Gas Injection Enhanced Oil Recovery Application in a Mature Naturally-Fractured-Carbonate Reservoir
Authors: Buenaventura J. E. , Alvarez R. S. , Flores J. G. , Martinez I. E. 
Published: June 2014

Abstract
The field of interest for this study consists of a complex sequence of naturally fractured carbonate reservoirs, located at a depth of 18040 ft (5500 m), in southeastern Mexico, specifically in the Chiapas-Tabasco basin, in the vicinity of Reforma, Chiapas. This field started producing in year 1972, reaching a peak production of 130M BOPD in 1978. According to a Discrete Fractured Network (DFN) analysis, the rock partitioning consists of approximately 80% matrix, with a porosity of 4% and a permeability of 1 mD on the average, and 20% fractures, with a porosity of 0.01% and a permeability of 800 mD on the average; with areas of nearly 100% matrix rock. The reservoir pressure has declined from its original value of 6750 psi (475 Kg/cm2) in year 1972 to 3300 psi (232 Kg/cm2) to date. For the last 20 years, this field has produced below the bubble point pressure of 4523 psi (318 Kg/cm2), despite pressure maintenance efforts, consisting mainly of water injection (from 1978 to 1983), which were not successful essentially due to the compartmentalization and severe heterogeneity of the reservoirs, but leaving zones with high water saturation in the upper reservoir sections. Currently, there are 47 wells drilled in this field, consisting of 11 producers, 2 intermittent water injectors, 5 wells closed but awaiting reactivation, and 29 wells either abandoned or about to be abandoned. This field has been extensively studied recently in order to identify options to increase production and the Recovery Factor (RF) from current values of 2850 BOPD and 26.4%, respectively. The prize consists of 453 MMBO left mostly in the matrix of the reservoir rocks.
Several Enhanced Oil Recovery (EOR) processes have been studied in order to identify options to maximize the recovery of the oil left in the matrix, estimated by simulation in 345 MMBO. Given the levels of uncertainty in the process, a workflow to characterize the static properties and dynamic behavior of the reservoir was constructed. A number of sensitivity analyses were made with the purpose of identifying the most relevant variables for EOR applications and their impact in the ultimate recovery of the field. The most favorable, and technical and financially viable, process considering the availability of gas, consists in using a stream that has in excess of 40% of CO2. Due to the low reservoir pressure, generalized miscibility is unlike to occur, however, it has been find out that this gas mixture could achieve multiple contact miscibility in some areas of the reservoir, and also help in other areas by changing the flow path of the oil by contacting new oil. The results of the analysis at the pilot level showed very encouraging results in terms of incremental production and recovery, as well as drop in the production of water. It was proposed to implement a pilot project, consisting of a 5-spot inverted pattern, to assess the Volumetric Sweep and Displacement efficiencies and to confirm the findings of the study. This will be the first EOR implementation in this part of Mexico using a gas of this nature. The expertise developed in this project will be a valuable input for similar efforts in other fields in the area and elsewhere.