23rd World Gas Conference, Amsterdam 6 THERMODYNAMIC ASPECTS OF DEVELOPMENT OF GAS RESERVES CONFINED TO COMPACT DEEP-SEATED RESERVOIR BEDS R.M. Ter-Sarkisov Russia
ABSTRACT The paper presents results of thermohydrodynamic investigations aimed at enhancement of efficiency of development of compact deep-seated reservoir beds. A number of factors lowering the gas-condensate recovery coefficients, particularly in the development of deposits in lowpermeability reservoirs are distinguished. For the study of mechanism and extent of influence of theses factors on the reservoir hydrocarbon recovery methods of physical and mathematical modeling of reservoir processes were used. On the basis of carried out research the known methods of maintaining of formation pressure were significantly improved. A solution algorithm for the task of enhancement of reservoir hydrocarbon recovery is proposed.
TABLE OF CONTENTS Abstract 1. Preamble 2. Methods 3. Results 4. Conclusion 5. List of Tables 6. List of Figures
1. PREAMBLE By natural gas reserves and production Russia is the largest player at the world energy market. Due to exhaustible character of hydrocarbon reserves their structure is continuously worsening. In the nearest years a significant and growing part of gas in the country will be produced from compressed deep-seated reservoir beds. The world practice has already a certain experience in developing natural gas fields confined to compact reservoirs. Even back in 195-196s in the USA gas condensate fields Hidley and Knox-Bromide (Carter-Knox) having reservoirs with permeability.1 to 4.5 md were successfully developed and operated. There are analogs to gas condensate fields Hidley and Knox-Bromide in Russia, including deep-seated objects. However, under conditions when such developments in Russia (for instance Achimovian deposits of a number of fields in West Siberia) should make a more significant than before contribution to the gas production volume, not only their normal exploitation will be required, but also the maximally possible reservoir hydrocarbon recovery. This calls for the necessity of profound scientific substantiation of the field development methods, taking into account factors which were not considered before. 2. METHODS Investigations carried out by VNIIGAZ showed that the following specific factors can be distinguished lowering the gas-condensate recovery coefficients, particularly in the development of deposits in low-permeability reservoirs: - "insufficient" content of ethane-propane-butane fraction in the mixture; - presence of bound oil in the reservoir gas saturation zone; - presence in the reservoir bed of filtration fields with initial pressure gradients (IPG); - structure-forming influence of the reservoir rock on the formation hydrocarbons; - lowering of relative phase permeability (RPP) values for hydrocarbons with typical "complex" gas-liquid saturation of the reservoir bed; - as a rule, significant heterogeneity of reservoir by collecting properties and fracturing, with occurrence of basic HC reserves in zones with hindered inflow to producing well. The mechanism and the level of negative influence of each of the listed factors on the reservoir beds HC recovery have been the subject of thorough study of VNIIGAZ's specialists. For this purpose methods of physical and mathematical modeling of reservoir processes were used. 3. RESULTS Thermodynamic calculations allow to assess the influence of the ethane-propane-butane fraction on the equilibrium condensate content (content of C 5+ ) in the formation gas. Figure 1 shows a triple diagram of gas-condensate mixture under different pressures. The curves in the figure separate the areas of single-phase state (above the curve) and two-phase state of the mixture. Under high pressures the enrichment of system with ethane-propane-butane fraction enhances the gas evaporation capability. If the injected gas is enriched with this fraction up to -4, it can create two-phase or completely miscible flow in the formation. Under low pressures the conditions of complete miscibility are lacking. Depletion of fraction C 2- C 4 in the system leads to increase in the gas evaporation capability.
3 L C 2 - C 4, mol 1 1 9 8 5 4 C1 7 6 1 C 1, mol 5 MPa 5 6 4 7 1 MPa 3 8 C 9 G 25 MPa 1 1 3 4 5 6 7 8 9 1 C 5+, mol Figure 1. Triple diagram of gas condensate mixture Curves subdivide areas of one phase (above the curve) and two phase state of mixture (t=8º). Table 1 gives characteristics of Achimovian reservoirs with regard to the level of initial pressure gradients (IPG). It shows that IPG values can reach 3-4 MPa. The point is that no less than a half of the Achimovian deposits are confined to reservoirs with permeability not exceeding 2 md. Group reservoirs of Table 1. Characteristics of Achimovian reservoirs of the Urengoi field Absolute permeability, md Formation gas saturation, Equilibrium gas saturation, 1. Mediumproductivity reservoir, mediumporous 2. Lowproductivity reservoir, finely-porous 3. Reservoir Non-reservoir, finelyultrafinelyporous *unsteady flow permeability 1-3 - 1-4 md Formation gas permeability, md Initial pressure gradient, MPa/m, at Share of different gas formation pressures: reserves, 6 38 MPа MPа > 1 >7 1-15 > 7-7 1-2 7-55 15-7-1-2-1 5-45 -25 1-.4 - -.3 17 1-.5 45-35 25-3.4.8 -.7.2-1 31.5-.3 35-3 3-5.8 -*.7-3 1-4 25 In connection with development of new objects in compact reservoirs the influence of pressure and saturation of rocks with liquid phase on permeability was studied.
