DIAGNOSTIC PLOTS FOR ANALYSIS OF WATER PRODUCTION AND RESERVOIR PERFORMANCE
The research is aimed at the understanding of the various diagnostic plots for the analysis of water production that are available as well as the application of these methods in a case study. It also aimed at the establishment of a work flow for the evaluation of water production mechanisms. A workflow was developed that combines numerical simulation and diagnostic plots to analyze the water production performance in a reservoir. This workflow was validated using a case study. The multi-layer reservoir model with varying vertical permeability was constructed using a numerical simulator with the reservoir properties of the case study. Trends from the field data were analyzed using the trends observed from the simulated data as templates. For the production wells, oil rate and water rate versus time plots as well as the X-plot were used to evaluate water production characteristics of the case study. The water-oil ratio (WOR), WOR derivative and X-Plot were used for the field production diagnosis while the Hall and the Hearn Plots were used for the water injection well diagnosis. The results of the diagnostic plots showed that multi-layered channelling was the controlling mechanism and the cause of the water production in the case study. For the injection wells, the plots indicated that some wells in the case study had the problem of extensive near wellbore fracturing while other wells had the problem of wellbore plugging. The workflow and results of this study can be applied by reservoir and production engineering teams to other reservoirs to diagnose water production mechanisms; identify sources of water production, and provide information for planning water management programmes to mitigate excessive water production problems.
1.1 DESCRIPTION OF PROBLEM
Produced water is any water that is present in a reservoir with the hydrocarbon resource and is produced to the surface with the crude oil or natural gas. This water could either come from an aquifer or from injection wells in water flooding process. The production of this water alongside the oil from any reservoir is a condition that is natural in all reservoirs. It is expected that water production would increase with the life of the reservoir. However, a premature increase in the production of water in any reservoir is an undesirable condition. Excess or premature water production, exists with associated cost implication on the surface facilities, artificial lift systems, corrosion and scale problems. Another effect that ensues is a decrease in the recovery factors as oil is left behind the displacement front, thereby reducing the performance of the reservoir. All these along with the decrease in the quantity and quality of the oil imply a reduced profitability.
Globally, as at 2002, analysis showed that three barrels of water is produced to one barrel of oil and the cost of water handling ranges from 5 to 50 cents, where this cost is a function of the water cut (Bailey et al, 2000). It is therefore imperative that actions be taken to reduce this adverse effect, as this will not just lead to potential savings but its greatest values comes from potential increase in oil production and recovery. To control the produced water effectively, the source or the mechanism of the water problem must be identified. Diagnostic plots have been used successfully to identify the mechanism of water production and that is the focus of this work.
1.2 STUDY OBJECTIVES
Reservoir simulation would most likely describe a reservoir adequately but a quicker and cheaper way to analyse the performance of a reservoir is by the use of analytical and diagnostic plots, therefore, this research work is aimed at:
· Developing a work flow for the evaluation of water production mechanisms.
· Presenting the workflow by considering detailed step by step approach on how water production problems in the reservoir can be diagnosed to support water management planning for mitigation actions.
· The use of a couple of case studies to demonstrate the application of the workflow and diagnostic plots to identify water production characteristics.
· Formulating guidelines on how to mitigate water production and thereby optimizing well performance and oil recovery.