A STUDY OF COMMONLY USED CONVENTIONAL METHODS FOR GAS RESERVE ESTIMATION

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1 A STUDY OF COMMONLY USED CONVENTIONAL METHODS FOR GAS RESERVE ESTIMATION Mohammad Shahedul Hossain and Mohammad Asad Ibne Hossain* Deartment of Petroleum & Mining Engineering, Shahjalal University of Science and Technology, Sylhet-3114, Bangladesh Engr. Salma Akhter Deartment of Chemical Engineering and Polymer Science, Shahjalal University of Science and Technology, Sylhet 3114, Bangladesh Reserve estimation is very crucial in order to gain the confidence on rofitable extraction of gas from a gas r available in the course of the develoment of a gas field. Volumetric, Material Balance and Traditional Decline curve analysis are commonly used methods to estimate the reservoirs. After discovery, during initial delineation, volumetric method is used. When adequate roduction data are available, material balance is used to estimate the reserve. As deletion roceeds decline curve analysis is used to determine the remnant amount of reserve. However, modern estimation ractice also uses tye curves such as Blasingame, Fetkovich, Agarwal-Gardner as imortant tool to get a lucid idea about the reserve. They rovide a more clear signal from the roduction data and quite handy to use and understand. Keywords: reserve definition, volumetric method, material balance method, decline curve analysis, tye curve. 1. INTRODUCTION Natural gas is the naturally occurring hydrocarbon gas redominantly comosed of methane. It is found in underground tras formed by structural and stratigrahic features called as reservoir. The rosect of accumulation of natural gas deends on coexistence of several factors in the subsurface which includes a) source rock, b) reservoir rock, c) tra, d) seal or ca rock etc. Once a reservoir is discovered, it is necessary to estimate the reserve of it. Reserve is the quantity of gas that is recoverable from the underground gas deosits. In many laces natural gas and crude oil are found coexisting because their origin and accumulation rocesses are similar in many accounts. Ws shall discuss about the estimation rocess of dry gas reservoir as they are common in Bangladesh. In 1986, the Society of Petroleum Engineers (SPE) adoted the following definition for reserves : be commercially recoverable and marketable from a given date forward, under existing economic conditions, by established oerating ractice, and under current government The rocess of estimating oil and gas reserves for a roducing field continues throughout the life of the field. There is always uncertainty in making such estimates. The level of uncertainty is affected by the following factors: a) Reservoir tye b) Source of reservoir energy, c) Quantity and quality of the geological, engineering, and geohysical data, d) Assumtions adoted when making the estimate, e) Available technology, and f) Exerience and knowledge of the evaluator. Due to the degree of uncertainty is inherent in condensate, natural gas, natural gas liquids and associated marketable substances anticiated to different categories on the basis of uncertainty such as: Page 54 / 79

2 a) Proved Reserve: u to 10% uncertainty b) Probable Reserve: 10 to 50% uncertainty. c) Possible Reserve: uncertainty level is as high as 90% Volume: The reservoir volume is obtained from geologic and fluid ressure analysis data. The geologist rovides contour mas of the to and base of the reservoir, as shown in Figure: CONVENTIONAL METHODS Different methods can be used to estimate the reserve deending on the maturity of the reservoir. The commonly used methods are: a) Volumetric Method: at early stages. b) Material Balance Method: when roduction data is available. c) Decline Curve Analysis: when the reservoir reaches decline mode. 2.1 Volumetric Method In the early stages of develoment, reserves estimates are restricted to the volumetric calculations. The volumetric method entails determining the hysical size of the reservoir, the ore volume within the rock matrix, and the fluid content within the void sace. This rovides an estimate of the hydrocarbons-in-lace, from which ultimate recovery can be estimated by using an aroriate recovery factor. For a gas reservoir: GIIP 7758 V B gi 1 S R F R Wi. Figure 1: Contour Ma Such mas have contour lines drawn for every 20 feet, or so, the elevation can be determined by determining the gas water contact (GOC) Porosity, : Effective orosity of a samle is defined as the following ratio: Effective Porosity Interconnected Bulk ore volume volume Effective orosity of the reservoir rock can be determined from study of core analysis. The orosity of clean sandstones may be calculated from electric logs Interstitial water saturation, S Wi : Water saturation is defined as the following ratio: Water saturation Volume of water Volume of resent in total ore ore saces saces GIIP = Gas initially in lace (SCF) V R S Wi = Reservoir volume (acre-ft) = Porosity (fraction) = Interstitial water saturation (fraction) B gi = Initial gas formation volume factor (res bbl/scf) Interstitial or connate water saturation may be determined from electric logging information or by laboratory determinations run on cores by restored state, evaoration mercury injection, or centrifuge methods Formation volume factor,b g,: Gas formation volume factor is the ratio of volume of gas at the reservoir condition to that of gas at the standard condition. Mathematically: R.F = Recovery factor (fraction) Page 55 / 79

