Gazprom VNIIGAZ: Specifics of Determination of Gas Condensate Characteristics in the Development of Deep-Seated Fields with Highly Productive Strata

This article will look at the examples of the Karachaganak and Vuktyl oil and gas-condensate fields, reviewing the specific findings of how to determine the gas and condensate characteristics in hydrocarbon fields with high net reservoir thickness, and namely, the component composition of the formation’s gas, potential content of C₅₊ hydrocarbons (HC), physicochemical properties of the condensate (density, molecular mass), PVT-characteristics of the formation gas and, predicted behavior of the C₅₊ HC content under formation drawdown conditions. It has been illustrated that the Karachaganak oil and gas-condensate field is a classic example of how the impact of gravitational forces brings significant changes in the composition and other gas-condensate characteristics across the depth of the stratum. The differentiation is vividly expressed on the example of C₅₊ HC. Concentration of the C₅₊ HC group grows 3+ times as the depth increases (3700m to 5200m). The density and molecular mass of condensate increases as well.

The two methods for calculating the compositional change across the formation’s depth are presented in this article: one was elaborated by V.F. Perepelichenko, the other by A.I. Brusilovsky and O.Y. Batalin, who built their approach on a strict correlation of thermodynamics and specifically accounted for the real properties of the fluids. It has been demonstrated that the estimated values of change in the content of C₅₊ HC taking place across formation depth, are somewhat different from the actual ones, which is obviously related to the fact that a simplified model of formation fluid mix was applied, where the C₅₊ HC group was simulated with the use of n-undecane (nC₁₁H₂₄) only.

The initial PVT-characteristics of the mode of occurrence have been displayed, including the component composition of the formation fluid mix at the Vuktyl oil and gas-condensate field. Differently to the Karachaganak oil and gas-condensate field, the content of C₅₊ HC at the Vuktyl oil and gas-condensate field varies across the profile of the deposit not so significantly, namely, from 308 to 382 g/m³, and the C₅₊ density and molecular mass increase as the depth grows. The analysis of findings of the researched phase transformations of gas-condensate fluid mix at the Vuktyl oil and gas-condensate field, carried out on PVT- units, made it possible to determine the phase curves of the formation fluid mix of this field. To evaluate the current content of C₅₊ HC and condensate recovery in the process of formation pressure drawdown, the C₅₊ distribution balance was calculated, which has been predicted in the process of development of the targets at the Karachaganak and Vuktyl oil and gas-condensate fields.

Due to the fact that gas condensate properties may vary differently across different depths at various oil and gas-condensate fields, depending on their formation environment, the concept of the variation rate (gradient) for gas condensate characteristics was introduced by the authors, to compare the degree with which gas-condensate characteristics vary as the occurrence depth grows. The C₅₊ HC gradient at the Karachaganak oil and gas-condensate field is 5 times higher than it is at the Vuktyl oil and gas-condensate field, and the change in density and molecular mass is 1.2 to 1.5 times more intensive. The inconsiderable increase in the content of C₅₊, circa 20%, with significant thickness (over 800 m) of the productive horizon at the Vuktyl oil and gas-condensate field, testifies that the gravity factor was not prevailing during formation of this occurrence.

Several gas-condensate and oil and gas-condensate fields, which are unique by their deep-seated reserves, were discovered and brought into development in recent years. They are featured either  with high thickness, such as the Karachaganak and Vuktyl oil and gas-condensate fields, or with vast area, such as , for instance, the Astrakhan gas-condensate field, the Achimov deposits of the Urengoy oil and gas-condensate field, the Orenburg oil and gas-condensate field and some others [1]. Determining the gas-condensate characteristics of these fields is related to the presence of abnormally high formation pressure, high temperatures, low reservoir characteristics of the penetrated formations. In the conditions when a deposit has considerable thickness, gas-condensate characteristics vary as the depth grows, due to gravity force. And if the deposit area is much greater than its thickness, gas-condensate characteristics may vary across the area as well. In several cases, there is probability that the gas-condensate characteristics change both with the depth and across their area.

The basic gas-condensate characteristics of the fluids and products of gas-condensate and oil and gas-condensate deposits include: the component composition of formation gas; potential content of C₅₊ (HC); physicochemical properties of condensate – density (ρ) and molecular mass (M, g/mole); formation gas condensation isotherms; the C₅₊ HC group distribution balance under formation pressure drawdown, etc.

