Abstract
The objective of this study is to understand the influence of the regional aquifer and surrounding fields over two clusters of giant oil, fractured carbonate reservoirs in the Mexican Gulf of Mexico (GoM). These reservoirs are referred to as Cluster-A and Cluster-B, and our goal is to predict the impact and influence of gas injection in Cluster-A and waterflooding in both clusters.
Field-B1 pressure is declining and if it follows with the current trend, there will be difficulties to continue production from the existing wells, which mainly use electrical submersible pumps, since the pumps would operate outside their pressure range. For this reason it is was imperative to consider the implementation of a pressure maintenance program that would enable to continue producing within operative ranges of the artificial lift system.
To implement a secondary recovery project of the large magnitude required, the different regional uncertainties must be reduced as much as possible. Since Field-B1 is hydraulically connected through a regional aquifer with other fields under operation, the following were considered:
Quantify the energy of the regional aquifer
Evaluate the dynamic behavior of the regional aquifer as it is influencing all fields simultaneously
Understand the historical pressure behavior of Field-B1 (which is to be waterflooded) to determine the influence of the surrounding fields
To fulfill these objectives, a simulation model including Cluster-A, Cluster-B, and surrounding fields was built.
The model encompasses a total area of 2,000 km2. The aquifer was modeled explicitly using the simulation grid cells using a border region with pore volume and transmissibility multipliers to represent a large area to the south.
The influences of production and water injection of surrounding fields (not explicitly modeled in this study) were included by representing these fields as pseudowells.
To speed up the analysis and understanding of drive mechanisms and the impact of sensitivity variables to the history match, a coarse model was built and used to perform a large number of sensitivity runs. Findings and lessons learned from the coarse model were applied to the fine-grid model.
The fine-grid model was history matched. As a result, the size and influence of the regional aquifer and connectivity of large limit faults located throughout the fields and aquifer was determined. By analyzing the time of flight along streamlines, the fluid transfer between fields and aquifer and relationship with events occurring in the fields was understood.
The model was subsequently used to predict primary recovery, waterflooding, and gas-injection scenarios. We focused on the impact on pressure and production of Cluster-B, which is to be developed. In the scenarios, water is injected in Cluster-B, while nitrogen and hydrocarbon sour gas is injected in Cluster-A. The gas-injection rates are set to honor different voidage replacement ratios.
As a result of the study, the aquifer size and characteristics were better understood, along with past and future interactions among fields.
The most important conclusion is that the aquifer water drive in combination with the pressure maintenance program underway in Cluster-A through nitrogen injection of 100% voidage replacement ratio will not suffice to maintain the Cluster-B pressure above the necessary levels. It is imperative to implement an additional secondary recovery process to provide pressure support to Cluster-B, and thus ensure a stable operating range for the electrical submersible pumps installed in Field-B1.