3 B gi V reservoir V base condition condition b Ti i Z T b i Z b c S c w wi f i G Bg Bgi We GBg Swi W B w B gi = Initial gas formation volume factor (res bbl/scf) b = Base ressure (sia) i = Initial reservoir ressure (sia) T b = Temerature at base conditions ( 0 R) T i = Temerature at initial conditions ( 0 R) Z b = Gas deviation factor at base conditions Z i = Gas deviation factor at initial reservoir conditions Recovery factor ( R.F) : Recovery factor is a number between zero and unity reresenting the fraction of recoverable oil. It deends on the following factors: a) Current economic circumstances. b) Environmental and ecological considerations. c) Governmental regulations as well as olitics. d) Physics of the reservoir-fluid system. Finally, GIIP can be obtained by using all the arameters in the initial equation. This equation can be reduced to a simlified form based on a few assumtions Without Water Influx : If there is no external energy from other sources, such as aquifer is alied, the reservoir is considered as comletely enclosed. If W= 0; W e = 0 and HCPV=constant, then the equation (3) reduces to: Z i Z i G i Z G This equation is alicable for: 1. Constant reservoir temerature 2. No hase change in the reservoir 3. No water influx 4. No rock comaction 5. No connate water exansion. From equation (4) a lot of /Z vs. G found. i can be Although volumetric method is the only method available at the early stage of the reservoir, the result might be harmed by erroneous determination of the reservoir area. Reservoir boundary is subject to large errors. Again, this method cannot rovide the EUR (Exected Ultimate Recovery). 2.2 Material Balance Method Material balance methods rovide a simle, but effective alternative to volumetric methods for estimating not only original gas in lace, but also gas reserve at any stage of reservoir deletion. Material balance equation is simly a statement of conservation of mass, or (Original hydrocarbon mass) (Produced hydrocarbon mass)=(remaining hydrocarbon mass) The general material balance equation reduces to the conventional gas material balance equation as : With water influx Figure 2: /z vs G curve If there is a water drive in the reservoir, then the ore volume is reduced by the amount equal to the volume of encroaching water. Now, considering water influx, equation (3) becomes: GB gi ( G G) Bg We WBw..(5) Now, using roduction data we can use equation (4) or (5) to determine the amount of gas remaining (G- G ) in the reservoir. Page 56 / 79

4 Material balance is a good tool to measure the reserve. But, for material balance it is needed to shut down the reservoir to determine the arameters. Thus, the data is as available as roduction data and the well manager is unwilling do it frequently as it not economically feasible. 3. Harmonic decline. 2.3 Decline Curve Analysis The roduction rate of wells, or grous of wells, generally declines with time. An emirical formula can sometimes be found that fits the observed data so well that it seems rather safe to use the formula to estimate future relationshis. In most case the roduction will decline at a decreasing rate, that is, dq/dt will decrease with time. The figure below shows an identical curve. The t = 0 oint can be chosen arbitrarily. q is the gas roduction rate and t is time. The area under the curve between the times t 1 and t 2 is a is a measure of the cumulative roduction during this time eriod since G t 2 t1 q dt Figure 4 Exonential, Hyerbolic and Harmonic Decline Curve Exonential Decline: data lots as a straight line on a semilog aer. The equation of the straight line on the semilog aer is given by: q=q i e -Dt (7) D = nominal exonential decline rate, 1/day t = time, day Hyerbolic Decline: y lotted on a semilog aer concaves uward, then it is modeled with a hyerbolic decline. The equation of the hyerbolic decline is given by: q=q i (1+bD i t) -1/b Figure 3: roduction rate vs time There are three commonly recognized tyes of decline curves. Each of these has a searate mathematical form that is related to the second factor, which characterizes a decline curve, that is, the curvature. These tyes are referred to as: 1. Constant-ercentage or exonential decline. 2. Hyerbolic decline. Di = initial nominal exonential decline rate (t = 0), 1/day b = hyerbolic exonent t = time, day Harmonic Decline : A secial case of the hyerbolic decline is harmonic decline to be equal to 1. Page 57 / 79

