The cost analysis of the separation of produced formation water from the hydrocarbon reservoir using the example of the Upper Miocene sandstone deposits of the Sava Depression

Formation water is produced during the whole lifetime of a hydrocarbon reservoir alongside the oil and/or gas and it represents the main part of the produced fl uid. The produced formation water is separated during the process of dehydration. This paper deals with the formation water separation costs regarding the fi elds A, B and C which are located in the western part of the Sava Depression. The dehydration process regarding fi eld A is executed in three locations, and in fi elds B and C, it is executed in one location. The technological system of formation water separation and the geological characteristics of the above-mentioned reservoirs is represented. A statistical analysis regarding the formation water separation costs has been made. The costs have been statistically estimated and a correlation between the costs relevant for the usual formation water separation process has also been made. The purpose of the analysis of the cost of the dehydration process is the optimization of the production system and cost control of the process.


Introduction
Formation water is produced during the production from a hydrocarbon reservoir together with oil and/or gas.Globally, oil wells produce about 220 million BWPD (ca.35 million m 3 /day) (Tajmiri & Reza Ehsani 2016).The ratio between the produced water and oil is 3:1, and the average share of water in the uids equals 70% (Fakhru'l-Razi et al. 2009).Separation of the formation water is carried out by the dehydration process.The ef ciency of formation water separation directly affects the formation water's quality, and its importance for the injection system has been described in the Western Sava depression water-injection system analysis (Ivšinovi 2017).This paper deals with the costs and the formation water separation process and the geological characteristic of the reservoirs in the oil and gas elds A, B and C.These are located in the western part of the Sava Depression.The dehydration process regarding eld A is executed at three locations, and in elds B and C, it is executed at one location.The statistical data was gathered between the years 2009 and 2015.This paper describes the formation water separation process and an estimate will be made regarding the formation water separation costs and the correlation between the important variables in the formation water separation process.

The Geographic Area of Research and the Basic Geological (Lithostratigraphic) Characteristics of the Area
The oil and gas elds described in this paper are located in the Croatian part of the Pannonian Basin System in the Sava Depression.The geotectonic position of the Sava Depression (blue) within the Pannonian Basin System is shown in Figure 1.
The oil and gas eld A is located 55 km south-east from Zagreb, and elds B and C are located approximately 90 km south-east from Zagreb.The general and common geographic locations of the elds in question are shown in Figure 2, (blue), while the observed reservoirs (blue) are shown in Figure 3.
The reservoir rocks of the oil and gas eld A are ne to middle grained quartz micaceous sands.On the preneogenic bottom rock, there lies a transgressive complex of neogenic sediments.Within this sediment complex, the main reservoirs are; the sandstones of Lower Pontian, Kloštar Ivani Formation, Pannonian, and Ivani -Grad Formation.
The reservoir rocks of oil and gas eld B are poorly to ne grained quartz micaceous sandstones.The reservoir structure (see Figure 4) is brachyanticline with northwest-southeast general orientation.In eld B, the reservoirs are formed in Pliocene and Miocene deposits.The total depth of the reservoirs is between 1 000 and 2 000 meters.
The Mining-Geology-Petroleum Engineering Bulletin and the authors ©, 2018, pp.35-43, DOI: 10.1177/rgn.2018.1.5 The reservoir rocks of oil and gas eld C are poorly to middle grained sands and poorly to ne grained quartz micaceous sandstones.The reservoir structure is an elongated anticline with northwest-southeast general orientation.There are two maximums in the central part of the structure.The reservoir rocks are interlayered with marls and sandy marls.Seal rocks are marls that turn into calcitic marls in the deeper reservoirs.
According to Veli et al. 2012, oil and gas elds are divided into: large elds (which produced more than 10 6 m 3 of oil/condensate or more than 10 9 m 3 of gas), medium elds (which produced 10 5 -10 6 m 3 of oil/condensate or 10 8 -10 9 m 3 of gas), small elds (which produced 10 4 -10 5 m 3 of oil, <10 5 condensate or 10 7 -10 8 m 3 of gas) and very small elds (which produced less than 10 4 m 3 of oil or less than 10 7 m 3 of gas).According to the above mentioned classi cation, the oil and gas elds B and C are medium elds, while the oil and gas eld A is classi ed as a large eld.

