually used in the application of Vapor-Liquid-Equilibrium (VLE).Just like the name, a liquid will "flashes" or partially evaporates at a system pressure, when the liquid temperature is higher than its bubble temperature, producing a two-phase system of vapor and liquid in equilibrium [6].Flash calculation is an integral part of process engineering calculations.Someone uses the flash calculation in order to determine the amounts (in moles) of hydrocarbon liquid and gas coexisting in a vessel at a given pressure and temperature.The flash calculation is also accomplished to determine the composition of the existing hydrocarbon phases [7].Flash calculation method is used to calculate evaporative loss.The three important parameters used in the flash calculation are pressure (P), temperature (T), and vapor fraction (n v ).Evaporation is indicated by the value of vapor fraction [7].The value of vapor fraction (n v ) ranges in between 0 to 1.If the vapor fraction equals 0, the fluid is in liquid phase, if it equals 1, the fluid is in gas phase.Elseif it is in between 0 and 1 (0<n v <1), the fluid is in mixed-liquid-vapor phase; in other words, part of light component in fluid evaporates; this causes oil loss due to flash phenomena [1].
Figure 6 shows the flash calculation algorithm.The data required in the flash calculation are hydrocarbon composition (z i ), pressure (P i ), and temperature (T i ) of each shipper fluid.The intended pressure (P i ) in this case is the fluid pressure in a storage tank.The fluids of all shippers are stored in atmospheric tanks with temperatures about 30°C (Table 1).The Antoine equation which is used to calculate flash correction factor (FCF, in %Vol) is taken from the equation "Anto5" in UniSim Design R451 Honeywell [8], and written as follows: No. Procedure Formula 1 Input data: vapor fraction n v = 0, pressure P, and HC composition z i of all shippers (i) Calculation of vapor pressure of component j with guessed temperature Calculation of objective function f(n vi ), where Repeat procedure number 2 to 4 with other value of T i until f(n vi ) = 0 same with no. 4 Notes: i = shipper; j = component of hydro carbon.
Calculation procedure of bubble point T b [1].
No. Procedure Formula Input data: vapor fraction n v = 1, pressure P, and HC composition z i of all shippers (i) Calculation of vapor pressure of component j with guessed temperature Calculation of objective function f(n vi ), where Repeat procedure number 2 to 4 with other value of T i until f(n vi )=0 same with no. 4 Notes: i = shipper; j = component of hydro carbon.

Table 6.
Calculation procedure of dew point T d [1].
where P vap j is vapor pressure of component j (in kPa), T is temperature of system (in K), and a j , b j , c j , d j , e j , f j are Antoine parameters for each component j and listed in Table 4.
The flash correction factor (FCF), in %Vol, must be calculated in order to know the oil losses due to flash phenomena.The FCF can be determined with following procedure: a. Inputting data: fluid pressure (P) and temperature (T) in tank, hydrocarbon composition (z).
b. Calculations of bubble point (T b ) at the atmospheric pressure.Bubble point is saturated condition at n v = 0. Calculation procedure of T b is written in Table 5.
c. Calculation of dew point (T d ) at the atmospheric pressure.Dew point is saturated condition at n v = 1.Calculation procedure of T d is written in Table 6.
d. Calculation of vapor fraction n v .As shown in Figure 6, n v will equal to zero if the fluid temperature in tank (T) is less than its bubble point (T b ), this shows that the fluid is in liquid phase.The value of n v will equal to one if T is greater than T d , this indicates that the fluid is in vapor phase.In other case, the value of n v is in between 0 and 1, if T is located in between T b and T d , this shows that the fluid is in the vapor-liquid mixture phase.Table 7 shows the calculation procedure of n v .
e. Flash correction factor (FCF) can be determined by the following equation: No. Procedure Formula 1 Input data: temperature T, pressure P, and HC composition z i of all shippers (i) Repeat procedure number 2 to 4 with other value of n v until f(n vi )=0 same with no. 4 Table 7.
Calculation procedure of vapor fraction n v [1].The constants of a, b, c are referenced from PSME of UPN "Veteran" Yogyakarta collaborated with LEMIGAS Jakarta [9].
e. Calculation of the 1st shrinkage volume (V sh1 ) g. Calculation of the 1st mixed SG (SG mix1 ) *The oil mixing stratification in each tank can be seen in Table 12.

Table 11.
Calculation procedure of shrinkage volume in tanks.* where FCF is in %Vol.The calculation results of T b and T d for all shippers are listed in Table 8.Since the fluid storage temperatures in tanks for all shippers are lower than their bubble points, all fluids are in liquid phase.There are no evaporative losses.

Calculation of individual loss
Individual loss consists of emulsion and evaporative losses.Individual loss for each shipper is listed in Table 9. Shipper B produces the biggest individual loss, i.e., 201.13 barrel, due to its high water cut and BS&W.The total individual loss (TIL) is 252.69 barrel.Finally, the net standard volume (NSV) that excludes sediment, water, and free water is 3747.31barrel.The NSV is then used to calculate the shrinkage volume factor in the group losses.