Figure 2 shows, as the example, the results of experimental measurements of relative permeabilities used in mathematical modeling. The left graph illustrates the dependence of relative permeabilities on saturation and pressure during filling, and the right graph illustrates the same dependence during draining. It is shown that relative permeabilities depend substantially on formation pressure with pressure influence being larger for gas as compared with liquid. 1 1 General phase ОФП, permeability 8 6 4 k газ k жидк General phase ОФП, permeability 8 6 4 k газ k жидк liquid.2.4.6.8 1 Насыщенность, доли единицы 25 МПа МПа 1 МПа 5 МПа Ряд5 Ряд6 Ряд7 Ряд8 Filling.2.4.6.8 1 Насыщенность, доли единицы 25 МПа МПа 1 МПа 5 МПа МПа 5 МПа 1 МПа 25 МПа Draining Figure 2. Influence of formation pressure upon relative phase permeability The successful prospecting and development, in particular, of Achimovian deposits will require application of known methods of maintaining of reservoir pressure (cycling process) with significant improvements made by VNIIGAZ's specialists neutralizing the negative influence on the HC recovery of all mentioned factors. The task solution algorithm envisages: - inclusion of the bound oil in the process of interfacial mass exchange and reservoir mass transfer; - taking account of the agent's composition and the level of the impact pressure on IPG, structure-forming effects in the system rock-fluid, RPP; - optimization of composition of the injected into reservoir gaseous agent, varying subject to the impact pressure, assessment of the required volume of injected agent; - designing of two systems of location of producing and injecting wells for zones of fractured reservoir and zones of compact reservoir. The assessment showed the possibility of the maximal increase of gas condensate and oil recovery from a compact deep-seated reservoir bed with use of the proposed algorithm. Figure 3 shows the results of physical modeling of the process of injecting non-equilibrium gas into the formation. The experiment was conducted under pressure of 3 bars and temperature of 62 C. The amount of ethane in the output was rap idly reducing after the injected gas breakthrough. The propane content was decreasing after pumping of one and half pore volumes through the formation model. The butane content remained at the initial level for a long period while a content of heavier hydrocarbons even slightly increased and remained so for a long time.
15 Сomponents content g/m 3 1 5 С 2 С 3 С 5+ С 4.5 1 1.5 2 2.5 3 V V inj,vo lume po r. (P = 3 Мпа, t = 62 o C) Figure 3. C 2 -C 5+ components content in the product at dry gas injection Figure 4 presents the experimental results of partial formation pressure maintenance by injecting enriched gas. The gas return factor was 5 percent. The left figure shows the change of condensate extraction factor during depletion and formation stimulation. Коэф ф ициент извлечения конденсата, Condensate Recover Coefficient 1 8 6 4 истощение Depletion закачка Injection q g\m, г/м 3 3 4 3 1 КГФ GCF Мо Molecular лекулярная mass масса 275 225 175 125 МС5+, г/мо g\mol ль.5 1 1.5 Relative Относительная gas recovery, добыча shares газа, of доли unitед..5 1 1.5 Relative Относительная gas recovery, добыча shares газа, доли of unit ед. 75 Figure 4. Partial maintaining of formation pressure by means of enriched gas The right figure shows the changes of condensate share in the output as well as its molecular weight. In the course of mass transfer a surge of liquid hydrocarbons is formed at the formation model exit and the condensate is efficiently extracted within the liquid phase. When this surge of liquid hydrocarbons approaches the model exit face, the share of the produced condensate and its molecular mass rise sharply. The condensate recovery factor in the process reached 85.
At present, at the Vuktyl field already almost half of the object is developed with gas injection. The results of stimulation are good enough for a secondary method. Figure 5 shows the dynamics of coefficients of additional HC recovery at one of sites (in the area of gas treatment unit 8). The diagrams show that the efficiency of extraction is higher if the molecular mass of hydrocarbon is lower, i.e. the interphase equilibrium constant of is higher. K и, initial reserves (С 1 ; С 2 -С 4 ) K и, initial reserves (С 5+ ) 18 1 16 14 С 5+.8 12 1 С 1 С 2 -С 4.6 8 6.4 4 2.2.5 1 1.5 2 2.5 3 Q зак, vol. of pores 1993 1995 1997 3 5 Figure 5. Vuktyl oil-gas condensate field, area close to gas pre-treatment Unit-8. Coefficients Kin change for additional hydrocarbon recovery by injecting dry gas 4. CONCLUSION It should be noted that the results obtained and at the Vuktyl field have an important significance for the gas producing industry in Russia. Already scores of gas condensate fields entered the final stage of reserves recovery. They are potential objects for implementation of the low-pressure gas technology. The simplicity of the technology and its proved efficiency allow considering it as a very promising for the industry. In our opinion, the described technology may be of practical interest to other gas producing countries.
5. LIST OF TABLES Table 1. Characteristics of Achimovian reservoirs of the Urengoi field 6. LIST OF FIGURES Figure 1. Figure 2. Figure 3. Figure 4. Figure 5. Triple diagram of gas condensate mixture Influence of formation pressure upon relative phase permeability C 2 -C 5+ components content in the product at dry gas injection Partial maintaining of formation pressure by means of enriched gas Vuktyl oil-gas condensate field, area close to gas pre-treatment Unit-8. Coefficients Kin change for additional hydrocarbon recovery by injecting dry gas