The Dehydration System Technology in the Oil and Gas Fields A, B & C
The dehydration process is performed in separators.These are throughput devices of cylindrical shape (vertical or horizontal).They are used to ef ciently separate gas from a liquid phase under a certain pressure and temperature.The retention of the uids in the processing vessels causes the formation water to be separated at the bottom of the vessel.The amount of the separated formation water regarding the time of its retention is shown in Figure 5.
The retention time of the produced water in the treatments vessels is from 3 to 30 minutes (Arnold & Stewart, 2008).The technological process of produced water separation in the oil and gas eld A is represented in Figure 6.
The formation water is processed at three gathering stations, and the dehydration process is done by using formation water separators and dehydrators.The average value of process parameters in eld A are: uid ow: 2 000 m 3 / day, pressure: 1.0-1.5 bar and temperature: 35-40 °C.The technological process of produced water separation in the oil and gas elds B and C is represented in Figure 7.
The produced uids are gathered from ve measuring stations at the dispatch station of the oil and gas elds B and C. Fields B and C have a common gathering system, and thus a common dehydration system.Due to this, the common dehydrating system for elds B & C may be viewed as a single technological process.The average value of process parameters in elds B & C are: uid ow: 800 m 3 /day, pressure: 1.0-1.5 bar and temperature: 40-45 °C.The formation water separation is performed in the gravity settling vessel and dehydrator.

The Formation Water Separation costs in the Oil and Gas Fields A, B & C
The Figure 8 shows an evident increase in the amount of the produced formation water from eld A. This is a consequence of the additional optimization of the wells in the analyzed oil and gas eld ( eld A).The optimization of the oil and gas eld A was achieved through well workovers.Capital workovers are made on six wells.Capital workover operations in production wells cover operations performed in formations (formation remedial operations), and in the wellbore (equipment repair op-erations, etc.).The consequence of these capital workovers was an increase of the produced uids, and the formation water.The oil and gas elds B and C were not optimized.To calculate the overall unit cost, data regarding the energy sources' price (electric energy and natural gas) is needed.These are shown in Table 1.
The data from Figure 8 and Table 1 was used for the calculation of the unit price of formation water separation according to the methodology of the authors Ivšinovi & Dekani from 2015.The calculated formation water separation costs regarding the elds A, B and C are shown in Table 2.
According to Table 2, the costs with the largest share in the overall costs of formation water separation are: energy ( eld A: 41.0%, elds B&C: 73.3%), heat exchangers and process vessels maintenance ( eld A: 32.3%, elds B&C: 10.5%) and chemicals ( eld A: 21.1%, elds B&C: 11.4%).A statistical evaluation of the above-mentioned data will be made and a correlation between the most important variables will be presented in the following chapters.

