Rate Transient Analysis and Flowing Material Balance for Oil & Gas Reservoirs

Estimation of initial hydrocarbon in place is a critical step for any investment in the oil and gas industry, based on which revenues and further developments are planned and designed. The conventional ways of estimating the hydrocarbon initially in place are: Material Balance Equation, Volumetric Methods, and Numerical Simulation Models. These approaches could require a considerable time of production and some tests to collect data about the average reservoir pressure profile in time and apply the conventional Material Balance Equation or to build the numerical simulation model. However, for some economic reasons, we require to have a first indication about the reserves within the first year of production to evaluate the development strategies for the reservoir; considering the cost of well testing, the time down when the well being shut in, the loss of production, the time needed to stabilize the pressure in the reservoir, the conventional material balance method becomes ineffective. In other cases, it becomes even difficult to perform well testing due to the low permeability, high viscosity liquids, or the large extension of the reservoir. These challenges encouraged reservoir engineers to extend Pressure Transient Analysis (Well testing) into what is called Production Analysis (PA) or Rate Transient Analysis (RTA) where a set of production data interpretation to obtain reasonable results about reservoir characteristics, completion effectiveness and hydrocarbon initially in place. What signifies RTA is that this approach aims to analyze rate as well as pressure. However, for pressure, testes are not performed but the usage of permanent downhole gauges (PDG) helped to have a continuous record of pressure in time. The high cost of using (PDG) in each well led to converting well head pressure into bottom hole pressure using VLPs for pressure surveillance. Production Analysis was first introduced by Arps as Decline Curve Analysis (DCA) to empirically estimate the ultimate production recovery. However, Arps Type-curves are only applicable after the transient period i.e. in pseudo steady state conditions when the bottom hole pressure is fairly constant. Later in 1980, Fetkovich introduced a type-curve combining the Arps decline curve with the fluid flow behavior in a closed reservoir to provide a technique valid for both the two periods, transient and boundary dominated flow periods. However, this is still only applicable under the condition of constant bottom hole flowing pressure. This was the limitation until the introduction of the material balance time by Blasingame and McCary to transform the variable rate/variable pressure solution into an equivalent constant pressure or constant rate solution. Flowing Material Balance (FMB) was introduced as a recent approach of Production Analysis in which production data (flow rate and bottom hole pressure) is used through an iterative approach to estimate the initial hydrocarbon in place. Through the development of these different approaches, reservoir engineers succeed to exploit the production data in an easier way to overcome the complexities of the conventional methods. The study provides a validation on the production data analysis approaches used to estimate the initial hydrocarbon in place (RTA & FMB) by developing synthetic data on 4 different cases (Dry gas & Dead oil, Single & Multi-well Models) and comparing the results obtained with the numerical simulation models values. The study demonstrates the importance of PVT in Production analysis by importing PVT data represents our models from the PVT tables in Production Analysis software programs to be implemented in the FMB approaches and see how this will affect the results obtained. Results obtained showed approximate values of Hydrocarbon initially in place compared with the numerical simulation model.