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Processing and interpretation of hydrochemical data for control of oil field development

   The methodological research basis lies in collecting and gauging of data, creation of data banks and applicable software, developing of theoretical concept and its adjustment to Belarusian oil-fields, build-up of new theoretical footing permitting to design original and most effective processing techniques of hydrochemical information and data interpretation with reference to the control of oil pool development.
  Mass hydrochemical data processing nowadays shows not only the expediency but also a high efficacy of the application of the suggested methods for solution of many rather difficult but very relevant present-day and retrospective oilfield problems [1].
  We are going to take up the essence of the hydrochemical control methods of oil pool development and to demonstrate their usage.
   Recently almost in all oil-and-gas bearing regions more and more attention has been given to the control of oil pool development. Traditional downhole logging and hydrodynamic methods for various reasons do not enable to control it properly. To enhance the efficiency of these methods rather large funds are spent to buy expensive equipment and software. However it doesn’t make the traditional control of oil pool development more qualitative. In view of this some years ago on the authors’ initiative a new research direction was formed and began developing. It is the possibility of using the hydrochemical data in petroleum engineering.
  The origin of the produced water can be easily identified when we know the representative elemental composition of in-situ, injected and produced with oil waters.
It’s important to take into account that on Belarusian oil-fields the injected water as it moves to the producers is appreciably (200 g/l) enriched with natrium chloride at the expense of dissolution of catagenetic halite. The usage of water density, salinity, amount of Na+ or Cl- ions for identification of the produced water nature according to the accepted in other oil and gas bearing regions methods can cause significant mistakes.
  General salinity and the content of different components in produced water are determined by the elemental composition and the ratio of mixed (in-situ and injected) waters. However we have found out considerable deviations of concentrations of a number of water-soluble elements from theoretically anticipated content. This fact helps to establish the wide-ranging dissolution of catagenetic halite and anhydrite fillings and the crystallisation of calcium carbonate as the injected water moves to the producers in oil reservoirs [1-3]. To estimate the influence of these processes on the filtration characteristics of rocks the techniques and the computer programs GALIT, SULFAT, CARBON, KANAL and EXPRESS were designed. They are based on the data processing of the injected, in-situ and produced water elemental composition. With the help of these techniques the balance calculation of NaCl, CaSO4 and CaCO3 in in-situ, injected and produced waters on     Belarusian oil pools was made. Let’s have some examples.
  More than 5,000 m3 of halite, about 14 m3 of anhydrite have been dissolved and washed out by injected water and more than 6 m3 of calcium carbonate have been crystallised from produced water in well 105 on Berezinskoe oil pool. The produced water in well 75 of Ostashkovichskoe oil pool throughout the exploitation has washed out more than 100,000 m3 of dissolved halite, 105 m3 of anhydrite and 134 м3 of calcite have been crystallised. On the whole in the intersalt reservoir of Ostashkovichskoe oil pool more than 1.2 million m3 of halite, about 650 m3 of anhydrite have been washed out by produced water and about 2,200 m3 of calcite have been crystallised.
   These considerable changes of the filtration channel net during the Belarusian oil pool development have a great influence on the character of the water-oil displacement and the coverage of an oil pool by the development process. The halite dissolution often results in abrupt increase of heterogeneity of oil reservoir, the break of injected water through the formed filtration channels and the increase of well production watering. This conclusion is confirmed not only by greater speed of injected water moving during the oil pool development (this fact is ascertained by indicator methods) but also by the character of reservoir pressure changes.
  For the majority of gas and oil producing regions it isn’t hard to estimate the portion of injected water in produced one.
  One of the main parameters identified by the program KANAL is the value of X. It is the portion of injected waters in produced brines.
  The volume of in-situ brines washed out with produced water is calculated by multiplication of the volume of produced waters in each well by the portion of in-situ water (1-X) in produced brines and then the adding up these volumes in all watered wells. To estimate the volume of in-situ water that intruded and remained in the pool we first of all identify the volume of all waters flowed the pool. (This volume is equal to the volume of produced oil corrected to the pool conditions multiplied by average prevailing value (1-X)).
   The results of these calculations are very essential for balance study of injected and produced fluids, especially while designing the hydrodynamic models of development of some neighbouring oil pools.
   For many oilfield problems (recalculation of supply, control of development, etc.) the method of material balance is used. This method takes into account the volumes of injected and produced fluids that are corrected to pool conditions. For realisation of such calculations in the most gas and oil producing regions there is no need to allow for changes in productive strata, concerned with dissolution or crystallisation of different minerals. The latter results from relatively small scale of such processes. However it doesn’t concern the oil pools of Belarus where on some pools up to 1.2 million m3 of halite has been dissolved. Besides the injected and remained in the oil pool waters have dissolved more amount of this mineral.
