Introduction

Bottom-water reservoirs are widely distributed in domestic oil and gas development, characterized by an extensive, interconnected aquifer at the base of the reservoir. During the initial phase of development, the bottom water provides a stable natural drive energy, effectively maintaining formation pressure and assisting in the migration of crude oil. However, during the production process, the near-wellbore pressure drawdown can easily induce the rise of bottom water, the development of water coning, and eventually water breakthrough, which directly impacts well productivity and overall reservoir recovery performance.

1. Basic Characteristics of Bottom-Water Reservoirs

The core characteristic of a bottom-water reservoir is the presence of a large-scale, interconnected aquifer at the base of the oil zone. Under initial formation conditions, controlled by gravity segregation, the distribution of the oil and water phases is stable. The lower-density crude oil accumulates in the upper part of the reservoir, while the higher-density formation water gathers at the bottom, maintaining an overall flat and continuous water-oil contact (WOC). During the early stages of reservoir development, the bottom water relies on its elastic energy to provide the natural drive mechanism for crude oil flow. However, as production continues and bottom-hole pressure drops, the original oil-water equilibrium is disrupted. Driven by the pressure gradient, the bottom water migrates and rises toward the near-wellbore region, creating the conditions for the formation of a water cone.

2. Mechanisms of Bottom-Water Coning

Bottom-water coning is the deformation of the oil-water interface resulting from an imbalance between hydrodynamic forces and gravity in the near-wellbore region.

Once an oil well is put into production, the bottom-hole flowing pressure decreases, creating a pressure gradient directed toward the wellbore. This generates an upward hydrodynamic driving force that prompts the bottom water to migrate upward. Concurrently, the gravitational force resulting from the density difference between oil and water continuously counteracts the upward movement of the water body, attempting to maintain the stability of the water-oil contact (WOC).

Because the viscosity of formation water is much lower than that of crude oil, the mobility of the water phase is significantly higher than that of the oil phase. When the field liquid production rate is high and the production drawdown is excessively large, the upward hydrodynamic force overcomes the gravitational constraint. This causes the bottom water directly beneath the wellbore to preferentially bulge and deform upward. The originally flat oil-water interface gradually transforms into a cone-shaped water body with the perforation interval at its apex, a phenomenon known as bottom-water coning.

3. Mechanisms and Development Impacts of Bottom-Water Breakthrough

Once formed, the water cone continues to rise slowly as production proceeds. When the apex of the water cone reaches the lower perforation interval of the oil well, bottom water directly enters the wellbore through the perforations. This process is defined as bottom-water breakthrough and marks a critical turning point in the deterioration of well production performance.

Following water breakthrough, the high-mobility bottom water preferentially channels into the wellbore, causing a rapid increase in the well's water cut and a sharp decline in crude oil production. Simultaneously, as bottom water continuously channels along preferential fluid pathways, it bypasses and traps un-swept crude oil in the surrounding reservoir. This forms extensive dead oil zones, reduces the volumetric sweep efficiency, and directly lowers the ultimate crude oil recovery factor of the reservoir.

4. Conventional Water Control and Oil Stabilization Strategies

To address the challenges of bottom-water coning and water breakthrough, field operations primarily focus on regulating production modes, optimizing wellbore structures, plugging water channeling pathways, and implementing refined well completion and artificial lift control. The ultimate goal is to delay water cone development and postpone water breakthrough.

By calculating the well's critical production rate and reasonably controlling the production drawdown, near-wellbore pressure disturbances can be minimized, thereby suppressing the rapid rise of the water cone at the source. Adopting horizontal well development transforms the point-source suction of a vertical well into a uniform displacement along a extended horizontal section. This effectively reduces the local pressure drawdown and significantly slows the rate of water cone development. For wells that have already experienced water breakthrough, chemical water shutoff can be applied to plug high-permeability water channeling pathways, or mechanical zonal isolation can be implemented using packers and bridge plugs to achieve water control and oil stabilization. Simultaneously, through adaptive water control completions and variable-frequency artificial lift optimization, wellbore pressure and fluid levels can be stabilized, preventing sudden bottom-water channeling induced by high-intensity pumping.

During the development of bottom-water reservoirs, production drawdown disrupts the gravitational equilibrium at the oil-water interface, inducing the bottom water to bulge upward and form a water cone. If this water cone continues to rise and reaches the perforation interval, bottom-water breakthrough occurs. This leads to an escalating water cut and the trapping of remaining oil, ultimately reducing the reservoir's recovery factor. Fundamentally, the core objective of all water control and oil stabilization strategies is to balance the hydrodynamic and gravitational forces in the near-wellbore region, thereby delaying water cone development and postponing the onset of bottom-water breakthrough.