5 q=q i (1+D i t) -1 (9) qi Di = initial nominal exonential decline rate (t = 0), 1/day b = hyerbolic exonent=1 t = time, day Decline curve relationshis are emirical, and rely on uniform, lengthy roduction eriods. Decline curves are the most common means of forecasting roduction. They have many advantages such as: a) Data is easy to obtain, b) They are easy to lot, c) They yield results on a time basis, and d) They are easy to analyze In decline curve analysis, usually the flowing ressure is assumed to be constant as the roduction rate declines. But in the real conditions, wellhead ressure also varies with time. So, the reserve estimated by decline curve analysis often varies from actual reserve. 3. MODERN TYPE CURVE METHODS: Tye curve matching is essentially a grahical technique for visual matching of roduction data using relotted curves on a log-log aer. From matching of field data with re lotted data, dimensionless reservoir radius is determined, which is then used to calculate the reserve. Tye curves are also used as a valuable diagnostic tool. It rovides a ractical interretation of data to qualitative conclusion about well roduction. For examle we can exlore the following samle tye curve. Figure 5: Tye Curve as a Diagonstic Tool It is evident that at the early stage the roduction data do not match with the tye curve and the trend indicates a damaged reservoir. Then the decline indicates the boundary effect. At the late stage the raise of the curve indicates the resence of ressure suort. The suort might be from water drive, artially sealing fault, multi layer effect, communicating gas reservoirs etc. If the reservoir resonse line was lower than re lotted decline line, it would have been an indication of liquid loading effect. Modern reserve estimation ractice is comlemented by various tye curves. Several tye curves for the same reservoir are used to confirm the reserve. Tye curve analysis are done from roduction data (flowing ressure, flow rate) which are available in the roducing reservoirs. Three major Tye Curve Analysis methods are described as below: 3.1 Fetkovich Analysis Fetkovich was the first to extend the concet of using tye curves to transient roduction. The Fetkovich methodology uses the deletion for the analysis of boundary-dominated flow and constant ressure tyecurves (originally develoed by VanEverdingen and Hurst) for transient roduction. The most valuable feature of tyecurves lies not in the analysis, but in the diagnostics. There are two sets of curves that converge in the centre. Matching data on the left side rovides information about the transient behaviour of the system (k, s) while the right side gives Page 58 / 79

6 information about the boundary dominated behavior of the reservoir (OGIP, Area). The left-hand side of the Fetkovich tye curves ais derived from the analytical solution to the flow of a well in the centre of a finite circular reservoir roducing at a constant wellbore flowing ressure. Fetkovich showed that, for all sizes of reservoirs, when transient flow ended, the boundary-dominated flow could be reresented by an exonential decline. one deletion stem for tyecurve matching; the harmonic stem. One imortant advantage of this method is the tyecurves used for matching are identical to those used for Fetkovich decline analysis, without the emirical deletion stems. When the tyecurves are lotted using analytical exonential stem of the Fetkovich tyecurve becomes harmonic. The significance of this may not be readily evident until considering that, if the inverse of the flowing ressure is lotted against time, seudo-steady state deletion at a constant flow rate follows a harmonic decline. tyecurves allow deletion at a constant ressure to aear as if it were deletion at a constant flow rate. In fact, Blasingame et. al. have shown that boundary-dominated flow with both declining rates and ressures aear as seudosteady state deletion at a constant rate, rovided the rate and ressure decline monotonically. Figure 6: Samle Fetkovich Tye Curve be matched with the curve roerly, because Fetkovich analysis assumes that roduction occurs under constant bottomhole flowing ressure conditions.this is the major source of error for Fetkovich analysis. Figure 7: Blasingame Tye Curve 3.2 Blasingame Analysis The roduction decline analysis techniques of Fetkovich is limited in that it does not account for variations in bottomhole flowing ressure in the transient regime, and only account for such variations emirically during boundary dominated flow. In addition, changing PVT roerties with reservoir ressure are not considered for gas wells. Blasingame tyecurve uses a form of suerosition time function that only requires style of roduction decline analysis are further enhanced by the introduction of two additional tyecurves which are lotted concurrently with obtaining a more unique match. The Flowing Material Balance method rovides convenient and sensitive way to estimate Gas in lace without shut-in ressure excet for Pi. This method has better resolution for boundary Page 59 / 79