Calculation of shrinkage correction factor
A shrinkage loss is a group loss in oils mixing.The modified equation of API 12.3 is used for calculating of shrinkage losses and defined as follows: where a, b, and c are constants of modified API 12.3 as listed in Table 10, L c is %light component, Δ o API is o API difference between o API of shipper one and other, and S h is shrinkage volume percentage (in %Vol).
As written in McCain [10], the API gravity for each shipper is defined as follows: where o API i is API gravity of shipper i, and SG i is specific gravity (60 o /60 o ) of shipper i. Calculation procedure of shrinkage volume in tanks is shown in Tables 11 and 12.

Determination of sharing oil losses
Sharing oil losses can be determined with 2 methods, they are Proportional Method, and Stratified Method [1].Hermawan et al. [1].

Table 12.
The oil mixing stratification in each tank.

Proportional method
In oil and gas industries, the proportional method is frequently utilized to determine sharing oil losses.The operator measures the total received volume of oil in TANK-3 at the last station (see Figure 2).This measured volume value represents the net corrected volume (NCV) which does not take into account the mixing event at the previous station.The total shrinkage volume (V sh-prop ) is the difference volume between the total volume sent from all shippers and the net corrected volume as written below: where V i is net standard volume of shipper i, and V nc (TANK-3) is the netcorrected-volume in TANK-3.The proportional shrinkage volume for each shipper (ξ prop i ) can be calculated as follows: where x i is volume fraction of shipper i as defined below: The proportional shrinkage correction factor (SCF prop i in %Vol) for each shipper can then be calculated as follows: The shrinkage correction factors (SCF) with proportional method are listed in Table 13.The total shrinkage loss is 4.49 barrel.The SCF for each shipper is almost the same, i.e., 0,12%-Vol.When compared with other shippers, the shipper D has the highest value of SCF, i.e., 0.13%-Vol, because the oil of shipper D is categorized as condensate.Condensate which is also known as a light oil or gas oil has different characteristics with the heavy oil.The light oil has a low density with small molecular size.The molecular size of light oil is smaller than heavy oil, so it is understandable that when they mix together, shrinkage will occur geometrically as shown in Figure 7.This phenomenon is in accordance with the observations of Erno et al. [11], James [12], Shanshool et al. [13], and Hermawan et al. [1].The proportional method is considered unfair since the last shipper who experienced a few mixing processes also bears losses of other upstream shippers.

Stratified Method
The stratified method is the new method proposed by Hermawan et al. [1] where the net corrected volume (NCV) is calculated stratify from tank to tank as shown in Tables 11 and 12.The shrinkage volume is calculated for every mixing in the tank.Therefore, more often oil mixes with others; its volume will be more decreased.
The shrinkage volume for shippers A and B in TANK-1 is written as follows: where ξ stÀIi is shrinkage volume for shipper i (A, B) in TANK-1, and V shg-I is the group shrinkage volume in TANK-1.The shrinkage volume for shipper C and TANK-1 (mix A-B) in TANK-2 can be calculated with the following equation: Gross rate (BFPD) 600,00 2.000,00 1.000,00 400,00 4.000,00 Water Cut, WC, (%Vol) -10,00 where ξ stÀIIi is shrinkage volume for shipper i (C, and mix A-B) in TANK-2, and V shg-II is the group shrinkage volume in TANK-2.Finally, the shrinkage volume for shipper D and TANK-2 (mix A-B-C) in TANK-3 can be determined as follows: where ξ stÀIIIi is shrinkage volume for shipper i (D, and mix A-B-C) in TANK-3, and V shg-III is the group shrinkage volume in TANK-3.
The total stratified shrinkage volume (ξ stÀtoti ) for shippers A and B are the summation of its shrinkage volume in TANK-1, TANK-2, and TANK-3, for shipper C is those in TANK-2 and TANK-3; while for shipper D is only once in the last tank of TANK-3.
where for shipper Cξ stÀIi ¼ 0, and for shipper Dξ stÀIi ¼ ξ stÀIIi ¼ 0 .The stratified shrinkage correction factor (SCF st i in %Vol) for each shipper can then be determined with the following equation: Table 14 shows the stratified sharing losses results.The subtotal oil losses in TANK-1, TANK-2, and TANK-3 are 0.89, 2.88, and 0.72 barrels, respectively.The stratified method produces the total oil loss of 4.49 barrels.This result is the same with the proportional result.The SCFs in TANK-1, TANK-2, and TANK-3 for each shipper are almost the same, i.e., 0.04%-Vol, 0.09%-Vol, and 0.02%-Vol, respectively.If compared with other shippers, the total SCF of shippers A and B are the biggest one, i.e., 0.14%Vol.The total SCF of shipper C is 0.10%Vol.While the total SCF of shipper D is the smallest one, i.e., 0.02%Vol.
Comparison between proportional and stratified results is shown in Table 15.More often oil mixes with others; its volume will be more decreased.The stratified method is therefore considered fair, since the oil volume shrinkage of each shipper is calculated according to the amount of the mixing phenomena.

