Adjustment of Differential Liberation Data to Separator Conditions

Abstract

Abstract The actual reservoir process is neither flash nor differential. Thus, regardless of the testing procedure, some adjustment needs to be made to the resultant data to approximate the fluid behavior in the oil production process. The conventional method of adjustment extrapolates to negative values of solution gas-oil ratio and formation volume factor leads to values less than one at low pressure. Both extrapolations do not conform to the physical behavior. This is due to the fact that the conventional approach does not observe that the oil relative density at reservoir conditions is the same regardless of the process. This paper presents a new approach to the adjustment of differential liberation data to separator conditions.This approach is based on the fact that both flash and differential data should give the same value for the oil relative density at the reservoir conditions. This is achieved by correcting all the properties, i.e. solution gas-oil ratio, formation volume factor, gas relative density and oil relative density. The new method overcomes the disadvantages and limitation of the conventional approach. This method is tested on 400 PVT files from all over the world and the result is consistent with physical behavior. A sample calculation is presented to outline the new method. Introduction The differential solution gas-oil ratio is not the same as the flash solution gas-oil ratio as shown in Fig. 1.Similarly, the differential and flash oil formation volume factors are not the same as depicted in Fig. 2. Thus, regardless of the testing procedures - flash or differential, some correction needs to be made on the resultant data to approximate the fluid behavior in the oil production process. The oil that leaves the reservoir is flashed to the separator, necessitating that the solution gas oil ratio and the formation volume factor should be determined by a flash process. In order to calculate the combination fluid properties from standard data analysis, several assumptions were stipulated, but these assumptions limit the range of application. This paper describes a new method to adjust the differential liberation data to separator conditions. This method overcomes the disadvantages and limitations of the current method and come up with a correction procedure that results in a consistent physical trend. Reservoir Process In differential liberation process, gas is removed from oil as it is released from solution. While, in flash liberation process, gas remains in contact with oil. Generally, petroleum engineers consider that the liberation process in the reservoir more closely approaches a differential process.The fluid produced from the reservoir to the surface is considered to undergo a flash process. The actual liberation process in the reservoir is neither flash nor differential. In certain localities, the process is flash, and in others, the process is differential. In some other localities the process does not match either of them. A combination test proposed by Dodson2 is the closest to the reservoir process. At each step of differential liberation test, a sample is taken and flash liberated to obtain Rs, ?o, Bo, and ?g. Here it can be seen that all properties including the ?api are different at different pressures. Although this combination test or composite liberation is an improvement and closest to reservoir behavior, it does not match the actual reservoir behavior. The appendix to reference 3 explains the differential and flash processes, and their combination. From the combination test, it is justified to correct all the properties obtained by differential liberation test to flash liberation including ?g and ?o.