This article, with the Karachaganak and Vuktyl oil and gas-condensate field examples (Table 1) reviews the specifics and findings of determining the initial characteristics at the fields with a high thickness of the productive strata.

Karachaganak Oil and Gas-Condensate Field

The Karachaganak oil and gas-condensate field is located in the subsalt layers of the Caspian Depression, in the interval of 3600 to 5600m. The deposit is confined to solid carbonate massif, 15×30 km in size, the height of productive thickness amounts to 1600m. The Devonian, Carboniferous, and Permian systems have been sufficiently delineated in the effective pay. The field was brought into development in 1984.

The formation pressure (P_r) value of the deposit ranges from 52 MPa, at the top (the depth of 3700m) to 60 MPa, at the bottom (the depth of 5200m), the formation temperature (T_r) value increases from 343K to 358K within this interval range (Fig.1). The initial component composition of the formation fluid mix recovered from the Karachaganak oil and gas-condensate field is presented in Table 2.

The deposit is a classic example of how the effect of gravity results in a significant change of the composition and other gas-condensate characteristics across the thickness of a reservoir. The differentiation on the content of the C5+ HC group is most signified; whose concentration grows three and more times as the depth grows; the P and M values of the condensate behave in a similar way (Fig.2). The content of the C₂…C₄ and CO₂ components practically does not change, while the hydrogen sulfide content grows inconsiderably.

If we have deposits with productive strata of high thickness, any traditional approach to determine the component composition may bring us to an inaccurate estimation of HC reserves and to false field development indicators, as a result of that. The initial estimations [2] of changes in the component composition across the profile of the productive strata at the Karachaganak oil and gas-condensate field were carried out according to the Boltzmann formula with the use of the expression:

     (1)

where η is a molar fraction of i-component of the formation fluid mix at depth h; h₀ is the depth of the occurrence top;  M_i is molecular mass of the i-component of the fluid mix; g is gravity acceleration, R is universal gas constant.

O.Y. Batalin, A.I. Brusilovsky, and some others [2-5], suggested a more accurate method for calculation of variation in the component composition and pressure across the depth of an occurrence. The method is based on strict correlations of thermodynamics and it takes more accurate account of the real properties of fluids, and in its final form, it can be presented with the following formula [5]:

   (2)

where f_i(h₁) is fugacity of the i-component at depth h₁; f_i(h₂) is the target value of fugacity at depth h₂.

Tables 3 and 4 illustrate the findings of the calculated composition of the model mixture which, by its composition, is similar to the formation fluid mix at the Karachaganak oil and gas-condensate field, with the depth values ranging from h₀=4000 mto h=5000m. The calculations, whose findings are presented in Table 3, were carried out by the authors according to the formula (1). Changes in the composition of the model mixture, presented in Table 4, were calculated according to the formula (2) [5]. Depth was estimated based on the values of temperature, presented in Tables 3 and 4, height size increment, h = 200 m.

The calculation findings (see Tables 3 and 4) have inconsiderable divergence which speaks for possibility to use a simpler estimation technique applied by VolgoUralNIPIgas.

Calculating with the use of a more accurate technique [5] testifies that  the content of nC₁₁H₂₄, which is used to simulate the C₅₊ HC group in the calculations, changes as the depth grows from  5.98 (h₀) to 8.95 (h₀+1000m), i.e. it actually increases 1.5 times. Density and molecular mass of stable condensate behaves in a similar way. Alongside with that, the molar fraction of the mixture’s methane decreases by 4.19 %. The content of homologous compounds of methane in the formation gas range in average from 9.44 to 10% of mole. One can see relative stability in the quantity of non-hydrocarbon components (H₂S and C0₂).

Fig.3 presents comparative findings of the estimated [2-5] and actual values of the C₅₊ HC content changing along with the depth of Karachaganak oil and gas-condensate field. It should be noted that the estimated values of the condensate potential content (PC) are a little different from the actual ones which is apparently related to the use of a simpler model of the formation fluid mixture, where the C₅₊ HC group was simulated with n-undecane (nC₁₁H₂₄) only.

Taking into account variations in PVT-properties of the gas condensate characteristics in the process of developing the Karachaganak oil and gas-condensate field, three development targets have been delineated: I – gas-condensate Low Permian target with the bottom boundary at the depth of 4550 m; II – gas-condensate target in the Carboniferous system with the bottom boundary at the depth of 5000m; III – oil target in the Carboniferous system at the depth of more than 5000m (Table 5).

