Development and Simulation of Huff-N-Puff and Single-Well Gas-Assisted Gravity Drainage Processes for the EOR Design in Offshore Oil Fields
Authors: Almansor A. M. , Al-Mudhafar W. J. , Rao D. N. 
Published: April 2024

Abstract
The Gas-Assisted Gravity Drainage (GAGD) has acquired widespread popularity as a promising technique to improve and enhance the oil recovery. A new modified version has been introduced to consider a single well of dual completion for gas injection and oil production. It is called the Single Well-GAGD process, which is particularly useful in offshore reservoirs, with the aim of substantially decreasing expenses. It involves the injection of gas through the annulus, while oil is extracted by a horizontal well. The gas injection capacity of this technique is constrained by the short perforation intervals, which are insufficient for sustaining long horizontal perforations. Therefore, this study proposes an updated approach to enhance the capacity for injecting gas and improve the efficiency of recovering oil. The upgraded method integrates the huff-and-puff technique, which involves cyclic injection and production on single horizontal tubing. This technology is referred to as the Huff and Puff Gas-Assisted Gravity Drainage (HnP-GAGD) process. In the HnP-GAGD process, gas is injected into the reservoir via horizontal tubes over a period of several weeks. Following a period of soaking, the well is accessed to initiate production, a process that can span many weeks or even months. During the HnP-GAGD process, the injection of gas into the reservoir leads to gravity segregation, causing the gas to go towards the highest zone and form a gas cap that helps maintain reservoir pressure. The oil subsequently moves downward to lower horizontal perforations to facilitate oil production. A reservoir flow simulation model was developed to simulate the injection of CO2 for the purpose of enhancing oil recovery and demonstrating the superior effectiveness of the HnP-GAGD method compared to the conventional approach. A heterogeneous clastic reservoir in southern Iraq's South Rumaila oil field was used for the compositional flow simulation. The prediction was based on a comparison of cumulative oil production, average reservoir pressure, gas-oil-ratio, and water cut over a period of 15 years. By the end of the prediction period, HnP-GAGD exhibited a slightly superior oil recovery compared to SW-GAGD process with a slightly lower than the GAGD process. Specifically, the HnP-GAGD process yields a total of 939.719 MMSTB of oil; the GAGD and SW-GAGD methods yield 958.89 MMSTB and 923.659 MMSTB, respectively. It should be noted that the HnP-GAGD process has a shorter period of gas injection than the continuous phase in the GAGD process. In addition, the injected gas through the SW-GAGD process has a limited impact on improving oil production due to the far location of the perforations for gas injection. The HnP-GAGD technique exhibits a low gas-oil ratio (GOR), which suggests a reduced occurrence of horizontal tubing gas breakthrough compared to alternative root procedures. The presence of an active water aquifer at the bottom of the reservoir resulted in a consistent water cut in the three enhanced oil recovery (EOR) methods. This was due to the rapid and continuous rise in oil production, particularly at the beginning of the prediction period. Artificial lift systems (ESPs) can be used to eliminate water cut levels, sustain production rates, and enhance oil recovery. This research presents the HnP-GAGD technique as a means to enhance injectivity and reduce water cut through gravity segregation. The modified procedure has the potential to be effective in offshore and low-thickness reservoirs.