# Roughneck City Blog

*My question concerns methane gas at formation temperatures. Are the pressures ever great enough, considering bottom hole temperatures to liquefy methane? Does anyone know of an equation that can be used to determine this?*

*What equation would we use to accurately calculate the pressure needed at various depths-mud weights and temperatures?*

*I am looking for something more easily understood than this below;*

**The Reservoir Engineering Handbook States the following;**

**Pressure-Temperature Diagram**

Figure 1-1 shows a typical pressure-temperature diagram of a multicomponent system with a specific overall composition. Although a different hydrocarbon system would have a different phase diagram, the

general configuration is similar.

These multicomponent pressure-temperature diagrams are essentially

used to:

• Classify reservoirs

• Classify the naturally occurring hydrocarbon systems

• Describe the phase behavior of the reservoir fluid

To fully understand the significance of the pressure-temperature diagrams,

it is necessary to identify and define the following key points on

these diagrams:

• Cricondentherm (Tct)—The Cricondentherm is defined as the maximum

temperature above which liquid cannot be formed regardless of

pressure (point E). The corresponding pressure is termed the Cricondentherm

pressure pct.

• Cricondenbar (pcb)—The Cricondenbar is the maximum pressure above

which no gas can be formed regardless of temperature (point D). The

corresponding temperature is called the Cricondenbar temperature Tcb.

• Critical point—The critical point for a multicomponent mixture is

referred to as the state of pressure and temperature at which all intensive

properties of the gas and liquid phases are equal (point C). At the

critical point, the corresponding pressure and temperature are called the

critical pressure pc and critical temperature Tc of the mixture.

• Phase envelope (two-phase region)—The region enclosed by the bubble-

point curve and the dew-point curve (line BCA), wherein gas and

liquid coexist in equilibrium, is identified as the phase envelope of the

hydrocarbon system.

• Quality lines—The dashed lines within the phase diagram are called

quality lines. They describe the pressure and temperature conditions for

equal volumes of liquids. Note that the quality lines converge at the

critical point (point C).

• Bubble-point curve—The bubble-point curve (line BC) is defined as

the line separating the liquid-phase region from the two-phase region.

• Dew-point curve—The dew-point curve (line AC) is defined as the

line separating the vapor-phase region from the two-phase region.

In general, reservoirs are conveniently classified on the basis of the

location of the point representing the initial reservoir pressure pi and temperature

T with respect to the pressure-temperature diagram of the reservoir

fluid.