To evaluate the current content and condensate recovery factor (CRF) of C₅₊ in the process of formation pressure (P_r) drawdown, the balance of C₅₊ HC distribution was calculated which has been predicted in the process of developing target I of the field under consideration (Table 6 and Fig.4). Table 6 and Fig.4 illustrate that CRF of the development target I may amount to 41.5%, and with account of backing up pressure (P_bu ~ 15MPa) it would hardly exceed 33%. Similar calculations testify that the CRF value for the development target II (initial C₅₊ content equals 640 g/m³) would correspondingly amount to circa 35%, with the formation pressure R_P=0.1 MPa, while, with account of the backing up pressure P_bu = 15 MPa, it would not even exceed 29%.

Vuktyl Oil and Gas-Condensate Field

The Vuktyl oil and gas-condensate field was brought into pilot development in 1968. The massive blanket deposit of gas and condensate comprises the carbonate coal and Low Permian deposits occurring at depths of 2100m to 3300m. The penetrated section (down to depth of 6.4km) is composed of Silurian, Devonian, Carboniferous, Permian, Triassic and Quarternary deposits. The major pool of the Vuktyl oil and gas-condensate deposit is confined to organic limestones, the productive thickness of which amounts to about 800m vertically. The limestones are covered with cap rock of 50 to 100m in length. This is a massive roof deposit, limited by faults. The reservoir occurs in the depth range of 2400 to 3300m. Oil fringe is present there.

The initial PVT properties for the occurrence of the formation fluid mix at the Vuktyl oil and gas-condensate field are as follows: P_r = 34.9….37.3 MPa, T_r = 320…338 K. Both the formation pressure and the temperature increase as the depth grows (Fig.5)

The initial component composition of the formation fluid mix is presented in Table 7, according to which the condensate content change is almost linear across the section of the occurrence (308 to 382.5 g/m³). The fraction of major components of C₂…C₄ remains, as the depth grows, practically unchanged, while density and molecular mass of C₅₊ HC increase.

Phase transformations of the formation gas-condensate fluid mix were explored with the use of PVT³ (УГК-3) units at the stages of exploration and initial development of the Vuktyl oil and gas-condensate field. The findings of that research (Table 8) are indicative of the following:

• dewpoint pressure (P_dp) changes from 32.6 to 34.6 MPa;

• maximum condensation pressure (P_mc) of unstable condensate ranges from 15 to 17.6 MPa;

• fallout of unstable condensate, under P_mc, ranges between 490…550 cm³/m³; while that of stable condensate lies in the range of  295…395 cm³/m³;

• undersaturation of the system ranges between 0.86 and 10.38%;

• undersaturation, or difference between P_r and P_dp, decreases as the depth grows

The curves, displaying the dynamics of potential content of the C₅₊, were calculated on the basis of the initial maximal and minimal values, which theoretically reflect the actual change in the content of condensate in the produced formation gas, taking place in the process of the field development (Fig.6). A similar approach to predicting the C₅₊ content, with depths increasing, makes it possible to more accurately describe the range of probably actual change in the content of C₅₊ in the process of the occurrence development.

R. M.Ter-Sarkisov and others [6] presented their predicted findings in the changes of the content of gas from some wells at the Vuktyl oil and gas-condensate field under formation pressure (P_r) drawdown (Table 9), which generally reflects the probable changes in the content of the separate components of the fluid mixture in the process of field development.

The authors of this article calculated the average balance of the C₅₊ hydrocarbon distribution in the process of developing the Vuktyl oil and gas-condensate field to its depletion state. (Table 10 and Fig.7).

At the Karachaganak field it was demonstrated that the occurrences with great mining level of productive horizon have significant change in their content of density and molecular mass of C₅₊ HC across the occurrence depth. Due to the gravity field effect, the formation fluid mixture may be a gas condensate system at the top, while at the bottom part it can be high-gravity oil with great amount of dissolved hydrocarbons. The transition from the gas condensate to the fluid system takes place without the formation of interfacial areas.

