DEVELOPMENT OF BREAKTHROUGH TIME CORRELATIONS FOR CONING IN BOTTOM WATER SUPPORTED RESERVOIRS
ABSTRACT
This research work mainly investigates the development and the behavior of cones (both water and gas cones) in oil reservoirs supported by strong aquifer, and from which analytical correlations are developed for quick engineering estimates of the time for water/gas cones to break into the perforations of the producing wells. The studies treated the cone development and breakthrough times in both horizontal and vertical well producing reservoirs and made analysis on them. The Ozkan and Raghavan (1990) method was employed as the base approach in the modeling of the cones; as well as their breakthrough times and then compare with that of Chaperon’s approach(1986) with both the horizontal and vertical well applied. The developed models were then run on field data, the results were graphically represented in both the horizontal and vertical well cases. Analytical correlations were then developed from the results obtained for breakthrough time estimation and compared with literature on example case. This work actually employs the dimensionless (or the normalized approach) system to curtail the units complexities and represent the results in a more generalized form. These analytical correlations can be leveraged on to plan better future recompletion strategy as they provide an engineering estimate of when water breaks into the production wells.
CHAPTER ONE
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.
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.