Introduction

In reservoir engineering, bubble point pressure (Pb) is a highly critical thermodynamic parameter. During the initial phase of oilfield development, formation pressure generally exceeds the bubble point; natural gas remains dissolved within the crude oil, and the fluid exists in a single-phase liquid state. However, with continuous production and the depletion of reservoir energy, the pressure inevitably declines. Once the formation pressure drops below the bubble point, gas begins to break out of the crude oil in large volumes, officially transitioning the fluid in both the formation and the wellbore from a single-phase liquid to a gas-liquid two-phase flow. This phase transition indicates that the reservoir has entered the solution gas drive stage, necessitating corresponding adjustments to the existing production engineering strategies.

1. Cascading Changes in Fluid Phase Behavior and Properties

Once the pressure drops below the bubble point, a series of cascading changes occur in the fluid properties, primarily manifested as the deterioration of the wellbore flow pattern. The massive breakout of free gas causes a sharp increase in the gas-liquid ratio (GLR). During the artificial lift process, the crude oil flow pattern often transitions from a relatively stable bubble flow to slug flow or even annular flow, resulting in a substantial increase in frictional resistance within the production string. Meanwhile, as light components such as methane and ethane escape, the proportion of heavy components in the degassed crude oil relatively increases. Macroscopically, this leads to a significant rise in crude oil viscosity and a drastic reduction in mobility. In microscopic pores, the breakout free gas bubbles trigger the Jamin effect at the pore throats, directly blocking the fluid channels. This causes a significant decrease in the relative permeability of the crude oil, ultimately leading to a severe deterioration in the well's inflow performance.

2. Physical Interference of Free Gas on Artificial Lift

This complex two-phase flow condition imposes severe physical interference on downhole artificial lift equipment.

For positive displacement equipment that relies on cavity volume changes, the high compressibility of gas is highly detrimental. When a high proportion of free gas is drawn into the pump, the pumping action is largely consumed by the compression and expansion of the gas. This makes it difficult to establish the effective pressure differential required for fluid discharge, resulting in a drastic drop in volumetric efficiency.

For centrifugal lift systems operating via high-speed impeller rotation, the massive density difference between gas and liquid makes free gas highly prone to accumulation in the low-pressure zones of the impeller. Once the local gas void fraction reaches a critical threshold, it blocks the continuous flow of the liquid phase, triggering a sudden drop in pump head or an underload shutdown (gas lock).

Furthermore, the relative motion of downhole mechanical components relies heavily on liquid-phase fluids for lubrication and cooling. High gas-cut conditions severely degrade the fluid's lubricity and specific heat capacity, exposing moving parts to the risk of dry friction and accelerating wear and fatigue.

3. Engineering Mitigation for Complex Two-Phase Flow Conditions

To address the complex phase behavior below the bubble point pressure, field operations typically require adjustments from both the reservoir and wellbore dimensions. The most direct approach is to supplement reservoir energy at the source through water or gas injection for pressure maintenance. By artificially maintaining the reservoir pressure near the bubble point, gas breakout is delayed, maximizing the single-phase flow state of the crude oil.

If massive free gas breakout within the wellbore is inevitable, a gas anchor must be installed below the intake of the artificial lift equipment. Utilizing the principles of fluid reversal and gravity segregation, the gas anchor separates the free gas before it enters the pump and vents it through the casing-tubing annulus.

Regarding the selection of artificial lift methods, gas lift technology is sometimes employed to transform the presence of gas into lifting power. By actively injecting high-pressure gas into the wellbore to reduce the mixture density and hydrostatic column pressure of the fluid within the tubing, the expansion energy of the gas is utilized to lift the crude oil to the surface, thereby effectively circumventing the vulnerability of mechanical equipment to gas lock.

For wells that still utilize mechanical lift, operational practices often involve deepening the pump setting depth and increasing pump submergence to elevate the pressure at the pump intake. This compresses the volume of free gas and strives to improve the fluid conditions entering the pump.

The decline of formation pressure below the bubble point is an inevitable stage in the development life cycle of the vast majority of oil and gas fields. From abrupt changes in fluid phase behavior to the artificial lift challenges posed by gas-liquid two-phase flow, only by accurately grasping the underlying physical principles and strategically adjusting reservoir management and artificial lift technologies can we truly guarantee stable and economic production during the middle and late stages of oilfield development.