The Chosen Statistical Methods for the Data Processing
The normal (Gauss) distribution is the most wellknown and, in nature, the most common distribution function.It is commonly used in geology and hydrocarbon reservoirs geology (e. g., Malvi & Meduni 2015).The Shapiro and Wilko (S-W) test is the most common one for testing the normal distribution of data.This test is based on the correlation of a sample of the "statistical order" which has a normal distribution.The null hypothesis is the normality i.e. the uniformity of data.The Sha- (1) Where: W is the test-value, y i is the data, m 1 stands for the arithmetic mean of data, a i is the calculated linear regression value for expected values from standard normal "statistical order".
The Shapiro-Wilko test is a regular tool in the statistical calculation in any statistical computational program so it is important to emphasize that the null hypothesis is not accepted if the p-value is inferior or equal to the threshold of signi cance (0.05).The sample size for individual costs for Fields A and B & C is seven, for each individual cost in Table 2.In order to calculate the interval estimation of expectations and choose the adequate method of correlation, the condition for the calculation is the existence of a normal data distribution.Table 3 shows the test results of formation water separation testing to the existence of normal distribution.
According to the data in Table 3, an interval estimation of expectations with a 0.95 con dence level for t-distribution will be made.The costs with no uniform distribution will be shown with a middle value and the belonging corrected standard deviation.
The interval estimation (IE) is calculated according to the following equation (e.g.Pfaff 2012; Benši & Šuvak 2013): (2) Where: stands for the arithmetic mean, t-the read value from table for t-distribution, s-corrected standard deviation, n-sample size.The non-integral estimation is used when it does not exist uniform distribution.The non-integral estimation is calculated according to the following formula: (3) Where: NIE stands for non-integral estimation, arithmetic mean, s -corrected standard deviation.The formation water separation costs are estimated in Table 4.
The estimated costs from Table 4 are used in the cost analysis regarding the separation system and a possible optimization and the separation system rationalization.The heat exchangers and process vessels maintenance costs as well as the energy and chemicals costs will correlate with the quantity for the produced formation water, while the energy and chemicals costs will correlate with the heat exchangers and process vessels maintenance costs.To calculate the correlation coef cient, the Pearson and Spearman correlation coef cients will be used.The condition for the application of the Pearson correlation is that the observed samples are normally (uniformly) distributed.The Pearson correlation coefcient is calculated according to the following equation (e.g.(5) Where: r s stands for the Spearman correlation coef cient, d i -the difference between the rankings for each x i and y i pair of data, n -sample size.
According to the results of the existence of normal distribution from Table 3, the correlation coef cients among the formation water separation costs for elds A, B and C variables have been calculated (see Table 5).
The positive correlation between variables showed a linear increase of both variables.Negative correlation between variables shows a linear increase of one variable, while the second variable records a linear decline.There is a correlation eld A (-0.821); elds B and C (0.962) between the heat exchangers and process vessels maintenance and the quantity of produced formation water.There is a middle correlation between the remaining variables pairs which comes as a consequence of the unevenness of the investment during the observed period due to the decrease of operational costs caused by the decrease in the price of crude oil on the markets.The non-correlation among the chemicals and energy and the heat exchangers and process vessels maintenance is the consequence of the decrease in the investments in the formation water separation system.

Conclusion
The formation water separation costs in the oil-gas eld A range from 0.14 USD/ m 3 to 0.59 USD/ m 3 .The average year amount of produced formation water is 478 000 m 3 .The formation water separation costs in the oil and gas elds B and C range from 0.68 USD/m 3 to 1.37 USD/m 3 while the average year amount of produced formation water is 152 000 m 3 .The difference in the unit cost is caused by the difference in the amount of pro- The Mining-Geology-Petroleum Engineering Bulletin and the authors ©, 2018, pp.35-43, DOI: 10.1177/rgn.2018.1.5duced formation water, energy sources, the physical and chemical content of the uids and the process maintenance.There is a correlation between the heat exchangers and process vessels maintenance and the quantity of produced formation water.There is a middle correlation between the energy and the quantity of produced formation water, and no correlation between the chemicals and the quantity of produced formation water, etc.The consequence of the lack of investment into the separation system is the zero correlation between chemicals and heat exchangers and process vessels maintenance.The lack of uniformity regarding the data is a consequence of the reduction of formation water separation costs.This however is the consequence of the lower price of crude oil on the world market.The formation water separation costs regarding mature oil and gas elds represent a signi cant share in the overall costs which can, in a certain moment and with a certain combination of technological factors and energy prices, be fundamental for the cost calculation and a business decision regarding the possible continuation of hydrocarbon exploitation in such elds.

Figure 5 :Figure 6 :
Figure 5: The separation of the formation water as a function of retention time (Arnold & Stewart 2008)

Figure 7 :Figure 8 :
Figure 7: The technological process of formation water separation and collection in the oil and gas elds B and C

Table 2 :
Unit costs of formation water separation in the process of dehydration on the oil-gas elds A, B and C

Table 3 :
Testing costs of separating formation water on the constancy of normal distribution

Table 4 :
Cost estimates (USD/m 3 ) for the separation of the formation water in oil and gas elds A, B and C

Table 5 :
The correlation between the costs of separated formation water for elds A, B and C The Spearman coef cient is used when there is no clear linear relation between the two variables if their original values are compared, but if the values are ranked, the dependence can be calculated.It is used in the correlation between the two independent variables.It had earlier been used in the hydrocarbon reservoir calculation in CPBS (e.g.,

Malvi 2006; Malvi & Prskalo 2008).
Due to the fact that the coef cient calculation technique is somewhat not affected by the extreme values and the gathering of data in certain regular intervals is not exclusively necessary, it can be applied to little sample "clusters" (e.g.,