7 dominated flow than any of the roduction analysis methods. A Blasingame tye-curve lot can be dislayed reresenting the resonse of a vertical well in a closed circle, with the data overlaid. This lot is also resent for gas where material balance time is relaced by a material balance seudo time. The loglog lot can be used as a diagnostic tool with excetionally clean data although even with scattered data, trends may be detected. The simulated model can be comared to the data on both of these lots. The Agarwal-Gardner Rate-Time analysis lot includes dimensionless tyecurves, based on the constant rate solution. Unlike Blasingame, these tyecurves are grahed using the welltest dimensionless variables q D and t DA. Agarwal and Gardner also include tyecurves for the rimary ressure derivative and semi-logarithmic (welltest) derivative (lotted as an inverse). The result is a diagnostic analysis that clearly shows the transition oint from transient to boundary dominated flow. 3.3 Agarwal-Gardner Rate-Time Analysis: Agarwal and Gardner have comiled and resented new decline tyecurves for analyzing roduction data. Their methods build uon the work of both Fetkovich and Blasingame, utilizing the concets of the equivalence between constant rate and constant ressure solutions. Agarwal and Gardner resent new tyecurves with dimensionless variables based on the conventional welltest definitions, as oosed to the Fetkovich dimensionless definitions used by Blasingame et. al.. They also include rimary and semi-log ressure derivative lots (in inverse format for decline analysis). Furthermore, they resent their decline curves in additional formats to the standard normalized rate vs. time lot. These include the rate vs. cumulative and cumulative vs. time analysis lots. 4. CONCLUSION Reserve estimation is very crucial for a reserve to make decisions. Estimation rovides the confidence about economical feasibility to roduce from a reservoir; whether it is cost worthy or not. In this aer a few conventional and tye curve methods are discussed. Other methods like analogy, reservoir simulation are also used to estimate the reserve. Unfortunately none of them is foolroof. It is thus recommended to use several methods at a time with consideration of their merits and drawbacks. The final reserve should be declared by averaging the valid outut of those methods. It should be noted that the total amount reserve may vary with resect to time due to accumulation or migration of gas. So reserve estimation should be done in a regular basis to have clear idea on current reserve. REFERENCES Figure 8: Agarwal Gardner Rate vs Time Tyecurve Analysis 1. Lee, J. and Wattenbargar, R.A.(1996), Gas Reserve Estimation, SPE Textbook Series, Volume 5, Imam, B.(2005), Energy Resources of Bangladesh, University Grants Commission of Bangladesh, Dake, L.P.(1991), Fundamentals of reservoir engineering(twelfth edition), Elsevier,,Amsterdam. 4. Craft, B.C. and Hawkins, M.F.(1991),Alied Petroleum Reservoir Engineering (second Edition),Prentice Hall, N.J 5. Selley, R.C., Elements of Petroleum Geology(second edition), Academic Press, San Diego Page 60 / 79

8 6. William, D. and Mccain, Jr., Petroleum Fluids(second Edition), Pennwell books, Tulsa,Oklahoma 7. Gaskari, R., Mohaghegh, S. D. and Jalali, J., New Method for Production Data Analysis to Identify New Oortunities in Mature Fields: Methodology and Alication. 8. Blasingame, T.A; McCray, T.L; Lee, W.J: "Decline Curve Analysis for Variable Pressure Dro/Variable Flowrate Systems," aer SPE resented at the SPE Gas Technology Symosium, January, Agarwal, R.G; Gardner, D.C; Kleinsteiber, S.W; and Fussell, D.D.: "Analyzing Well Production Data Using Combined Tye Curve and Decline Curve Analysis Concets," aer SPE resented at the 1998 SPE Annual Technical Conference and Exhibition, New Orleans, Setember. 10. Fetkovich, M.J.: "Decline Curve using Tye Curves," JPT (June 1980), Fetkovich, M.J; Fetkovich, E.J; and Fetkovich, M.D.: "Useful Concets for Decline Curve Forecasting, Reserve Estimation, and Analysis," SPERE (February 1996), 13. Page 61 / 79