The balance changes in an oil pool caused by concerned lithohydrochemical processes are determined by two constituents: the volume of dissolved halite and the volume decrease at the expense of electrostriction. The fulfilled calculations show that the total volume decrease caused by halite dissolution, for example, on the intersalt reservoir of Ostashkovichskoe oil deposit is more than 2 million m3. This figure should be taken into account when calculating the material balance.
   To solve this problem we use computer programs KANAL and EXPRESS. One of the main parameters is the NaClexc value - excess content of sodium chloride in produced water, which results from dissolution of catagenetic halite filling of cracks, pores and caverns.
  The low concentrations of NaClexc usually testify either to fine washing out of main filtration channels from halite or to high degree of development of oil pools within the given area or indicate the areas of inflow of in-situ water that is not able to solve halite in significant amounts. Therefore the determining of areas that were washed and are being washed out from halite is made by maps of individual portion of injected waters in produced brines and by maps of excess content of NaCl in produced waters within a period of time.
  The chemical composition and the density of injected water differ considerably from the in-situ brines. It is possible to study the direction and velocity of injected water. We illustrate it with the intersalt reservoir of Ostashkovichskoe oil pool. There was no influence of peripheral waterflooding pattern on the reservoir. That was why the spot injection of fresh water directly into the reservoir had been organised. The majority of producing wells gave not only more watered oil but also the produced water with lower density. The regular retard of the time when the produced water of lower density appears as we move from the injected wells to the producers allows to determine the direction and to calculate the velocity of filtration flows within the limits of the reservoir for a period of time.
  The statistic data processing testifies that the average velocity of fluid is 1-2 m/day. The considerable changes of injected water chemical composition have caused the chemical changes of produced water as a rule within one-two months. It indicates that the velocity of filtration flows has increased considerably comparing with the initial stage of development.
  As the chemical composition data (especially for produced water) exceeds many times the oil pool pressure data and as the hydrochemical changes in an oil pool depend on the hydrodynamic processes the authors studied the possibility of using hydrochemical information for modification and making maps of isobar.
The maps of average density of injected water for the most watered reservoirs, several diagrams shoving the change of this parameter were made and the chemical composition of in-situ, injected and produced waters were analysed. It helped to modify the isobars on the existed maps and to specify the formation pressure changes in oil pools. The fulfilled research has shown the expediency of usage of chemical composition and density data for the modifying of retrospective and present-day hydrochemical conditions in oil pools.               
  The problem of salt-crystallising control is acute in many gas and oil producing regions. The main crystallising salts in the majority of regions are calcite and anhydrite. In Belarus there is one more crystallising salt -- halite that is the main calmotant.
The main method of salt crystallisation control in wells is the adding of fresh water. But the carried research shows that for this purpose the spot injection into the part where there is the intensive crystallisation is, the decrease of injected water salinity, pressure maintenance at the proper level (that prevents the in-situ water intrusion) are as well can be used.
  According to our research the change of produced water density can often testify to the quality of water isolation measures (change of filtration flows).
In most cases the change of filtration and underground water flows results in essential density change of produced water. The abrupt change of produced water density after GТМ testifies to the change of filtration flows and a good enough quality of GTM.
   To make final conclusions about the efficiency of performed GTM it is necessary to consider one important condition. The point is that immediately after water isolation operation in most cases the produced water density decreases or increases abruptly and some time later is stabilised at a definite level. The time needed for the stabilisation of the parameter in question indicates the period when wells start to work in normal regime.
  One of the most relevant problems arising in the course of control of oil pool development and oil production is the forecast of time of oil production watering. To present day for this problem the content of salts in oil in waterless operation period has been used. The authors first note that in many wells before and at the beginning of their watering there is the increase of oil density. This fact can be used for the forecasting the time of oil production watering in different oil pools. As the above-mentioned methods have different efficiency of prediction we suggest making up so-called " the triple graphs" for more detailed study. They have synchronic time axes and show the change of the following parameters in the course of oil pool development:
• Content of chloride salts in oil with mapping of oil production watering;
• Density of oil;
• Density of produced water.
   Summering the aforesaid information up we’d like to note that the fulfilled oil field research shows that the hydrochemical techniques of data processing and control of Belarusian oil pool development essentially differ from the offered earlier methods in other oil and gas bearing regions. They are more effective in practice.
The offered techniques can be successfully applied not only in Belarus but also in other oil and gas bearing regions (Irkutsk amphitheatre, Tungus basin, Triassic province in Algeria, Pre-Caspian and Eastern Pre-Caucasia regions, sedimentary rocks of Michigan and Illinois in the USA).






Author: V. V. Mulyak