1.1 Introduction
Coning is the mechanism describing the movement of water/gas into the perforations of producing wells. For water coning the movement is upwards for the case of bottom water, side wards for edge water, but it is downwards for gas coning. The production of water from oil wells is a common occurrence which increases the cost of producing operations and may reduce the efficiency of the depletion mechanism and the recovery of reserves. The objective of this research work is to model the behaviour of this coning(mainly water coning, from bottom water) and then use it to evaluate the time it would take a cone to break into the producing well in reservoir of well-defined boundary conditions.
As a prelude to other parts of this study, this chapter will discuss the background upon which this study was initiated, the statement of problems that led to this study, the Aim and Objectives of the study. Others are Significance of the study, Scope of work, Limitations of the Study and Definition of technical terms.
1.2 Background of Study
The coning of water into production wells is caused by pressure gradients established around the wellbore by the production of fluids from the well. These pressure gradients can raise the water-oil contact near the well where the gradients are dominant. The gravity forces that arise from fluid density differences counterbalance the flowing pressure gradients and tend to keep the water out of the oil zone. Hence, at any given time, there is a balance between the gravitational and the viscous forces at any point on and away from the completion interval. The water cone formed will break eventually into the well to produce water along with the oil when the viscous forces exceed that of the gravitational forces. This basic visualization of coning can be expanded further by introduction of the concept of stable cone, unstable cone and critical production rate.
Stable Cone: If a well is produced at a constant rate and the pressure gradient in the drainage system have constant, a steady state condition is reached, if at this condition, the dynamic forces (viscous forces) at the well are less than the gravity forces then the water or gas cone that has formed will not extend to the well .Moreover, the cone will not advance nor recede, thus establishing what is known as stable cone.
Unstable Cone: Conversely, if the pressure in the system is in an unsteady-state condition, then the cone that will be formed is unstable and it will continue to advance until the steady-state condition takes over. If the flowing pressure drop is sufficient to overcome the gravity forces, the unstable cone will mushroom and ultimately break into the well. In actual sense therefore, stable cones may only be ‘pseudo-stable' because the drainage system and the pressure distribution generally change. For a example, during reservoir depletion, the water-oil contact may advance toward the completion interval, thereby increasing coning tendencies. Another one is reduction in productivity due to well damage requires a corresponding increase in the flowing pressure drop to maintain a given production rate. This increase in pressure drop may force an otherwise stable cone into the well.
Critical Production: The critical production rate is the rate beyond and above which the flowing pressure gradient at the well causes water (or gas) to cone into the well. It is therefore, the maximum rate of oil production without concurrent production of water by coning. A build-up is stable at the critical rate but is at a position of incipient breakthrough.
One assumption in critical production rate is that the cone has built-up to just before the breakthrough into the well. But, these analyses reveal nothing directly about the time it takes for the cone to build-up to this incipient breakthrough position. Thus, water-free oil can be produced from a well for a prolonged period at rates above critical rate before the well reaches the condition to which the critical applies. Theoretically, the basic coning equation for a water-oil system can be developed by applying the conservation of mass to each of the phases relating flow velocities with pressure by Darcy's law, and relating pressure across water-oil contact interfaces by capillary pressure. With the usual boundaries at the well and reservoir the solutions of the resulting equations for the time behaviour of a water- oil interface constitutes a free-surface, boundary value problem.
Study Approach:
The main study of this work is focused on engineering cone breakthrough time prediction in bottom water supported reservoirs either produced by horizontal well or vertical well with the application of Ozkan and Raghavan method (1990). This application would be compared with the chaperon (1986) model to investigate their behaviour on this study model.
1.3 Statement of Problems
Investigation reveals the following problems of the development of breakthrough time correlations for coning in bottom water supported reservoirs research work;
- Insufficient information relating to the model of water/gas behaviour in bottom water/gas supported reservoir,
- Improper simplification of analytical correlations for engineering estimates of breakthrough time to enable better future recompletion strategy or prior to detailed simulation study,
- Lack of information relating to the breakthrough times in horizontal and vertical wells.
1.4 Aim and Objectives of Study
The aim of the study is development of breakthrough time correlations for coning in bottom water supported reservoirs. In achieving this aim, the following specific objectives are set out as follows;
- To model the water/gas behaviour in bottom water/gas supported reservoir,
- To develop simplified analytical correlations for engineering estimates of breakthrough time to enable better future recompletion strategy or prior to detailed simulation study,
- To compare the breakthrough times in horizontal and vertical wells.
1.5 Scope of Study
The study focuses on the development of breakthrough time correlations for coning in bottom water supported reservoirs.
1.6 Limitations of the Study
During the course of this study, many things militated against its completion, some of which are:
- Time Constraint: The time frame given to accomplish this project was very short due to school academic calendar and it was carried out under pressure which made the researcher not to implement some necessary features.
- Establishment Policies: Establishment policies posed a serious limitation as most staffs are not ready to release information needed for this project work. There were lots of information needed from the staffs of this institution to enhance the study which took them time to release or they did not release at all for security purposes, hence the scope was reduced.
- Research material: availability of research material is a major setback to the scope of the study.
- Frequent power failure: This made the researcher append more money on fuel to ensure sustainable power.
- Financial Constraint: Insufficient fund tends to impede the efficiency of the researcher in sourcing for the relevant materials, literature or information and in the process of data collection (internet, questionnaire and interview).
1.7 Significance of Study
This study will serve as reference material for subsequent researcher in the field or related topics and will be beneficial to Petroleum Engineers.
1.8 Organization of Research Work
This research study comprises of five chapters. Chapter one introduces the problem of this study, the objectives and the approach of this study employs in tackling this problem. Chapter two is essentially, the literature reviews of related studies previously done on this problem. Chapter three describes the mechanics and the behaviours of cones. Chapter four tackles the model assumptions and mathematical formulations, and shows some field applications and the results and discussion of the study. And finally, the conclusion and recommendations are covered in chapter five.
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