Daily join report
A Joint Report is created when several Cooperation Contract Contractors or Shippers use shared facilities, so that a distribution mechanism for oil losses due to evaporation, emulsion, and shrinkage is required [14,15].Daily join report needs to be compiled to find out the distribution of oil losses for each shipper.The stratified method is chosen to determine sharing oil losses since this method gives the fair results.The daily join report for all shippers is shown in Table 16.

Sources of oil losses
Causes of oil losses that can be minimized are as follows: • Limited number of tanks in the field (both storage and handover tanks), so this will limit the settling time for separation of water and sediment.
• The condition of tank that allows the loss of petroleum due to the evaporation of light component.
• Unavailability of adequate flowmeter devices, so that the measurements are made manually.
• Improper heating process operating condition.
• Improper setting of interface level in 3-phase-separator.
Inevitable oil losses are as follows: • The presence of oil and water emulsions with a high degree of stability, so the separation of oil and water is difficult in a short time.In this case, the special demulsifier should be injected at an early stage, such as the farthest or mid oil cluster, and the wash tank inlet, in order to break the emulsion.By injecting demulsifier at the early stage, it is hoped that the oil-water turbulence in the pipe will not form a strong emulsion.If the fluid travel time from the farthest cluster to the wash tank is long enough, then midway injection of demulsifier is necessary [16].
• Shrinkage of oil volume caused by mixing two or more different characteristics of petroleum in the same pipe or tank.
• Unstable crude oil due to the evaporation of light component

Conclusion
A case study on oil losses in the oil and gas industries has been discussed for some shippers namely, shipper A, B, C, and D with typical diagram of oil distribution and mixing process.The individual loss includes loss due to water cut, emulsion and evaporation phenomena.The parameters of water cut (%-vol) and BS&W (%-Vol) need to be identified to calculate the net volume free of water and sediment.However, shipper D does not contribute emulsion loss because its oil is typically condensate with BS&W=0.All shippers do not produce evaporative loss, because the oil temperatures in tanks are lower than its bubble temperature.
The group loss happens during mixing oil in the same storage tank.The parameter of oil specific gravity must be determined to calculated the group loss.The oil volume will shrink when two or more oils mix together in the same storage tank.In this case, the proportional and stratified methods have been utilized to calculate the sharing oil losses.The proportional method gives almost the same of shrinkage correction factor (SCF) for all shippers.However, the proportional method is considered unfair, since the downstream shipper, e.g., shipper D, bear the losses of the upstream shippers (shippers A, B, and C).Therefore, the stratified method is considered fair for determining the sharing oil losses, since the oil loss of each shipper is calculated based on the amount of the mixing event.
According to the analysis of oil losses case study with typical oil distribution flow diagram in the oil and gas industries, the considered several ways to prevent oil losses include the following: • Provide the adequate tanks in the field (both storage and handover tanks), so this will give enough settling time for separation of water and sediment.The available tank should meet predetermined criteria such as material and good welding so as not to leak.
• Provide the adequate flowmeter devices to prevent measurement errors.
• Use demulsifier to solve the emulsion.The demulsifier will destroy the interfacial films that cover the water droplets, so that the water droplets can coalesce and separate from the oil.Demulsifier should be injected in several location such as the farthest and mid oil cluster, and the wash tank inlet.
• Use heating treatment to solve the emulsion.There is no standard solution for breaking the emulsion, for example, the higher the heating temperature, the faster the emulsion breaking process, and the less demulsifier required.But the heating temperature must be below the boiling temperature of the oil to avoid evaporation.
• Keep the tank operating conditions so that there is no evaporation of the light components.By maintaining low oil temperature minimizes evaporative loss from storage tank.
• Maintain the interface level in 3-phase-separator below the weir height to prevent offset.

Figure 1 .
Figure 1.Pipeline system and storage tank for oil gathering activity.(a) Pipeline System.(b) Storage Tank.

Figure 2 .
Figure 2. Typical oil mixing phenomena in gathering station.

Figure 4 .
Figure 4.The typical block diagram of oil distribution and mixing phenomena.

Figure 3 .
Figure 3.Typical 3-phase-separator in the oil and gas gathering station.

Figure 5 . 6 Crude
Figure 5. Emulsion profile of Shippers A, B, and C.

Figure 7 .
Figure 7. Shrinkage volume illustration from mixing phenomenon of light and heavy oils [1]. 0

Table 1 .
The required data: production rate, tank's condition, properties.

Table 4 .
Antoine parameters for hydrocarbon: T in K; P in kPa.

Table 8 .
Normal bubble and dew points of crude oils.
10Crude Oil -New Technologies and Recent Approaches
GROUPS h (%) = a x L c x (100 -L c ) b x (ΔAPI) c

Table 10 .
Parameters a, b, c in API 12.3 equations.

Table 13 .
Proportional sharing losses results.

Table 15 .
Comparison between proportional and stratified results.

Table 16 .
Daily join report.