Relevance, Use and Abuse of Material Balance Equations - Coursework Example

Introduction Material balance equation which is abbreviated as (MBA) is considered to be important for reservoir engineers in the exploration of gas and oil. By use of MBA it is possible to have the approximate initial hydrocarbon available without using geological interpretation in addition to being able to help in verification of volume estimates. Material balance equation come in handy in the determination extent of aquifer influence, it is used to understand the drive mechanism that may be applied and may further be used in estimation of recovery factor and recoverable reserves. MBE application involves putting into consideration the time intervals in the entire time of production of the reservoir and maintains that a volume balance would always exist in the reservoir in time intervals. There are a number of methods that have been established on application of MBE to a variety of reservoirs and giving solution to the equation so as to get the initial oil-in-place (N),or initial gas-in-place (G) and ratio of the initial free gas to oil (m) present in the reservoir. The straight line method that has been made popular by Havlena and Odeh(2,3) is one of the methods which does not consider each and every time interval and the associated data as different other time intervals, it puts together all time intervals thus coming up with a solution that is satisfactory to all intervals. However, in the use of straight line method, it is necessary for there to be a separate way of m value determination. In most of the applications the m used has root in geological data on relative ratio of gas cap to oil column volumes. Having the average reservoir pressure at the time intervals accurately estimated is also considered to be another important consideration. In the standard way of doing things the average reservoir pressure is estimated from the test made on the individual wells. In what is considered to be a thick formation where permeability is high and hydrocarbons with low viscosity, the average pressures from the well tests will serve as good estimates of reservoir pressures from well’s drainage area. In cases where there are thinner formations with low permeability and the hydrocarbons being highly viscous, the reservoir pressure will show huge variations in the entire reservoir and in order for an average drainage area reservoir to be obtained, it would mean longer testing hours with the resulting values will often not be accurate. Accuracy in the determination of the average reservoir pressure determines if the reserve estimates resulting from the MBE will be accurate. MBE may be analyzed dynamically through the introduction of a time variable as a MBE derivative. The solution of a combination of original MBE and associated time derivatives makes it possible to have simultaneous solution of initial oil-in-place (N), the ratio of initial gas to oil (m), permeability of reservoir (K) and average decline history of pressure just from cumulative production history and PVT data with limited pressure data or even without. There are three major approaches that MBE has conventionally applied. In the first approach MBE is applied individually at successive time intervals with the intervals being expected to present a substantial decline in pressure in the reservoir and the second case involve application of MBE at the time interval by use of XY plot that was made popular by Havlena and Odeh. In the two approaches calculation of N and m is accomplished by utilization of observed pressure and production in MBE and aquifer model. It is necessary to have a means in which m and aquifer model can be estimated is necessary, alternatively iteration may be used in the estimation of m and properties of aquifer. The inherent problem with the methods is non applicability in situations where available data for the field is inadequate with production rates that are erratic. Advancement in computer technology made a third method to be popular where calculation of average reservoir pressures was achieved through aquifer model, observed production together with the assumed values of N and m in the MBE (Canadian Institute of Mining, Metallurgy and Petroleum. 1994). The challenge associated with the use of this method is that the estimation of initial hydrocarbon in place by geological interpretation is required but there is difficulty in confirming the link between the volume to production. Through introduction of a time variable in the static tank model, it is possible to simultaneously determine the initial oil-in-place (N) or initial gas-in-place (G), ratio of initial gas to oil (m), and average pressure history of reservoir from the production history data and PVT data only. Material balance techniques give another alternative which is independent method that is used in the estimation of the original hydrocarbons in place thus acting as a supplement to direct volumetric calculations where production is sufficient. Material balance that has pool’s history may be helpful in identification of drive mechanism and the expected recovery factor range, owing to the fact that different drive mechanism has a tendency of displaying different pressure behaviours in situations where we have same cumulative production. Material balance equation find common application in finding solution to reservoir development questions in addition to being useful in reservoir geometry interpretation (Otis, 1997).. Through geological and geophysical mapping a picture of pool’s shape and orientation is given but in typical cases the in-place volume confidence will not be high not unless there is abundance of the well or seismic control. On the other hand material balance has been found to reveal a lot concerning reservoir volume but with little highlight on the shape or orientation. With the two working in conjunction there is great improvement on the understanding and making of interpretation on pool’s parameters. There is great acceptance of material balance as being the most accurate procedure in the estimation of original gas in place as it involves the actual reservoir data. The general guide is that there should be production of minimum range of 10-20% of the in-place volume so as to be able to have sufficient data to use in establishing a trend and be able to have a reliable extrapolation to the original in-place volume by material balance (Bachu, S. ,2008a.). This makes material balance to be directly useful in development of a geologist who is keen on identifying infill and step-out drilling areas that can help in optimization of pool depletion. From the explorer stand point material balance would likely be used in the description of manner of production similar to producing pools. Through material balance procedure it is possible to fully describe expansion of water, oil, gas and rock over time during the pool production process (Hassanzadeh, H.et al , 2008). Removal of a fluid from a reservoir more often results to pressure reduction with the fluid that is left behind expanding to fill the original space. Waterflooding or gas storage are injection situations whose handling involve treatment of the injection volumes as negative production. The material balance equation is considered to be an inventory of the mass of all the materials that goes in, those going out and the material that will remain in the reservoir where for convenience purpose, the mass balance expression is given in terms of reservoir voidage (Dean, L. ,2008).. References Bachu, S. (2008a.) CO2, Storage and Geological Media: Role, means, status, and barriers to deployment. Progress in Energy and Combustion Science – An International Review Journal, v. 34, p. 254-273. Canadian Institute of Mining, Metallurgy and Petroleum. (1994). Determination of Oil and Gas Reserves, Petroleum Society Monograph Number 1, Chapter 6. Canadian Institute of Mining, Metallurgy and Petroleum. (2004). Determination of Oil and Gas Reserves, Petroleum Society Monograph Number 1, Chapter 6. Dean, L. (2008). Reservoir Engineering For Geologists, Part 3: Volumetric Estimation. Canadian Society of Petroleum Geologists Reservoir, v. 34, no. 11, p. 20-23. Hassanzadeh, H.et al ( 2008). Accelerating CO2 Dissolution in Saline Aquifers for Geological Storage – Mechanistic and Sensitivity Studies. Paper submitted (June 2008) to Journal of Petroleum Science and Engineering. Mireault, R. and Dean, L. (2008). Reservoir Engineering For Geologists, Part 8b: Monte Carlo Simulation / Risk Assessment. Canadian Society of Petroleum Geologists Reservoir, v. 35, no. 7, p. 14-19. Otis, Robert M. and Schneidermann, N. (1997). A Process for Evaluating Exploration Prospects. AAPG Bulletin, vol. 81 no. 7. p. 1087-1109 Zuber, M.D. 1996. A Guide to Coalbed Methane: Basic Reservoir Engineering for Coal. Gas Research Institute Chicago, Illinois, GRI Reference No. GRI-94/0397, Chapter 3. Read More