However, changes in the gas condensate characteristics taking place across the depth of an occurrence may behave differently at various oil and gas-condensate fields, depending on their formation environment. The authors of the article, using the technique developed by VolgoUralNIPIgas, carried out the calculation of changes in the content of C₅₊ HC across the depth of the Vuktyl oil and gas-condensate field (Table 11, Fig. 8). According to Fig. 8, the actual distribution of the C₅₊ HC values, across the occurrence depth at the Vuktyl oil and gas-condensate field, significantly differs from the estimated one.

To compare the degree of change in gas condensate characteristics, across the depth of occurrences, the authors introduced the concept of the gradient of gas condensate characteristics, i.e. the intensity in change of gas condensate characteristics of an occurrence. In particular, the gradients of the following gas condensate characteristics have been reviewed: pressure (ΔP_r/Δh)), temperature (ΔT_r/Δh), density (Δρ/Δh), molecular mass (ΔМ/Δh) and content (ΔС₅₊/Δh) of condensate С₅₊. The actual and simulated (estimated) values of the above mentioned parameters are presented in Table 12, which brings us to the following conclusions:

1) the average gradients of pressure and temperature are equal to 0.0054 MPa/m and 0.0154 k/m for the Karachaganak oil and gas-condensate field, and to 0.0027 MPa and 0.0174 k/m for the Vuktyl oil and gas-condensate field, correspondingly, i.e. the gradient of pressure is higher at the first field than it is at the second one, while the gradients of temperature remain much the same;

2) the intensity of change in density and molecular mass of condensate at the Karachaganak oil and gas-condensate field are correspondingly 1.2 and 1.5 times higher than they are at the Vuktyl field;

3) the gradient (ΔС₅₊/Δh) is 5 times higher at the Karachaganak field than it is at the Vuktyl field.

Insignificant, an approximately 20% increase of the C₅₊ content with the actually examined thickness of the productive horizon of the Vuktyl oil and gas-condensate field being 800m, indicates that the gravity factor does not actually prevail in the process of the occurrence formation. Hence, the techniques for calculating the C₅₊ HC content across the depth of an occurrence, taking into account the gravity constituent [2-6], (and the findings of the calculation are sufficient to adequately describe the C₅₊ HC distribution across the depth of the occurrence at Karachaganak oil and gas-condensate field), may not always be applied with regard to other fields.

References

1. V.I. Lapshin. Formation, composition and component yield of formation fluid systems in deep-seated carbonate deposits, survey info / V.I. Lapshin, V.A.Nikolayev, D.V. Izyumchenko and others. – M., Gasprom VNIIGAS, 2010. – 118 pp.

2. O.Y. Batalin Predicting formation fluid mix and pressure across the depth of an occurrence / O.Y. Batalin, A.I. Brusilovsky, N.G. Vafina and others // “Neftepromyslovoye delo I transport nefti” (Oil Field Business and Oil Transportation) – 1984. -# 10, pp.9-11.

3. O.Y. Batalin Phase Equilibrium of Multi-component Fluid Mixtures in Gravity Field / О. Y. Batalin, S.L. Kritskaya, N.G. Vafina // Gubkin High School. – 1985. – # 192. – pp.
96-101.

4. O.Y. Batalin. Phase equilibrium in the systems of natural hydrocarbons / O.Y. Batalin, А.I. Brusilovsky, M.Y. Zakharov. – М. Nedra, 1992. – 224 pp.

5. O.Y. Batalin The Experience of Research on the Thermodynamic Properties of Multi-component Fluid Mixtures: Survey info / O.Y. Batalin, S.L. Kritskaya // Development And Exploitation Of Gas And Gas-Condensate Fields. – М. VNIIEgasprom, 1987. -# 12. – 50 pp.

6. R.M. Ter-Sarkisov. Scientific Basics of the Efficiency Enhancement in Development of Gas-Condensate Fields / R.M. Ter-Sarkisov, V.G. Podyuk, V.A. Nikolayev. – М. Nedra, 1998. – 344 pp.

Authors:

V.I. Lapshin¹, A.G.Posevich¹, A.A. Konstantinov¹, A.N.Volkov²

¹ Gazprom VNIIGAZ LLC, Proektiruemyj proezd 5537, 15, 1, Razvilka, s.p. Razvilkovskoe, Leninsky dist., Moscow region, 142717, Russia

² Branch of Gazprom VNIIGAZ in Ukhta, Sevastopolskaya str., 1A, Ukhta, Komi Republic, 169300, Russia