Oil Losses Problem
Oil Losses Problem in Oil and Gas Industries
Abstract
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.
Keywords: Emulsion, Flash, Offset, Oil losses, Proportional, Shrinkage, Stratified
1. 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 themeasured 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.
1.1 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.
1.2 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
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].
2. 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.
2.1 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 (SGo), the formation water specific gravity (SGw), 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&Wvolume. In order to calculate the total sharing oil losses, the individual losses such as emulsion and evaporative losses must first be calculated.
2.2 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 (a1, b1, a2, b2) for each shipper are 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:
Y1 ¼ a1X1 þ b1 (1)
where X1 is the percentage of the addition of the volume of formation water (in %vol), Y1 is the calculated SG, a1 and b1 are constants. The calculated SG is:
SGcalculated ¼ ð1 XwÞSGo þ XwSGw (2)
where Xw is water volume fraction in oil, SGw is specific gravity of formation water, and SGo 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.
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